Matt Williams, Author at Universe Today

December 2, 2015July 25, 2017 by Matt Williams

Zodiac Signs and Their Dates

A chart of the constellations and signs that make up the zodiac. Credit: NASA

Did you know that there are 88 constellations in the night sky? Over the course of several thousand years, human beings have cataloged and named them all. But only 12 of them are particularly famous and continue to play an active role in our astrological systems. These are known as the zodiac signs, 12 constellations that correspond to the different months of the year.

Each of these occupies a sector of the sky which makes up 30° of the ecliptic, starting at the vernal equinox – one of the intersections of the ecliptic with the celestial equator. The order of these astrological signs is Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces. Here are all the zodiac signs and their dates. If your birthday falls within one of those date ranges, that’s your zodiac sign.

Granted, modern science has shown astrology to be an ancient fallacy, a way of connecting patterns in celestial movements to events and behaviors here on Earth. But for ancient people, such patterns were necessary given the fact that they lacked an understanding of human psychology, astronomy, and that Earth was not the center of the universe.

The concept of the zodiac originated in Babylon in the 2nd millennium BCE, and was later influenced by Hellenistic (Ancient Greek), Roman, and Egyptian culture. This resulted in a mix of traditions, where the 12 zodiac symbols were associated with the 12 Houses – different fields of experience associated with the various planets – and the four classical elements (Earth, Wind, Water and Fire).

*The symbols of the zodiac. Credit: what-is-astrology.com*

In essence, astrology maintains that celestial phenomena are related to human activity, so the signs are held to represent certain characteristics of behavior and personality traits. What we know today as astrology comes from the 2nd century AD, as it was formally described by Ptolemy in his work, Tetrabiblos.

This book was responsible for the spread of astrology’s as we know it across Europe and the Middle East during the time of the Roman Empire. These traditions have remained relatively unchanged for over seventeen centuries, though some alterations have been made due to the subsequent discoveries of the other planets in our Solar System.

Naturally, the birth of the modern psychology, biology and astronomy has completely discredited the notion that our personalities are determined by birth signs, the position of the stars or the planets. Given what we know today of the actual elements, the movements of the planets, and the forces that govern the universe, astrology is now known for being little more than superstition.

What’s more, the dates of the ‘star signs’ were assigned over 2,000 years ago, when the zodiac was first devised. At that time, astronomers believed that the Earth’s position was fixed in the universe, and did not understand that the Earth is subject to precession – where Earth’s rotational and orbital parameters slowly change with time. As such, the zodiac signs no longer correspond to constellations of stars that appear in night sky.

The constellations Ophiuchus. Credit: — *The constellations Ophiuchus, represented as a man grasping a snake. Credit: chandra.harvard.edu*

And last, but certainly not least, there is the issue of the missing 13th sign, which corresponds to the constellation Ophiuchus. Over 2000 years ago, this constellation was deliberately left out, though the Sun clearly passes in front of it after passing in front of Scorpius (aka. Scorpio) and before reaching Sagittarius.

It is unclear why ancient astrologers would do this, but it is a safe bet that they wanted to divide the 360° path of the Sun into 12 equal parts. But the true boundaries that divide the constellations, as defined by the International Astronomical Union (IAU), are not exact. And Ophiuchus actually spends more time behind the Sun than its immediate neighbor (19 days compared to Scorpius’ 12).

To find out what zodiac sign you were really born under, check out this story from BBC’s iWonder. And in the meantime, here are the zodiac signs, listed in order along with what they mean, and some interesting facts associated with their respective constellations:

Aries: March 21 – April 19

The sign of Aries, which covers 0° to 30° of celestial longitude, is represented by The Ram, which is based on the Chrysomallus – the flying ram that provided the Golden Fleece in Greek mythology. Aries is associated with the First House, known traditionally as Vita (Latin for life) and in the modern context as the “House of Self”. Aries is associated with Fire, and the ruling celestial body of Aries is Mars.

The Aries constellation is also home to Teegarden’s Star, one of Sun’s closest neighbors, located approximately 12 light years away. It appears to be a red dwarf, a class of low temperature and low luminosity stars. And then there’s Alpha Areitis, which is easily spotted by the naked eye. Also known as “Hamal” – literally “head of the sheep” in Arabic – this star is located at the point where constellations angles downward to form an arc.

*The constellation Aries. Credit: iau.org*

For those with telescopes, several galaxies can be spotted within the Aries constellation as well. These include the spiral galaxy NGC 772 and the large 13th magnitude NGC 697 spiral galaxy. NGC 972 is another, which is faint (at magnitude 12) and part of a galaxy group. And then there’s the dwarf irregular galaxy NGC 1156, which is considered a Magellanic-type galaxy with a larger than average core.

Aries is also home to several meteor showers, such as the May Arietids. This daylight meteor shower begins between May 4th and June 6th with maximum activity happening on May 16th. The Epsilon Arietids are also a daylight occurrence, and are active between April 25th to May 27th with peak activity on May 9th. And then there are the Daytime Arietids, which occur from May 22nd to July 2nd with a maximum rate of one a minute on June 8th.

To top it off, the Aries constellation contains several stars with extrasolar planets. For example, HIP 14810, a G5 type star, is orbited by three confirmed exoplanets, all of them giant planets (all Super-Earths). HD 12661, also a G-type main sequence star, has two orbiting planets (which appear to be Super-Jupiters). And HD 20367, a G0 type star, has one orbiting gas giant that roughly the same size as Jupiter.

Taurus: April 20 – May 20

The sign of Taurus, which covers 30° to 60° of celestial longitude, is represented by The Bull – which is based on the Cretan Bull that fathered the Minotaur and was killed by Theseus. Taurus is associated with the Second House, known by the Latin name of Lucrum (wealth) and by the modern name, “House of Value”, and the element Earth. The ruling celestial body of Taurus is Venus.

*The constellation Taurus. Credit: iau.org*

Taurus’ brightest star, Alpha Tauri, is also known by its traditional name, Al Dabaran (which was Latinized to become Aldebaran). The name, which is Arabic, literally means “the Follower” because of the way the Taurus constellation appears to follow the Pleiades star cluster across the sky. In Latin, it was traditionally known as Stella Dominatrix, but to Medieval English astronomers, it was known as Oculus Tauri – literally the “eye of Taurus.”

There is one major annual meteor shower associated with the constellation of Taurus: the annual Taurids, which peak on or about November 5th of each year and have a duration period of about 45 days. The maximum fall rate for this meteor shower is about 10 meteors per hour, with many bright fireballs often occurring when the parent comet – Encke – has passed near perihelion.

And speaking of Pleiades (aka. Messier 45, The Seven Sisters) this cluster of stars is located perpendicular to Aldebaran in the night sky, and is visible to the unaided eye. Although it is made up of over 1000 confirmed stars, this object is identifiable by its seven particularly bright blue stars (though as many as 14 up can be seen with the naked eye depending on local observing conditions).

Gemini: May 21 – June 20

The sign Gemini covers 60° to 90° of the celestial longitude, and is represented by The Twins. These are based on the Dioscuri of Greek mythology, two mortals that were granted shared godhood after death. Gemini is part of the Third House, traditionally named Fratres (Brothers) and currently known as the House of Communications. The associated element for Geminis is Air, and the ruling celestial body is Mercury.

*The constellation Gemini. Credit: iau.org*

Gemini’s alpha and beta stars – aka. Castor and Pollux (“The Twins”) – are the easiest to recognize and can be spotted with the naked eye. Pollux is the brighter of the two, an orange-hued giant star of magnitude 1.2 that is 34 light-years from Earth. Pollux has an extrasolar planet revolving around it, as do two other stars in Gemini, a super-Jupiter which was confirmed in 2006.

There are two annual meteor showers associated with the constellation of Gemini. The first is the March Geminids, which peaks on or around March 22nd. The average fall rate is generally about 40 per hour (but this varies) and the meteors appear to be very slow, entering our atmosphere unhurriedly and leaving lasting trails.

The second meteor shower are the Geminids themselves, which peak on or near the date of December 14th, with activity beginning up to two weeks prior and lasting for several days. The Geminids are one of the most beautiful and mysterious showers, with a rate of about 110 per hour during a moonless night.

The Gemini constellation is also associated with Messier 35, a galactic open star cluster that is easily spotted with the naked eye. The star cluster is quite young, having formed some 100 million years ago, and is quite bright due to it having blown away most of its leftover material (i.e. nebular dust and gas) that went into the star formation process. Other open clusters in Gemini include NGC 2158, which lies directly southwest of M35 in the night sky.

The open star clusters Messier 35 and NGC 2158, photographed at La Palma, Roque de los Muchachos (Degollada de los Franceses). Credit: estelar.de/Oliver Stein — *The open star clusters Messier 35 and NGC 2158, photographed at La Palma, Roque de los Muchachos. Credit: estelar.de/Oliver Stein*

Cancer: June 21 – July 22

Cancer, which covers 90° to 120° of celestial longitude, is represented by The Crab – or Karkinos, a giant crab from Greek mythology that harassed Hercules during his fight with the Hydra. The sign is associated with the Fourth House – Genitor (Parent) in Latin, or the House of Home and Family in modern times. In terms of the elements, Cancers are characterized by the element of Water, and the ruling celestial body of Cancer is The Moon.

Cancer’s best known star is Beta Cancri, also known by its Arab name Altarf (“the End”). This 3.5 magnitude star is located 290 light-years from Earth and is a binary star system that consists of a spectral type K4III orange giant and a magnitude 14 red dwarf. This system is also home to a confirmed exoplanet, beta Cancri b, which is a Super-Jupiter with an orbital period of over 600 days.

In terms of deep-sky objects, Cancer is best known as being the home of Messier Object 44 (aka. Praesepe, or the Beehive Cluster), an open cluster located in the center of the constellation. Located 577 light-years from Earth, it is one of the nearest open clusters to our Solar System. M44 contains about 50 stars, the brightest of which are of the sixth magnitude.

The smaller, denser open cluster of Messier Object 67 can also be found in Cancer, which is 2500 light-years from Earth and contains approximately 200 stars. And so can the famous quasar, QSO J0842+1835, which was used to measure the speed of gravity in the VLBI experiment conducted by Edward Fomalont and Sergei Kopeikin in September 2002.

*The location of the Caner constellation. Credit: IAU*

The active galaxy OJ 287 is also found in the Cancer constellation. Located 3.5 billion light years away from Earth, this galaxy has a central supermassive black hole that is one of the largest known (with 18 billion solar masses), and produces quasi-periodic optical outbursts. There is only one meteor shower associated with the constellation of Cancer, which is the Delta Cancrids. The peak date for this shower is on or about January 16t, and has been known to average only about 4 comets per hour (and the meteors are very swift).

Leo: July 23 – Aug. 22

Those born under the sign of Leo, which covers 120° to 150° of celestial longitude, carry the sign of The Lion – which is based on the Nemean Lion of Greek mythology, a lion that had an impenetrable hide. The sign of Leo is tied to the Fifth House, known in Latin as Nati (Children), or by its modern name, House of Pleasure. The sign of Leo is also associated with the element of Fire and the ruling celestial body of Leo is The Sun.

There are five annual meteor showers associated with the constellation Leo. The first is the Delta Leonid meteor stream, which begins between February 5th through March 19th every year. The activity peaks in late February, and the maximum amount of meteors averages around 5 per hour. The next is the Sigma Leonid meteor shower, which begins on April 17th. This is a very weak shower, with activity rates no higher than 1 to 2 per hour.

The next is the November Leonids, the largest and most dependable meteor shower associated with the Leo constellation. The peak date is November 17th, but activity occurs around 2 days on either side of the date. The radiant is near Regulus and this is the most spectacular of modern showers.

*The constellation Leo. Credit: iau.org*

The shower is made more spectacular by the appearance of the Temple-Tuttle comet, which adds fresh material to the stream when it is at perihelion. The last is the Leo Minorids, which peak on or about December 14th, which is believed to produce around 10 faint meteors per hour.

Leo is also home to some of the largest structures in the observable universe. This includes many bright galaxies, which includes the Leo Triplet (aka. the M60 group). This group of objects is made up of three spiral galaxies – Messier 65, Messier 66, and NGC 3628.

The Triplet is at a distance of 37 million light-years from Earth and has a somewhat distorted shape due to gravitational interactions with the other members of the Triplet, which are pulling stars away from M66. Both M65 and M66 are visible in large binoculars or small telescopes, but seeing them in all of their elongated glory requires a telescope.

In addition, it is also home to the famous objects Messier 95, Messier 96, and Messier 105. These are spiral galaxies, in the case of M95 and M96 (with M95 being a barred spiral), while Messier 105 is an elliptical galaxy which is known to have a supermassive black hole at its center. Then there is the Leo Ring (aka. Cosmic Horseshoe) a cloud of hydrogen and helium gas, that orbits two galaxies found within this constellation.

*The notable gravitational lens known as the Cosmic Horseshoe is found in Leo. Credit: NASA/ESA/Hubble*

Virgo: Aug. 23 – Sept. 22

The sign of Virgo, which covers 150° to 180° of celestial longitude, is represented by the The Maiden. Based on Astraea from Greek mythology, the maiden was the last immortal to abandon Earth at the end of the Silver Age, when the gods fled to Olympus. Virgo is part of the Sixth House – Valetudo (Health) in Latin, or House of Health in modern times. They are also associated with the element of Earth and the ruling celestial body of Virgo is Mercury.

The brightest star in the Virgo constellation is Spica, a binary and rotating ellipsoidal variable – which means the two stars are so close together that they are egg-shaped instead of spherical – located between 240 and 260 light years from Earth. The primary is a blue giant and a variable star of the Beta Cephei type.

Besides Spica, other bright stars in Virgo include Beta Virginis (Zavijava), Gamma Virginis (Porrima), Delta Virginis (Auva) and Epsilon Virginis (Vindemiatrix). Other fainter stars that were also given names are Zeta Virginis (Heze), Eta Virginis (Zaniah), Iota Virginis (Syrma) and Mu Virginis (Rijl al Awwa). Virgo’s stars are also home to a great many exoplanets, with 35 verified exoplanets orbiting 29 of its stars.

The star 70 Virginis was one of the first planetary systems to have a confirmed exoplanet discovered orbiting it, which is 7.5 times the mass of Jupiter. The star Chi Virginis has one of the most massive planets ever detected, at a mass of 11.1 times that of Jupiter. The sun-like star 61 Virginis has three planets: one is a super-Earth and two are Neptune-mass planets.

*The constellation Virgo. Credit: iau.org*

Libra: Sept. 23 – Oct. 22

The sign of Libra covers 180° to 210° of celestial longitude. It is represented by the symbol of The Scales, which is based on the Scales of Justice held by Themis, the Greek personification of divine law and custom and the inspiration for modern depictions of Lady Justice. Libra is part of the Seventh House – Uxor (Spouse) or House of Partnership, are associated with the element of Air, and the ruling celestial body is Venus.

Two notable stars in the Libra constellation are Alpha and Beta Librae – also known as Zubenelgenubi and Zubeneschamali, which translates to “The Southern Claw” and “The Northern Claw”. Alpha Libae is a double star consisting of an A3 primary star with a slight blue tinge and a fainter type F4 companion, both of which are located approximately 77 light years from our Sun.

Beta Librae is the brighter of the two, and the brightest star in the Virgo constellation. This is a blue star of spectral type B8 (but which appears somewhat greenish) which is located roughly 160 light years from Earth. Then there’s Gamma Librae (also called Zubenelakrab, which means “the Scorpion’s Claw”) which completes the Scorpion sign. It is an orange giant of magnitude 3.9, and is located 152 light-years from Earth.

*The constellation Libra. Credit: iau.org*

Libra is home to the star Gliese 581, which has a planetary system consisting of at least 6 planets. Both Gliese 581 d and Gliese 581 g are considered to be some of the most promising candidates for life. Gliese 581 c is considered to be the first Earth-like exoplanet to be found within its parent star’s habitable zone. All of these exoplanets are of significance for establishing the likelihood of life outside of the Solar System.

Libra is also home to one bright globular cluster, NGC 5897. It is a fairly large and loose cluster, has an integrated magnitude of 9, and is located 40,000 light-years from Earth.

Scorpio: Oct. 23 – Nov. 21

The sign of Scorpio covers 210° to 240° of celestial longitude. Scorpio is represented by The Scorpion, which is based on Scorpius – a giant scorpion in Greek mythology sent by Gaia to kill Orion. Scorpio is part of the Eighth House – Mors (Death), known today as the House of Reincarnation – and is associated with the element of Water. Traditionally, the ruling celestial body of Scorpio was Mars, but has since become Pluto.

The Scorpius constellations includes many bright stars, the brightest being Alpha Scorpii (aka. Antares). The name literally means “rival of Mars” because of its distinct reddish hue. Other stars of note include Beta Scorpii (Acrab, or “the scorpion”), Delta Scorpii (Dschubba, or “the forehead”), Xi Scorpii (Girtab, also “the scorpion”), and Sigma and Tau Scorpii (Alniyat, “the arteries”).

Lambda Scorpii (Shaula) and Upsilon Scorpii (Lesath) – whose names both mean “sting”- mark the tip of the scorpion’s curved tail. Given their proximity to one another, Lambda Scorpii and Upsilon Scorpii are sometimes referred to as “the Cat’s Eyes”.

*The constellation Scorpius. Credit: iau.org*

The Scorpius constellation, due to its position within the Milky Way, contains many deep-sky objects. These include the open clusters Messier 6 (the Butterfly Cluster) and Messier 7 (the Ptolemy Cluster), the open star cluster NGC 6231 (aka. Northern Jewel Box), and the globular clusters Messier 4 and Messier 80 (NGC 6093).

The constellation is also where the Alpha Scorpiids and Omega Scorpiids meteor showers take place. The Alphas begin on or about April 16th and end around May 9th, with a peak date of most activity on or about May 3rd. The second meteor shower, the Omega (or June) Scorpiids peaks on or about June 5th of each year. The radiant for this particular shower is closer to the Ophiuchus border and the activity rate on the peak date is high – with an average of about 20 meteors per hour and many reported fireballs.

Sagittarius: Nov. 22 – Dec. 21

The sign of Sagittarius covers 240° to 270° of celestial longitude and is represented by The Archer. This symbol is based on the centaur Chiron, who according to Greek mythology mentored Achilles in the art of archery. Sagittarius is part of the Ninth House – known as Iter (Journeys) or the House of Philosophy. Sagittarius’ associated element is Fire (positive polarity), and the ruling celestial body is Jupiter.

Stars of note in the Sagittarius constellation include Alpha Sagittarii, which is also known as Alrami or Rukbat (literally “the archer’s knee”). Then there is Epsilon Sagittarii (“Kaus Australis” or “southern part of the bow”), the brightest star in the constellation – at magnitude 1.85. Beta Sagittarii, located at a position associated with the forelegs of the centaur, has the traditional name Arkab, which is Arabic for “achilles tendon.”

*The Sagittarius constellation. Credit: iau.org*

The second-brightest star is Sigma Sagittarii (“Nunki”), which is a B2V star at magnitude 2.08, approximately 260 light years from our Sun. Nunki is the oldest star name currently in use, having been assigned by the ancient Babylonians, and thought to represent the sacred Babylonian city of Eridu. Then there’s Gamma Sagittarii, otherwise known as Alnasl (the “arrowhead”). This is actually two star systems that share the same name, and both stars are actually discernible to the naked eye.

The Milky Way is at its densest near Sagittarius, since this is the direction in which the galactic center lies. Consequently, Sagittarius contains many star clusters and nebulae. This includes Messier 8 (the Lagoon Nebula), an emission (red) nebula located 5,000 light years from Earth which measures 140 by 60 light years.

Though it appears grey to the unaided eye, it is fairly pink when viewed through a telescope and quite bright (magnitude 3.0). The central area of the Lagoon Nebula is also known as the Hourglass Nebula, so named for its distinctive shape. Sagittarius is also home to the M17 Omega Nebula (also known as the Horseshoe or Swan Nebula).

This nebula is fairly bright (magnitude 6.0) and is located about 4890 light-years from Earth. Then there’s the Trifid Nebula (M20 or NGC 6514), an emission nebula that has reflection regions around the outside, making its exterior bluish and its interior pink. NGC 6559, a star forming region, is also associated with Sagittarius, located about 5000 light-years from Earth and showing both emission and reflection regions (blue and red).

Capricorn: Dec. 22 – Jan. 19

The sign of Capricorn spans 270° to 300° of celestial longitude and is represented by the Mountain Sea-Goat. This sign is based on Enki, the Sumerian primordial god of wisdom and waters who has the head and upper body of a mountain goat, and the lower body and tail of a fish. The sign is part of the Tenth House – Regnum (Kingdom), or The House of Social Status. Capricorns are associated with the element Earth, and the ruling body body is Saturn.

*The constellation Capricornus. Credit: iau.org*

The brightest star in Capricornus is Delta Capricorni, also called Deneb Algedi. Like other stars such as Denebola and Deneb, it is named for the Arabic word for “tail”, which in this case translates to “the tail of the goat’. Deneb Algedi is a eclipsing binary star with a magnitude of 2.9, and which is located 39 light-years from Earth.

Another bright star in the Capricorni constellation is Alpha Capricorni (Algedi or Geidi, Arabic for “the kid”), which is an optical double star (two stars that appear close together) – both o which are binaries. It’s primary (Alpha² Cap) is a yellow-hued giant of magnitude 3.6, located 109 light-years from Earth, while its secondary (Alpha¹ Cap) is a yellow-hued supergiant of magnitude 4.3, located 690 light-years from Earth.

Beta Capricorni is a double star known as Dabih, which comes from the Arabic phrase “the lucky stars of the slaughter” a reference to ritual sacrifices performed by ancient Arabs. Its primary is a yellow-hued giant star of magnitude 3.1, 340 light-years from Earth, while the secondary is a blue-white hued star of magnitude 6.1. Another visible star is Gamma Capricorni (aka. Nashira, “bringing good tidings”), which is a white-hued giant star of magnitude 3.7, 139 light-years from Earth.

Several galaxies and star clusters are contained within Capricornus. This includes Messier 30 (NGC 7099) a centrally-condensed globular cluster of magnitude 7.5. Located approximately 30,000 light-years from our Sun, it cannot be seen with the naked eye, but has chains of stars extending to the north that can be seen with a telescope.

*The globular cluster Messier 30, imaged by the Hubble Telescope. Credit: NASA/Wikisky*

And then there is the galaxy group known as HCG 87, a group of at least three galaxies located 400 million light-years from Earth. It contains a large elliptical galaxy, a face-on spiral galaxy, and an edge-on spiral galaxy. These three galaxies are interacting, as evidenced by the high amount of star formation in the face-on spiral, and the connecting stream of stars and dust between edge-on spiral and elliptical galaxy.

The constellation of Capricornus has one meteor shower associated with it. The Capricornid meteor stream peaks on or about July 30th and is active for about a week before and after, with an average fall rate is about 10 to 30 per hour.

Aquarius: Jan. 20 – Feb. 18

Aquarius, which spans 300° to 330° of celestial longitude, is represented by the Water Bearer. In ancient Greek mythology, Aquarius is Ganymede, the beautiful Phrygian youth who was snatched up by Zeus to become the cup-bearer of the Gods. Aquarius is part of the Eleventh House – Benefacta (Friendship), or House of Friendship, is associated with the element of Air. Traditionally, the ruling celestial body of Aquarius was Saturn, but has since changed to Uranus.

While Aquarius has no particularly bright stars, recent surveys have shown that there are twelve exoplanet systems within the constellation (as of 2013). Gliese 876, one of the nearest stars (15 light-years), was the first red dwarf start to be found to have a planetary system – which consists of four planets, one of which is a terrestrial Super-Earth. 91 Aquarii is an orange giant star orbited by one planet, 91 Aquarii b, a Super-Jupiter. And Gliese 849 is a red dwarf star orbited by the first known long-period Jupiter-like planet, Gliese 849 b.

*The constellation Aquarius. Credit: iau.org*

Aquarius is also associated with multiple Messier objects. M2 (NGC 7089) is located in Aquarius, which is an incredibly rich globular cluster located approximately 37,000 light-years from Earth. So is the four-star asterism M73 (which refers to a group of stars that appear to be related by their proximity to each other). Then there’s the small globular cluster M72, a globular cluster that lies a degree and half to the west of M73.

Aquarius is also home to several planetary nebulae. NGC 7293, also known as the Helix Nebula, is located at a distance of about 650 light years away, making it the closest planetary nebula to Earth. Then there’s the Saturn Nebula (NGC 7009) so-named because of its apparent resemblance to the planet Saturn through a telescope, with faint protrusions on either side that resemble Saturn’s rings.

There are five meteor showers associated with the constellation of Aquarius. The Southern Iota Aquarids begin around July 1st and end around September 18th, with the peak date occurring on August 6th with an hourly rate of 7-8 meteors average. The Northern Iota Aquarids occur between August 11th to September 10th, their maximum peak occurring on or about August 25th with an average of 5-10 meteors per hour.

Image of the Helix Nebula, combining from information from NASA's Spitzer Space Telescope and the Galaxy Evolution Explorer (GALEX). Credit: NASA — *Image of the Helix Nebula, combining from information from NASA’s Spitzer Space Telescope and the Galaxy Evolution Explorer (GALEX). Credit: NASA*

The Southern Delta Aquarids begin about July 14th and end around August 18th with a maximum hourly rate of 15-20 peaking on July 29th. The Northern Delta Aquarids usually begin around July 16th, and last through September 10th. The peak date occurs on or around August 13th with a maximum fall rate of about 10 meteors per hour.

Then there is the Eta Aquarid meteor shower, which begins about April 21th and ends around May 12th. It reaches its maximum on or about May 5th with a peak fall rate of up to 20 per hour for observers in the northern hemisphere and perhaps 50 per hour for observers in the southern hemisphere. Last, there is the March Aquarids, a daylight shower that may be associated with the Northern Iota Aquarid stream.

Pisces: Feb. 19 – March 20

The sign of Pisces covers 330° to 360° of celestial longitude and is represented by the The Fish. This symbol is derived from the ichthyocentaurs – a pair of centaurian sea-gods that had the upper body of a male human, the lower front of a horse, and the tail of a fish – who aided Aphrodite when she was born from the sea. Pisces is part of the Twelfth House of Carcer (Prison), or The House of Self-Undoing, and are associated with the element of Water. The ruling celestial body of Pisces is traditionally Jupiter, but has since come to be Neptune.

*The constellation Pisces. Credit: iau.org*

Beta Piscium, also known as Samakah (the “Fish’s Mouth”), is a B-class hydrogen fusing dwarf star in the Pisces constellation. Located 495 light years from Earth, this star produces 750 times more than light than our own Sun and is believed to be 60 million years old. The brightest star in the constellation, Eta Piscium, is a bright class B star that is located 294 years away from our Solar System.

This star is also known by its Babylonian name, Kullat Nunu (which translates to “cord of the fish”), the Arab name Al Pherg (“pouring point of water”), and the Chinese name Yòu Gèng – which means “Official in Charge of the Pasturing“, referring to an asterism consisting of Eta Piscium and its immediate neighbors – Rho Piscium, Pi Piscium, Omicron Piscium, and 104 Piscium.

And then there’s van Maanen’s Star (aka. Van Maanen 2) a white dwarf that is located about 14 light years from our Sun, making it the third closest star of its kind to our system (after Sirius B and Procyon B). Gamma Piscium is a yellow-orange giant star located about 130 light years away, and is visible with just binoculars.

The Pisces constellationis also home to a number of deep-sky objects. These include M74, a loosely-wound spiral galaxy that lies at a distance of 30 million light years from our Sun. It has many clusters of young stars and the associated nebulae, showing extensive regions of star formation. Also, there’s CL 0024+1654, a massive galaxy cluster that is primarily made up of yellow elliptical and spiral galaxies. CL 0024+1654 lies at a distance of 3.6 billion light-years from Earth and lenses the galaxy behind it (i.e. it creates arc-shaped images of the background galaxy).

Last, there the active galaxy and radio source known as 3C 31. Located at a distance of 237 million light-years from Earth, this galaxy has a supermassive black hole at its center. In addition to being the source of its radio waves, this black hole is also responsible for creating the massive jets that extend several million light-years in both directions from its center – making them some of the largest objects in the universe.

There is one annual meteor shower associated with Pisces which peaks on or about October 7 of each year. The Piscid meteor shower has a radiant near the Aries constellation and produces an average of 15 meteors per hour which have been clocked at speeds of up to 28 kilometers per second. As always, the meteoroid stream can begin a few days earlier and end a few days later than the expected peak and success on viewing depends on dark sky conditions.

Currently, the Vernal Equinox is currently located in Pisces. In astronomy, equinox is a moment in time at which the vernal point, celestial equator, and other such elements are taken to be used in the definition of a celestial coordinate system. Due to the precession of the equinoxes, the Vernal Equinox is slowly drifting towards Aquarius.

Astrology is a tradition that has been with us for thousands of years and continues to be observed by many people and cultures around the world. Today, countless people still consult their horoscope to see what the future has in store, and many more consider their birth sign to be of special significance.

And the fact that many people still consider it to be valid is an indication that superstitious and “magical” thinking is something we have yet to completely outgrow. But this goes to show how some cultural traditions are so enduring, and how people still like to ascribe supernatural powers to the universe.

We have a complete guide to all 88 constellations here at Universe Today. Research them at your leisure, and be sure to check out more than just the “zodiac sign” ones!

We also have a comprehensive list of all the Messier Objects in the night sky.

Astronomy Cast also has an episode on Zodiac Signs – Episode 319: The Zodiac

November 23, 2015March 14, 2018 by Matt Williams

Who was Stephen Hawking?

In honor of Dr. Stephen Hawking, the COSMOS center will be creating the most detailed 3D mapping effort of the Universe to date. Credit: BBC, Illus.: T.Reyes

When we think of major figures in the history of science, many names come to mind. Einstein, Newton, Kepler, Galileo – all great theorists and thinkers who left an indelible mark during their lifetime. In many cases, the full extent of their contributions would not be appreciated until after their death. But those of us that are alive today are fortunate to have a great scientist among us who made considerable contributions – Dr. Stephen Hawking.

Considered by many to be the “modern Einstein”, Hawking’s work in cosmology and theoretical physics was unmatched among his contemporaries. In addition to his work on gravitational singularities and quantum mechanics, he was also responsible for discovering that black holes emit radiation. On top of that, Hawking was a cultural icon, endorsing countless causes, appearing on many television shows as himself, and penning several books that have made science accessible to a wider audience.

Early Life:

Hawking was born on January 8th, 1942 (the 300th anniversary of the death of Galileo) in Oxford, England. His parents, Frank and Isobel Hawking, were both students at Oxford University, where Frank studied medicine and Isobel studied philosophy, politics and economics. The couple originally lived in Highgate, a suburb of London, but moved to Oxford to get away from the bombings during World War II and give birth to their child in safety. The two would go on to have two daughters, Philippa and Mary, and one adopted son, Edward.

The family moved again in 1950, this time to St. Albans, Hertfordshire, because Stephen’s father became the head of parasitology at the National Institute for Medical Research (now part of the Francis Crick Institute). While there, the family gained the reputation for being highly intelligent, if somewhat eccentric. They lived frugally, living in a large, cluttered and poorly maintained house, driving around in a converted taxicab, and constantly reading (even at the dinner table).

*Stephen Hawking as a young man. Credit: gazettereview.com*

Education:

Hawking began his schooling at the Byron House School, where he experienced difficulty in learning to read (which he later blamed on the school’s “progressive methods”.) While in St. Albans, the eight-year-old Hawking attended St. Albans High School for Girls for a few months (which was permitted at the time for younger boys). In September of 1952, he was enrolled at Radlett School for a year, but would remain at St. Albans for the majority of his teen years due the family’s financial constraints.

While there, Hawking made many friends, with whom he played board games, manufactured fireworks, model airplanes and boats, and had long discussions with on subjects ranging from religion to extrasensory perception. From 1958, and with the help of the mathematics teacher Dikran Tahta, Hawking and his friends built a computer from clock parts, an old telephone switchboard and other recycled components.

Though he was not initially academically successfully, Hawking showed considerable aptitude for scientific subjects and was nicknamed “Einstein”. Inspired by his teacher Tahta, he decided to study mathematics at university. His father had hoped that his son would attend Oxford and study medicine, but since it was not possible to study math there at the time, Hawking chose to study physics and chemistry.

*Stephen Hawking (holding the handkerchief) and the Oxford Boat Club. Credit: focusfeatures.com*

In 1959, when he was just 17, Hawking took the Oxford entrance exam and was awarded a scholarship. For the first 18 months, he was bored and lonely, owing to the fact that he was younger than his peers and found the work “ridiculously easy”. During his second and third year, Hawking made greater attempts to bond with his peers and developed into a popular student, joining the Oxford Boat Club and developing an interest in classical music and science fiction.

When it came time for his final exam, Hawking’s performance was lackluster. Instead of answering all the questions, he chose to focus on theoretical physics questions and avoided any that required factual knowledge. The result was a score that put him on the borderline between first- and second-class honors. Needing a first-class honors for his planned graduate studies in cosmology at Cambridge, he was forced to take a via (oral exam).

Concerned that he was viewed as a lazy and difficult student, Hawking described his future plans as follows during the viva: “If you award me a First, I will go to Cambridge. If I receive a Second, I shall stay in Oxford, so I expect you will give me a First.” However, Hawking was held in higher regard than he believed, and received a first-class BA (Hons.) degree, thus allowing him to pursue graduate work at Cambridge University in October 1962.

*Hawking on graduation day in 1962. Credit: telegraph.co.uk*

Hawking experienced some initial difficulty during his first year of doctoral studies. He found his background in mathematics inadequate for work in general relativity and cosmology, and was assigned Dennis William Sciama (one of the founders of modern cosmology) as his supervisor, rather than noted astronomer Fred Hoyle (whom he had been hoping for).

In addition, it was during his graduate studies that Hawking was diagnosed with early-onset amyotrophic lateral sclerosis (ALS). During his final year at Oxford, he had experienced an accident where he fell down a flight of stairs, and also began experiencing difficulties when rowing and incidents of slurred speech. When the diagnosis came in 1963, he fell into a state of depression and felt there was little point in continuing his studies.

However, his outlook soon changed, as the disease progressed more slowly than the doctors had predicted – initially, he was given two years to live. Then, with the encouragement of Sciama, he returned to his work, and quickly gained a reputation for brilliance and brashness. This was demonstrated when he publicly challenged the work of noted astronomer Fred Hoyle, who was famous for rejecting the Big Bang theory, at a lecture in June of 1964.

*Stephen Hawking and Jane Wilde on their wedding day, July 14, 1966. Credit: telegraph.co.uk*

When Hawking began his graduate studies, there was much debate in the physics community about the prevailing theories of the creation of the universe: the Big Bang and the Steady State theories. In the former, the universe was conceived in a gigantic explosion, in which all matter in the known universe was created. In the latter, new matter is constantly created as the universe expands. Hawking quickly joined the debate.

Hawking became inspired by Roger Penrose’s theorem that a spacetime singularity – a point where the quantities used to measure the gravitational field of a celestial body become infinite – exists at the center of a black hole. Hawking applied the same thinking to the entire universe, and wrote his 1965 thesis on the topic. He went on to receive a research fellowship at Gonville and Caius College and obtained his PhD degree in cosmology in 1966.

It was also during this time that Hawking met his first wife, Jane Wilde. Though he had met her shortly before his diagnosis with ALS, their relationship continued to grow as he returned to complete his studies. The two became engaged in October of 1964 and were married on July 14th, 1966. Hawking would later say that his relationship with Wilde gave him “something to live for”.

Scientific Achievements:

In his doctoral thesis, which he wrote in collaboration with Penrose, Hawking extended the existence of singularities to the notion that the universe might have started as a singularity. Their joint essay – entitled, “Singularities and the Geometry of Space-Time” – was the runner-up in the 1968 Gravity Research Foundation competition and shared top honors with one by Penrose to win Cambridge’s most prestigious Adams Prize for that year.

In 1970, Hawking became part of the Sherman Fairchild Distinguished Scholars visiting professorship program, which allowed him to lecture at the California Institute of Technology (Caltech). It was during this time that he and Penrose published a proof that incorporated the theories of General Relativity and the physical cosmology developed by Alexander Freidmann.

Based on Einstein’s equations, Freidmann asserted that the universe was dynamic and changed in size over time. He also asserted that space-time had geometry, which is determined by its overall mass/energy density. If equal to the critical density, the universe has zero curvature (i.e. flat configuration); if it is less than critical, the universe has negative curvature (open configuration); and if greater than critical, the universe has a positive curvature (closed configuration)

According to the Hawking-Penrose singularity theorem, if the universe truly obeyed the models of general relativity, then it must have begun as a singularity. This essentially meant that, prior to the Big Bang, the entire universe existed as a point of infinite density that contained all of the mass and space-time of the universe, before quantum fluctuations caused it to rapidly expand.

Per the Friedmann equations, the geometry of the universe is determined by its overall mass/energy density. If equal to the critical density, ?0 the universe has zero curvature (flat configuration). If less than critical, the universe has negative curvature (open configuration). If greater than critical, the universe has positive curvature (closed configuration). Image credit: NASA/GSFC — *Per the Friedmann equations, the geometry of the universe is determined by its overall mass/energy density, and can have either flat, negative, or positive curvature. Credit: NASA/GSFC*

Also in 1970, Hawking postulated what became known as the second law of black hole dynamics. With James M. Bardeen and Brandon Carter, he proposed the four laws of black hole mechanics, drawing an analogy with the four laws of thermodynamics.

These four laws stated that – for a stationary black hole, the horizon has constant surface gravity; for perturbations of stationary black holes, the change of energy is related to change of area, angular momentum, and electric charge; the horizon area is, assuming the weak energy condition, a non-decreasing function of time; and that it is not possible to form a black hole with vanishing surface gravity.

In 1971, Hawking released an essay titled “Black Holes in General Relativity” in which he conjectured that the surface area of black holes can never decrease, and therefore certain limits can be placed on the amount of energy they emit. This essay won Hawking the Gravity Research Foundation Award in January of that year.

In 1973, Hawking’s first book, which he wrote during his post-doc studies with George Ellis, was published. Titled, The Large Scale Structure of Space-Time, the book describes the foundation of space itself and the nature of its infinite expansion, using differential geometry to examine the consequences of Einstein’s General Theory of Relativity.

Hawking was elected a Fellow of the Royal Society (FRS) in 1974, a few weeks after the announcement of Hawking radiation (see below). In 1975, he returned to Cambridge and was given a new position as Reader, which is reserved for senior academics with a distinguished international reputation in research or scholarship.

The mid-to-late 1970s was a time of growing interest in black holes, as well as the researchers associated with them. As such, Hawking’s public profile began to grow and he received increased academic and public recognition, appearing in print and television interviews and receiving numerous honorary positions and awards.

In the late 1970s, Hawking was elected Lucasian Professor of Mathematics at the University of Cambridge, an honorary position created in 1663 which is considered one of the most prestigious academic posts in the world. Prior to Hawking, its former holders included such scientific greats as Sir Isaac Newton, Joseph Larmor, Charles Babbage, George Stokes, and Paul Dirac.

His inaugural lecture as Lucasian Professor of Mathematics was titled: “Is the end in sight for Theoretical Physics”. During the speech, he proposed N=8 Supergravity – a quantum field theory which involves gravity in 8 supersymmetries – as the leading theory to solve many of the outstanding problems physicists were studying.

Hawking’s promotion coincided with a health crisis which led to Hawking being forced to accept some nursing services at home. At the same time, he began making a transition in his approach to physics, becoming more intuitive and speculative rather than insisting on mathematical proofs. By 1981, this saw Hawking begin to focus his attention on cosmological inflation theory and the origins of the universe.

Inflation theory – which had been proposed by Alan Guth that same year – posits that following the Big Bang, the universe initially expanded very rapidly before settling into to a slower rate of expansion. In response, Hawking presented work at the Vatican conference that year, where he suggested that their might be no boundary or beginning to the universe.

During the summer of 1982, he and his colleague Gary Gibbons organized a three-week workshop on the subject titled “The Very Early Universe” at Cambridge University. With Jim Hartle, an American physicist and professor of physics at the University of California, he proposed that during the earliest period of the universe (aka. the Planck epoch) the universe had no boundary in space time.

In 1983, they published this model, known as the Hartle-Hawking state. Among other things, it asserted that before the Big Bang, time did not exist, and the concept of the beginning of the universe is therefore meaningless. It also replaced the initial singularity of the Big Bang with a region akin to the North Pole which (similar to the real North Pole) one cannot travel north of because it is a point where lines meet that has no boundary.

This proposal predicted a closed universe, which had many existential implications, particularly about the existence of God. At no point did Hawking rule out the existence of God, choosing to use God in a metaphorical sense when explaining the mysteries of the universe. However, he would often suggest that the existence of God was unnecessary to explain the origin of the universe, or the existence of a unified field theory.

In 1982, he also began work on a book that would explain the nature of the universe, relativity and quantum mechanics in a way that would be accessible to the general public. This led him to sign a contract with Bantam Books for the sake of publishing A Brief History of Time, the first draft of which he completed in 1984.

After multiple revisions, the final draft was published in 1988, and was met with much critical acclaim. The book was translated into multiple languages, remained at the top of bestseller lists in both the US and UK for months, and ultimately sold an estimated 9 million copies. Media attention was intense, and Newsweek magazine cover and a television special both described him as “Master of the Universe”.

Further work by Hawking in the area of arrows of time led to the 1985 publication of a paper theorizing that if the no-boundary proposition were correct, then when the universe stopped expanding and eventually collapsed, time would run backwards. He would later withdraw this concept after independent calculations disputed it, but the theory did provide valuable insight into the possible connections between time and cosmic expansion.

During the 1990’s, Hawking continued to publish and lecture on his theories regarding physics, black holes and the Big Bang. In 1993, he co-edited a book with Gary Gibbons on on Euclidean quantum gravity, a theory they had been working on together in the late 70s. According to this theory, a section of a gravitational field in a black hole can be evaluated using a functional integral approach, such that it can avoid the singularities.

That same year, a popular-level collection of essays, interviews and talks titled, Black Holes and Baby Universes and Other Essays was also published. In 1994, Hawking and Penrose delivered a series of six lectures at Cambridge’s Newton Institute, which were published in 1996 under the title “The Nature of Space and Time“.

It was also in 1990s that major developments happened in Hawking’s personal life. In 1990, he and Jane Hawking commenced divorce proceedings after many years of strained relations, owing to his disability, the constant presence of care-givers, and his celebrity status. Hawking remarried in 1995 to Elaine Mason, his caregiver of many years.

Stephen Hawking lectured regularly throughout the 90s and 2000s. Credit: educatinghumanity.com — *Stephen Hawking lectured regularly throughout the 90s, many of which were collected and published in “The Nature of Space and Time” in 1996. Credit: educatinghumanity.com*

In the 2000s, Hawking produced many new books and new editions of older ones. These included The Universe in a Nutshell (2001), A Briefer History of Time (2005), and God Created the Integers (2006). He also began collaborating with Jim Hartle of the University of California, Santa Barbara, and the European Organization for Nuclear Research (CERN) to produce new cosmological theories.

Foremost of these was Hawking’s “top-down cosmology”, which states that the universe had not one unique initial state but many different ones, and that predicting the universe’s current state from a single initial state is therefore inappropriate. Consistent with quantum mechanics, top-down cosmology posits that the present “selects” the past from a superposition of many possible histories.

In so doing, the theory also offered a possible resolution of the “fine-tuning question”, which addresses the possibility that life can only exist when certain physical constraints lie within a narrow range. By offering this new model of cosmology, Hawking opened up the possibility that life may not be bound by such restrictions and could be much more plentiful than previously thought.

In 2006, Hawking and his second wife, Elaine Mason, quietly divorced, and Hawking resumed closer relationships with his first wife Jane, his children (Robert, Lucy and Timothy), and grandchildren. In 2009, he retired as Lucasian Professor of Mathematics, which was required by Cambridge University regulations. Hawking has continued to work as director of research at the Cambridge University Department of Applied Mathematics and Theoretical Physics ever since, and has made no indication of retiring.

“Hawking Radiation” and the “Black Hole Information Paradox”:

In the early 1970s, Hawking’s began working on what is known as the “no-hair theorem”. Based on the Einstein-Maxwell equations of gravitation and electromagnetism in general relativity, the theorem stated that all black holes can be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum.

In this scenario, all other information about the matter which formed a black hole or is falling into it (for which “hair’ is used as a metaphor), “disappears” behind the black-hole event horizon, and is therefore preserved but permanently inaccessible to external observers.

In 1973, Hawking traveled to Moscow and met with Soviet scientists Yakov Borisovich Zel’dovich and Alexei Starobinsky. During his discussions with them about their work, they showed him how the uncertainty principle demonstrated that black holes should emit particles. This contradicted Hawking’ second law of black hole thermodynamics (i.e. black holes can’t get smaller) since it meant that by losing energy they must be losing mass.

What’s more, it supported a theory advanced by Jacob Bekenstein, a graduate student of John Wheeler University, that black holes should have a finite, non-zero temperature and entropy. All of this contradicted the “no-hair theorem” about black boles. Hawking revised this theorem shortly thereafter, showing that when quantum mechanical effects are taken into account, one finds that black holes emit thermal radiation at a temperature.

From 1974 onward, Hawking presented Bekenstein’s results, which showed that black holes emit radiation. This came to be known as “Hawking radiation”, and was initially controversial. However, by the late 1970s and following the publication of further research, the discovery was widely accepted as a significant breakthrough in theoretical physics.

However, one of the outgrowths of this theory was the likelihood that black holes gradually lose mass and energy. Because of this, black holes that lose more mass than they gain through other means are expected to shrink and ultimately vanish – a phenomena which is known as black hole “evaporation”.

In 1981, Hawking proposed that information in a black hole is irretrievably lost when a black hole evaporates, which came to be known as the “Black Hole Information Paradox”. This states that physical information could permanently disappear in a black hole, allowing many physical states to devolve into the same state.

This was controversial because it violated two fundamental tenets of quantum physics. In principle, quantum physics tells us that complete information about a physical system – i.e. the state of its matter (mass, position, spin, temperature, etc.) – is encoded in its wave function up to the point when that wave function collapses. This in turn gives rise to two other principles.

The first is Quantum Determinism, which states that – given a present wave function – future changes are uniquely determined by the evolution operator. The second is Reversibility, which states that the evolution operator has an inverse, meaning that the past wave functions are similarly unique. The combination of these means that the information about the quantum state of matter must always be preserved.

By proposing that this information disappears once a black evaporates, Hawking essentially created a fundamental paradox. If a black hole can evaporate, which causes all the information about a quantum wave function to disappear, than information can in fact be lost forever. This has been the subject of ongoing debate among scientists, one which has remained largely unresolved.

However, by 2003, the growing consensus among physicists was that Hawking was wrong about the loss of information in a black hole. In a 2004 lecture in Dublin, he conceded his bet with fellow John Preskill of Caltech (which he made in 1997), but described his own, somewhat controversial solution to the paradox problem – that black holes may have more than one topology.

In the 2005 paper he published on the subject – “Information Loss in Black Holes” – he argued that the information paradox was explained by examining all the alternative histories of universes, with the information loss in those with black holes being cancelled out by those without. As of January 2014, Hawking has described the Black Hole Information Paradox as his “biggest blunder”.

Other Accomplishments:

In addition to advancing our understanding of black holes and cosmology through the application of general relativity and quantum mechanics, Stephen Hawking has also been pivotal in bringing science to a wider audience. Over the course of his career, he has published many popular books, traveled and lectured extensively, and has made numerous appearances and done voice-over work for television shows, movies and even provided narration for the Pink Floyd song, “Keep Talking”.

Stephen Hawking's theories on black holes became the subject of many television specials, such as . Credit: discovery.com — *Stephen Hawking’s theories on black holes became the subject of television specials, such as “Stephen Hawking’s Universe” on PBS. Credit: discovery.com*

A film version of A Brief History of Time, directed by Errol Morris and produced by Steven Spielberg, premiered in 1992. Hawking had wanted the film to be scientific rather than biographical, but he was persuaded otherwise. In 1997, a six-part television series Stephen Hawking’s Universe premiered on PBS, with a companion book also being released.

In 2007, Hawking and his daughter Lucy published George’s Secret Key to the Universe, a children’s book designed to explain theoretical physics in an accessible fashion and featuring characters similar to those in the Hawking family. The book was followed by three sequels – George’s Cosmic Treasure Hunt (2009), George and the Big Bang (2011), George and the Unbreakable Code (2014).

Since the 1990s, Hawking has also been a major role model for people dealing with disabilities and degenerative illnesses, and his outreach for disability awareness and research has been unparalleled. At the turn of the century, he and eleven other luminaries joined with Rehabilitation International to sign the Charter for the Third Millennium on Disability, which called on governments around the world to prevent disabilities and protect disability rights.

Professor Stephen Hawking during a zero-gravity flight. Image credit: Zero G. — *Professor Stephen Hawking participating in a zero-gravity flight (aka. the “Vomit Comet”) in 2007. Credit: gozerog.com*

Motivated by the desire to increase public interest in spaceflight and to show the potential of people with disabilities, in 2007 he participated in zero-gravity flight in a “Vomit Comet” – a specially fitted aircraft that dips and climbs through the air to simulate the feeling of weightlessness – courtesy of Zero Gravity Corporation, during which he experienced weightlessness eight times.

In August 2012, Hawking narrated the “Enlightenment” segment of the 2012 Summer Paralympics opening ceremony^. In September of 2013, he expressed support for the legalization of assisted suicide for the terminally ill. In August of 2014, Hawking accepted the Ice Bucket Challenge to promote ALS/MND awareness and raise contributions for research. As he had pneumonia in 2013, he was advised not to have ice poured over him, but his children volunteered to accept the challenge on his behalf.

During his career, Hawking has also been a committed educator, having personally supervised 39 successful PhD students.He has also lent his name to the ongoing search for extra-terrestrial intelligence and the debate regarding the development of robots and artificial intelligence. On July 20th, 2015, Stephen Hawking helped launch Breakthrough Initiatives, an effort to search for extraterrestrial life in the universe.

Also in 2015, Hawking lent his voice and celebrity status to the promotion of The Global Goals, a series of 17 goals adopted by the United Nations Sustainable Development Summit to end extreme poverty, social inequality, and fixing climate change over the course of the next 15 years.

President Barack Obama talks with Stephen Hawking in the Blue Room of the White House before a ceremony presenting him and 15 others the Presidential Medal of Freedom, August 12, 2009. The Medal of Freedom is the nation's highest civilian honor. (Official White House photo by Pete Souza) — President Barack Obama talks with Stephen Hawking in the Blue Room of the White House before a ceremony presenting him and 15 others the Presidential Medal of Freedom, August 12th, 2009. Credit: Pete Souza/White House photo stream

Honors and Legacy:

As already noted, in 1974, Hawking was elected a Fellow of the Royal Society (FRS), and was one of the youngest scientists to become a Fellow. At that time, his nomination read:

Hawking has made major contributions to the field of general relativity. These derive from a deep understanding of what is relevant to physics and astronomy, and especially from a mastery of wholly new mathematical techniques. Following the pioneering work of Penrose he established, partly alone and partly in collaboration with Penrose, a series of successively stronger theorems establishing the fundamental result that all realistic cosmological models must possess singularities. Using similar techniques, Hawking has proved the basic theorems on the laws governing black holes: that stationary solutions of Einstein’s equations with smooth event horizons must necessarily be axisymmetric; and that in the evolution and interaction of black holes, the total surface area of the event horizons must increase. In collaboration with G. Ellis, Hawking is the author of an impressive and original treatise on “Space-time in the Large.

Other important work by Hawking relates to the interpretation of cosmological observations and to the design of gravitational wave detectors.

On 12 November Peter Higgs and Stephen Hawking visited the "Collider" exhibition at London's Science Museum (Image: c. Science Museum 2013) — *Peter Higgs and Stephen Hawking visiting the “Collider” exhibition at London’s Science Museum in 2013, in honor of the discovery of the Higgs Boson. Credit: sciencemuseum.org.uk*

In 1975, he was awarded both the Eddington Medal and the Pius XI Gold Medal, and in 1976 the Dannie Heineman Prize, the Maxwell Prize and the Hughes Medal. In 1977, he was appointed a professor with a chair in gravitational physics, and received the Albert Einstein Medal and an honorary doctorate from the University of Oxford by the following year.

In 1981, Hawking was awarded the American Franklin Medal, followed by a Commander of the Order of the British Empire (CBE) medal the following year. For the remainder of the decade, he was honored three times, first with the Gold Medal of the Royal Astronomical Society in 1985, the Paul Dirac Medal in 1987 and, jointly with Penrose, with the prestigious Wolf Prize in 1988. In 1989, he was appointed Member of the Order of the Companions of Honour (CH), but reportedly declined a knighthood.

In 1999, Hawking was awarded the Julius Edgar Lilienfeld Prize of the American Physical Society. In 2002, following a UK-wide vote, the BBC included him in their list of the 100 Greatest Britons. More recently, Hawking has been awarded the Copley Medal from the Royal Society (2006), the Presidential Medal of Freedom, America’s highest civilian honor (2009), and the Russian Special Fundamental Physics Prize (2013).

Several buildings have been named after him, including the Stephen W. Hawking Science Museum in San Salvador, El Salvador, the Stephen Hawking Building in Cambridge, and the Stephen Hawking Center at Perimeter Institute in Canada. And given Hawking’s association with time, he was chosen to unveil the mechanical “Chronophage” – aka. the Corpus Clock – at Corpus Christi College Cambridge in September of 2008.

Stephen Hawking being presented by his daughter Lucy Hawking at the lecture he gave for NASA's 50th anniversary. Credit: NASA/Paul Alers — *Stephen Hawking being presented by his daughter Lucy Hawking at the lecture he gave for NASA’s 50th anniversary. Credit: NASA/Paul Alers*

Also in 2008, while traveling to Spain, Hawking received the Fonseca Prize – an annual award created by the University of Santiago de Compostela which is awarded to those for outstanding achievement in science communication. Hawking was singled out for the award because of his “exceptional mastery in the popularization of complex concepts in Physics at the very edge of our current understanding of the Universe, combined with the highest scientific excellence, and for becoming a public reference of science worldwide.”

Multiple films have been made about Stephen Hawking over the years as well. These include the previously mentioned A Brief History of Time, the 1991 biopic film directed by Errol Morris and Stephen Spielberg; Hawking, a 2004 BBC drama starring Benedict Cumberbatch in the title role; the 2013 documentary titled “Hawking”, by Stephen Finnigan.

Most recently, there was the 2014 film The Theory of Everything that chronicled the life of Stephen Hawking and his wife Jane. Directed by James Marsh, the movie stars Eddie Redmayne as Professor Hawking and Felicity Jones as Jane Hawking.

Death:

Dr. Stephen Hawking passed away in the early hours of Wednesday, March 14th, 2018 at his home in Cambridge. According to a statement made by his family, he died peacefully. He was 76 years old, and is survived by his first wife, Jane Wilde, and their three children – Lucy, Robert and Tim.

When all is said and done, Stephen Hawking was the arguably the most famous scientist alive in the modern era. His work in the field of astrophysics and quantum mechanics has led to a breakthrough in our understanding of time and space, and will likely be poured over by scientists for decades. In addition, he has done more than any living scientist to make science accessible and interesting to the general public.

*Stephen Hawking holding a public lecture at the Stockholm Waterfront congress center, 24 August 2015. Credit: Public Domain/photo by Alexandar Vujadinovic*

To top it off, he traveled all over the world and lectured on topics ranging from science and cosmology to human rights, artificial intelligence, and the future of the human race. He also used the celebrity status afforded him to advance the causes of scientific research, space exploration, disability awareness, and humanitarian causes wherever possible.

In all of these respects, he was very much like his predecessor, Albert Einstein – another influential scientist-turned celebrity who was sure to use his powers to combat ignorance and promote humanitarian causes. But what was especially impressive in all of this is that Hawking has managed to maintain his commitment to science and a very busy schedule while dealing with a degenerative disease.

For over 50 years, Hawking lived with a disease that doctor’s initially thought would take his life within just two. And yet, he not only managed to make his greatest scientific contributions while dealing with ever-increasing problems of mobility and speech, he also became a jet-setting personality who travelled all around the world to address audiences and inspire people.

His passing was mourned by millions worldwide and, in the worlds of famed scientist and science communicator Neil DeGrasse Tyson , “left an intellectual vacuum in its wake”. Without a doubt, history will place Dr. Hawking among such luminaries as Einstein, Newton, Galileo and Curie as one of the greatest scientific minds that ever lived.

We have many great articles about Stephen Hawking here at Universe Today. Here is one about Hawking Radiation, How Do Black Holes Evaporate?, why Hawking could be Wrong About Black Holes, and recent experiments to Replicate Hawking Radiation in a Laboratory.

And here are some video interviews where Hawking addresses how God is not necessary for the creation of the Universe, and the trailer for Theory of Everything.

Astronomy Cast has a number of great podcasts that deal with Hawing and his discoveries, like: Episode 138: Quantum Mechanics, and Questions Show: Hidden Fusion, the Speed of Neutrinos, and Hawking Radiation.

For more information, check out Stephen Hawking’s website, and his page at Biography.com

November 15, 2015September 18, 2019 by Matt Williams

Who was Albert Einstein?

Albert Einstein's Inventions — Albert Einstein in 1947. Credit: Library of Congress

At the end of the millennium, Physics World magazine conducted a poll where they asked 100 of the world’s leading physicists who they considered to be the top 10 greatest scientist of all time. The number one scientist they identified was Albert Einstein, with Sir Isaac Newton coming in second. Beyond being the most famous scientist who ever lived, Albert Einstein is also a household name, synonymous with genius and endless creativity.

As the discoverer of Special and General Relativity, Einstein revolutionized our understanding of time, space, and universe. This discovery, along with the development of quantum mechanics, effectively brought to an end the era of Newtonian Physics and gave rise to the modern age. Whereas the previous two centuries had been characterized by universal gravitation and fixed frames of reference, Einstein helped usher in an age of uncertainty, black holes and “scary action at a distance”.

Continue reading “Who was Albert Einstein?”

November 11, 2015December 12, 2015 by Matt Williams

Who was Gerard Kuiper?

Gerard Kuiper, founder of the Lunar and Planetary Laboratory. Credit: lpl.arizona.edu

In the outer reaches of the Solar System, beyond the orbit of Neptune, lies a region permeated by celestial objects and minor planets. This region is known as the “Kuiper Belt“, and is named in honor of the 20th century astronomer who speculated about the existence of such a disc decades before it was observed. This disc, he reasoned, was the source of the Solar Systems many comets, and the reason there were no large planets beyond Neptune.

Gerard Kuiper is also regarded by many as being the “father of planetary science”. During the 1960s and 70s, he played a crucial role in the development of infrared airborne astronomy, a technology which led to many pivotal discoveries that would have been impossible using ground-based observatories. At the same time, he helped catalog asteroids, surveyed the Moon, Mars and the outer Solar System, and discovered new moons.

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November 10, 2015November 30, 2016 by Matt Williams

Who Was Sir Isaac Newton?

The 17th century was an auspicious time for the sciences, with groundbreaking discoveries being made in astronomy, physics, mechanics, optics, and the natural sciences. At the center of all this was Sir Isaac Newton, the man who is widely recognized as being one of the most influential scientists of all time and as a key figure in the Scientific Revolution.

An English physicist and mathematician, Newton made several seminal contributions to the field of optics, and shares credit with Gottfried Leibniz for the development of calculus. But it was Newton’s publication of Philosophiæ Naturalis Principia Mathematica (“Mathematical Principles of Natural Philosophy”), for which he is most famous. Published in 1687, this treatise laid the foundations for classical mechanics, a tradition which would dominate scientists’ view of the physical universe for the next three centuries.

Early Life:

Isaac Newton was born on January 4th, 1643, – or December 25th, 1642 according to the Julian Calendar (which was in use in England at the time) – in Woolsthorpe-by-Colsterworth, a hamlet in the county of Lincolnshire. His father, for whom he was named, was a prosperous farmer who had died three months before his birth. Having been born prematurely, Newton was small as a child.

His mother, Hannah Ayscough, remarried when he was three to a Reverend, leaving Newton in the care of his maternal grandmother. His mother would go on to have three more children with her new husband, which became Newton’s only siblings. Because of this, Newton apparently had a rocky relationship with his stepfather and mother for some time.

William Blake's Newton (1795), depicted as a divine geometer. Credit: William Blake Archive/Wikipedia — *William Blake’s Newton (1795), depicted as a divine geometer. Credit: William Blake Archive/Wikipedia*

By the time Newton was 17, his mother was widowed again. Despite her hopes that Newton would become a farmer, like his father, Newton hated farming and sought to become an academic. His interests in engineering, mathematics and astronomy were evident from an early age, and Newton began his studies with an aptitude for learning and inventing that would last for the rest of his life.

Education:

Between the ages of 12 and 21, Newton was educated at The King’s School, Grantham, where he learned Latin. While there, he became the top-ranked student, and received recognition for his building of sundials and models of windmills. By 1661, he was admitted to Trinity College, Cambridge, where he paid his way by performing a valet’s duties (what was known as a subsizar).

During his first three years at Cambridge, Newton was taught the standard curriculum, which was based on Aristotelian theory. But Newton was fascinated with the more advanced science and spent all his spare time reading the works of modern philosophers and astronomers, such as René Descartes, Galileo Galilei, Thomas Street, and Johannes Kepler.

The result was a less-than-stellar performance, but his dual focus would also lead him to make some of his most profound scientific contributions. In 1664, Newton received a scholarship, which guaranteed him four more years until he would get his Masters of Arts degree.

David Loggan's print of 1690 showing Nevile's Great Court (foreground) and Nevile's Court with the then-new Wren Library (background) — New Court had yet to be built. Credit: Cantabrigia Illustrata/Cambridge (1690) — *David Loggan’s print of Trinity College, Cambridge, showing Nevile’s Great Court (foreground) and the then-new Wren Library (background). Credit: Cantabrigia Illustrata/Cambridge (1690)*

In 1665, shortly after Newton obtained his B.A., the university temporarily closed due to the outbreak of the Great Plague. Using this time to study at home, Newton developed a number of ideas he had which would eventually cement to become his theories on calculus, optics and the law of gravitation (see below).

In 1667, he returned to Cambridge and was elected as a fellow of Trinity, though his performance was still considered less than spectacular. However, in time, his fortunes improved and he gained recognition for his abilities. In 1669, he received his M.A. (before he had turned 27), and published a treatise expounding on his mathematical theories for dealing with infinite series.

By 1669, he succeeded his one-time teacher and mentor Isaac Barrow – a theologian and mathematician who discovered the fundamental theorem of calculus – and became the Lucasian Chair of Mathematics at Cambridge. In 1672, he was elected a Fellow of the Royal Society, which he would remain a part of until the end of his life.

Scientific Achievements:

While studying at Cambridge, Newton maintained a second set of notes which he entitled “Quaestiones Quaedam Philosophicae” (“Certain Philosophical Questions“). These notes, which were the sum total of Newton’s observations about mechanical philosophy, would lead him to discover the generalized binomial theorem in 1665, and allowed him to develop a mathematical theory that would lead to his development of modern calculus.

Newton's experiment using a prism revealed that light is refracted based on color. Credit: britannica.com — *Newton’s experiment using a prism revealed that light is refracted based on color. Credit: britannica.com*

However, Newton’s earliest contributions were in the form of optics, which he delivered during annual lectures while holding the position of Lucasian Chair of Mathematics. In 1666, he observed that light entering a prism as a circular ray exits in the form of an oblong, demonstrating that a prism refracts different colors of light at different angles. This led him to conclude that color is a property intrinsic to light, a point which had been debated in prior years.

In 1668, he designed and constructed a reflecting telescope, which helped him prove his theory. From 1670 to 1672, Newton continued to lecture on optics and investigated the refraction of light, demonstrating that the multicoloured spectrum produced by a prism could be recomposed into white light by a lens and a second prism.

He also demonstrated that colored light does not change its properties, regardless of whether it is reflected, scattered, or transmitted. Thus, he observed that color is the result of objects interacting with already-colored light, rather than objects generating the color themselves. This is known as Newton’s theory of color.

The Royal Society asked for a demonstration of his reflecting telescope in 1671, and the organization’s interest encouraged Newton to publish his theories on light, optics and color. This he did in 1672 in a small treatise entitled Of Colours, which would later be published in a larger volume containing his theories on the “corpuscular” nature of light.

Replica of Newton's second Reflecting telescope that he presented to the Royal Society in 1672. — *Replica of Newton’s second Reflecting telescope that he presented to the Royal Society in 1672. Credit: Wikipedia Commons*

In essence, Newton argued that light was composed of particles (or corpuscles), which he claimed were refracted by accelerating into a denser medium. In 1675, he published this theory in a treatise titled “Hypothesis of Light“, in which he also posited that ordinary matter was composed of larger corpuscles and about the existence of an ether that transmitted forces between particles.

After discussing his ideas with Henry More, an English theosophist and a member of the Cambridge Platonists, Newton’s interest in alchemy was revived. He then replaced his theory of an ether existing between particles in nature with occult forces, based on Hermetic ideas of attraction and repulsion between particles. This reflected Newton’s ongoing interest in both the alchemical and scientific, for which there was no clear distinction at the time.

In 1704, Newton published all of his theories on light, optics and colors into a single volume entitled Opticks: Or, A treatise of the Reflections, Refractions, Inflections and Colours of Light. In it, he speculated that light and matter could converted into one another through a kind of alchemical transmutation, and verged on theories of sound waves in order to explain repeated patterns of reflection and transmission.

While later physicists favored a purely wavelike explanation of light to account for the interference patterns and the general phenomenon of diffraction, their findings owed a great deal to Newton’ theories. Much the same is true of today’s quantum mechanics, photons, and the idea of wave–particle duality, which bear only a small resemblance to Newton’s understanding of light.

This set of papers documents some of Newton's early mathematical thinking, work that would develop into his theories of calculus. The texts include extracts he made from books he was reading and various notes and calculations - often on small scraps of paper he had to hand. Credit: newtonproject.sussex.ac.uk — *Excerpt from Newton’s “Quaestiones Quaedam Philosophicae”, a collection of observations that would lead to his development of modern calculus. Credit: newtonproject.sussex.ac.uk*

Though both he and Leibniz are credited with having developed calculus independently, both men became embroiled in a controversy over who discovered it first. Though Newton’s work in developing modern calculus began in the 1660s, he was reluctant to publish it, fearing controversy and criticism. As such, Newton didn’t publish anything until 1693 and did not give a full account of his work until 1704, whereas Leibniz began publishing a full account of his methods in 1684.

However, Newton earlier works in mechanics and astronomy involved extensive use of calculus in geometric form. This includes methods involving “one or more orders of the infinitesimally small” in his 1684 work, De motu corporum in gyrum (“On the motion of bodies in orbit”), and in Book I of the Principia, which he referred to as “the method of first and last ratios”.

Universal Gravitation:

In 1678, Newton suffered a complete nervous breakdown, most likely due to overwork and an ongoing feud with fellow Royal Society member Robert Hooke (see below). The death of his mother a year later caused him to become increasingly isolated, and for six years he withdrew from correspondence with other scientists, except where they initiated it.

During this hiatus, Newton renewed his interest in mechanics and astronomy. Ironically, it was thanks to a brief exchange of letters in 1679 and 1680 with Robert Hooke that would lead him to make his greatest scientific achievements. His reawakening was also due to the appearance of a comet in the winter of 1680–1681, about which he corresponded with John Flamsteed – England’s Astronomer Royal.

Thereafter, Newton began considering gravitation and its effect on the orbits of planets, specifically with reference to Kepler’s laws of planetary motion. After his exchanges with Hooke, he worked out proof that the elliptical form of planetary orbits would result from a centripetal force inversely proportional to the square of the radius vector.

Newton communicated his results to Edmond Halley (discoverer of “Haley’s Comet”) and to the Royal Society in his De motu corporum in gyrum. This tract, published in 1684, contained the seed that Newton would expand to form his magnum opus, the Principia. This treatise, which was published in July of 1687, contained Newton’s three laws of motion. These laws stated that:

When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.
The vector sum of the external forces ( $F)$ on an object is equal to the mass ( $m)$ of that object multiplied by the acceleration vector ( $a)$ of the object. In mathematical form, this is expressed as: F=ma
When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

Together, these laws described the relationship between any object, the forces acting upon it and the resulting motion, laying the foundation for classical mechanics. The laws also allowed Newton to calculate the mass of each planet, calculate the flattening of the Earth at the poles and the bulge at the equator, and how the gravitational pull of the Sun and Moon create the Earth’s tides.

In the same work, Newton presented a calculus-like method of geometrical analysis using ‘first and last ratios’, worked out the speed of sound in air (based on Boyle’s Law), accounted for the precession of the equinoxes (which he showed were a result of the Moon’s gravitational attraction to the Earth), initiated the gravitational study of the irregularities in the motion of the moon, provided a theory for the determination of the orbits of comets, and much more.

This volume would have a profound effect on the sciences, with its principles remaining canon for the following 200 years. It also informed the concept of universal gravitation, which became the mainstay of modern astronomy, and would not be revised until the 20th century – with the discovery of quantum mechanics and Einstein’s theory of General Relativity.

Newton and the “Apple Incident”:

The story of Newton coming up with his theory of universal gravitation as a result of an apple falling on his head has become a staple of popular culture. And while it has often been argued that the story is apocryphal and Newton did not devise his theory at any one moment, Newton himself told the story many times and claimed that the incident had inspired him.

In addition, the writing’s of William Stukeley – an English clergyman, antiquarian and fellow member of the Royal Society – have confirmed the story. But rather than the comical representation of the apple striking Netwon on the head, Stukeley described in his Memoirs of Sir Isaac Newton’s Life (1752) a conversation in which Newton described pondering the nature of gravity while watching an apple fall.

“…we went into the garden, & drank thea under the shade of some appletrees; only he, & my self. amidst other discourse, he told me, he was just in the same situation, as when formerly, the notion of gravitation came into his mind. “why should that apple always descend perpendicularly to the ground,” thought he to himself; occasion’d by the fall of an apple…”

John Conduitt, Newton’s assistant at the Royal Mint (who eventually married his niece), also described hearing the story in his own account of Newton’s life. According to Conduitt, the incident took place in 1666 when Newton was traveling to meet his mother in Lincolnshire. While meandering in the garden, he contemplated how gravity’s influence extended far beyond Earth, responsible for the falling of apple as well as the Moon’s orbit.

Sapling of the reputed original tree that inspired Sir Isaac Newton to consider gravitation. Credit: Wikipedia Commons/Loodog — *Sapling of the reputed original tree that inspired Sir Isaac Newton to consider gravitation, located on the grounds of Cambridge. Credit: Wikipedia Commons/Loodog*

Similarly, Voltaire wrote n his Essay on Epic Poetry (1727) that Newton had first thought of the system of gravitation while walking in his garden and watching an apple fall from a tree. This is consistent with Newton’s notes from the 1660s, which show that he was grappling with the idea of how terrestrial gravity extends, in an inverse-square proportion, to the Moon.

However, it would take him two more decades to fully develop his theories to the point that he was able to offer mathematical proofs, as demonstrated in the Principia. Once that was complete, he deduced that the same force that makes an object fall to the ground was responsible for other orbital motions. Hence, he named it “universal gravitation”.

Various trees are claimed to be “the” apple tree which Newton describes. The King’s School, Grantham, claims their school purchased the original tree, uprooted it, and transported it to the headmaster’s garden some years later. However, the National Trust, which holds the Woolsthorpe Manor (where Newton grew up) in trust, claims that the tree still resides in their garden. A descendant of the original tree can be seen growing outside the main gate of Trinity College, Cambridge, below the room Newton lived in when he studied there.

Feud with Robert Hooke:

With the Principia, Newton became internationally recognized and acquired a circle of admirers. It also led to a feud with Robert Hooke, with whom he had a troubled relationship in the past. With the publication of his theories on color and light in 1671/72, Hooke criticized Newton in a rather condescending way, claiming that light was composed of waves and not colors.

*A historical reconstruction of what Robert Hooke looked like. Credit: Wikipedia/Rita Greer/FAL*

While other philosophers were critical of Newton’s idea, it was Hooke (a member of the Royal Society who had performed extensive work in optics) that stung Newton the worst. This led to the acrimonious relationship between the two men, and to Newton almost quitting the Royal Society. However, the intervention of his colleagues convinced him to stay on and the matter eventually died down.

However, with the publication of the Principia, matters once again came to a head, with Hooke accusing Newton of plagiarism. The reason for the charge had to do with the fact that earlier in 1684, Hooke had made comments to Edmond Halley and Christopher Wren (also members of the Royal Society) about ellipses and the laws of planetary motion. However, at the time he did not offer a mathematical proof.

Nevertheless, Hooke claimed that he had discovered the theory of inverse squares and that Newton had stolen his work. Other members of the Royal Society believed the charge to be unfounded, and demanded that Hooke release the mathematical proofs to substantiate this claim. In the meantime, Newton removed all reference to Hooke in his notes and threatened to withdraw the Principia from subsequent publishing altogether.

Edmund Halley, who was a friend to both Newton and Hooke, tried to make peace between the two. In time, he was able to convince Newton to insert a joint acknowledgement of Hooke’s work in his discussion of the law of inverse squares. However, this did not placate Hooke, who maintained his charge of plagiarism.

Newton's own copy of his Principia, with hand-written corrections for the second edition. Credit: Trinity Cambridge/Andrew Dunn — *Newton’s own copy of his Principia, with hand-written corrections for the second edition. Credit: Trinity Cambridge/Andrew Dunn*

As time moved on, Newton’s fame continued to grow while Hooke’s continued to diminish. This caused Hooke to become increasingly embittered and more protective of what he saw as his work, and he spared no opportunity to lash out at his rival. The feud finally ended in 1703, when Hooke died and Newton succeeded him as president of the Royal Society.

Other Accomplishments:

In addition to his work in astronomy, optics, mechanics, physics and alchemy, Newton also had a keen interest in religion and the Bible. During the 1690’s, he wrote several religious tracts that addressed literal and symbolic interpretations of the Bible. For instance, his tract on the Holy Trinity – sent to the famous political philosopher and social theorist John Locke and unpublished until 1785 – questioned the veracity of 1 John 5:7, the description which the Holy Trinity is based on.

Later religious works – like The Chronology of Ancient Kingdoms Amended (1728) and Observations Upon the Prophecies of Daniel and the Apocalypse of St. John (1733) – also remained unpublished until after his death. In Kingdoms, he dealt with the chronology of various ancient kingdoms – the First Ages of the Greeks, ancient Egyptians, Babylonians, Medeans and Persians – and offered a description of the Temple of Solomon.

In Prophecies, he addressed the Apocalypse, as foretold within the Book of Daniel and Revelations, and espoused his belief that it would take place in 2060 CE (though other possible dates included 2034 CE). In his textual criticism titled An Historical Account of Two Notable Corruptions of Scripture (1754), he placed the crucifixion of Jesus Christ on April 3rd, AD 33, which agrees with a traditionally accepted date.

*Portrait of Newton in 1702, painted by Godfrey Kneller. Credit: National Portrait Gallery, London*

In 1696, he moved to London to take up the post of warden of the Royal Mint, where he took charge of England’s great recoining. Newton would remain in this post for 30 years, and was perhaps the best-known Master of the Mint. So serious was his commitment to the role that he retired from Cambridge in 1701 to oversee the reform of England’s currency and the punishment of counterfeiters.

As Warden, and afterwards Master, of the Royal Mint, Newton estimated that 20 percent of the coins taken in during the Great Recoinage of 1696 were counterfeit. Conducting many investigations personally, Newton traveled to taverns and bars in disguise to gather evidence, and conducted more than 100 cross-examinations of witnesses, informers, and suspects – which led to the successful prosecution of 28 counterfeit coiners.

Newton was a member of the Parliament of England for Cambridge University in 1689–90 and 1701–2. In addition to being President of the Royal Society in 1703, he was an associate of the French Académie des Sciences. In April 1705, Queen Anne knighted Newton during a royal visit to Trinity College, Cambridge, making him the second scientist to be knighted (after Sir Francis Bacon).

Death and Legacy:

Towards the end of his life, Newton took up residence at Cranbury Park near Winchester with his niece and her husband, where he would stay until his death. By this time, Newton had become one of the most famous men in Europe and his scientific discoveries were unchallenged. He also had become wealthy, investing his sizable income wisely and bestowing sizable gifts to charity.

Newton's tomb in Westminster Abbey. Credit: Wikipedia Commons/Klaus-Dieter Keller — *Newton’s tomb in Westminster Abbey. Credit: Wikipedia Commons/Klaus-Dieter Keller*

At the same time, Newton’s physical and mental health began to decline. By the time he reached 80 years of age, he began experiencing digestive problems and had to drastically change his diet and lifestyle. His family and friends also began to worry about his mental stability, as his behavior became increasingly erratic.

Then, in 1727, Newton experienced severe pain in his abdomen and lost consciousness. He died in his sleep on the next day, on March 2oth, 1727 (Julian Calendar; or March 31st, 1727, Gregorian Calendar) at the age of 84. He was buried in tomb at Westminster Abbey. And as a bachelor, he had divested much of his estate to relatives and charities during his final years.

After his death, Newton’s hair was examined and found to contain mercury, probably resulting from his alchemical pursuits. Mercury poisoning has been cited as a reason for Newton’s eccentricity in later life, as well as the nervous breakdown he experienced in 1693. Isaac Newton’s fame grew even more after his death, as many of his contemporaries proclaimed him to be the greatest genius who ever lived.

These claims were not without merit, as his laws of motion and theory of universal gravitation were unparalleled in his the time. In addition to being able to bring the orbits of the planets, the Moon, and even comets into one coherent and predictable system, he also invented modern calculus, revolutionized our understanding of light and optics, and established scientific principles that would remain in use for the following 200 years.

Einstein Lecturing — *Albert Einstein was one of Newton’s many admirers, claiming that to Newton, nature was “an open book, whose letters he could read without effort”. Credit: Ferdinand Schmutzer/Public Domain*

In time, much of what Newton espoused would be proven wrong, thanks largely to Albert Einstein. With his General Theory of Relativity, Einstein would prove that time, distance and motion were not absolutes, but dependent on the observer. In so doing, he overturned one of the fundamental precepts of universal gravitation. Nevertheless, Einstein was one of Newton’s greatest admirers and acknowledged a great debt to his predecessor.

In addition to calling Newton a “shining spirit” (in a eulogy delivered in 1927 on the 200th anniversary of Newton’s death), Einstein also remarked that “Nature to him was an open book, whose letters he could read without effort.” On his study wall, Albert Einstein is said to have kept a picture of Newton, alongside pictures of Michael Faraday and James Clerk Maxwell.

A survey of Britain’s Royal Society was also conducted in 2005, where members were asked who had the greater effect on the history of science: Newton or Einstein. The majority of the Royal Society’s members agreed that overall, Newton had a greater impact on the sciences. Other polls conducted in recent decades have produced similar results, with Einstein and Newton vying for first and second place.

It is not easy thing to be living during one of the most auspicious times in history. Moreover, it is not easy in the midst of all of that to be blessed with an insight that will lead one to comes up with ideas that will revolutionize the sciences and forever alter the course of history. But throughout it all, Newton maintained a humble attitude, and summarized his accomplishments best with the famous words: “If I have seen further it is by standing on the shoulders of giants.^“

We have written many articles about Isaac Newton for Universe Today. Here’s an article about what Isaac Newton discovered, and here’s an article about the inventions of Isaac Newton.

Astronomy Cast also has a wonderful episode, titled Episode 275: Isaac Newton

For more information, check out this article from the Galileo Society on Isaac Newton, and the non-profit group known as The Newton Project.

We’ve also recorded an entire episode of Astronomy Cast all about Gravity. Listen here, Episode 102: Gravity.

November 5, 2015October 8, 2019 by Matt Williams

Who Was Galileo Galilei?

Portrait of Galileo Galilei by Giusto Sustermans (1636). Credit: nmm.ac.uk

When it comes to scientists who revolutionized the way we think of the universe, few names stand out like Galileo Galilei. A noted inventor, physicist, engineer and astronomer, Galileo was one of the greatest contributors to the Scientific Revolution. He build telescopes, designed a compass for surveying and military use, created a revolutionary pumping system, and developed physical laws that were the precursors of Newton’s law of Universal Gravitation and Einstein’s Theory of Relativity.

But it was within the field of astronomy that Galileo made his most enduring impact. Using telescopes of his own design, he discovered Sunspots, the largest moons of Jupiter, surveyed The Moon, and demonstrated the validity of Copernicus’ heliocentric model of the universe. In so doing, he helped to revolutionize our understanding of the cosmos, our place in it, and helped to usher in an age where scientific reasoning trumped religious dogma.

Early Life:

Galileo was born in Pisa, Italy, in 1564, into a noble but poor family. He was the first of six children of Vincenzo Galilei and Giulia Ammannati, who’s father also had three children out of wedlock. Galileo was named after an ancestor, Galileo Bonaiuti (1370 – 1450), a noted physician, university teacher and politician who lived in Florence.

His father, a famous lutenist, composer and music theorist, had a great impact on Galileo; transmitting not only his talent for music, but skepticism of authority, the value of experimentation, and the value of measures of time and rhythm to achieve success.

The Camaldolese Monastery at Vallombrosa, 35 km southeast of Florence, where Galileo was educated from to. Credit: nobility.org — *The Camaldolese Monastery at Vallombrosa, 35 km southeast of Florence, where Galileo was educated until 1581. Credit: nobility.org*

In 1572, when Galileo Galilei was eight, his family moved to Florence, leaving Galileo with his uncle Muzio Tedaldi (related to his mother through marriage) for two years.When he reached the age of ten, Galileo left Pisa to join his family in Florence and was tutored by Jacopo Borghini -a mathematician and professor from the university of Pisa.

Once he was old enough to be educated in a monastery, his parents sent him to the Camaldolese Monastery at Vallombrosa, located 35 km southeast of Florence. The Order was independent from the Benedictines, and combined the solitary life of the hermit with the strict life of a monk. Galileo apparently found this life attractive and intending to join the Order, but his father insisted that he study at the University of Pisa to become a doctor.

Education:

While at Pisa, Galileo began studying medicine, but his interest in the sciences quickly became evident. In 1581, he noticed a swinging chandelier, and became fascinated by the timing of its movements. To him, it became clear that the amount of time, regardless of how far it was swinging, was comparable to the beating of his heart.

When he returned home, he set up two pendulums of equal length, swinging one with a large sweep and the other with a small sweep, and found that they kept time together. These observations became the basis of his later work with pendulums to keep time – work which would also be picked up almost a century later when Christiaan Huygens designed the first officially-recognized pendulum clock.

Galileo Demonstrating the New Astronomical Theories at the University of Padua by Félix Parra (1873). Credit: munal.gob.mx — *Galileo Demonstrating the New Astronomical Theories at the University of Padua, by Félix Parra (1873). Credit: munal.gob.mx*

Shortly thereafter, Galileo accidentally attended a lecture on geometry, and talked his reluctant father into letting his study mathematics and natural philosophy instead of medicine. From that point onward, he began a steady processes of inventing, largely for the sake of appeasing his father’s desire for him to make money to pay off his siblings expenses (particularly those of his younger brother, Michelagnolo).

In 1589, Galileo was appointed to the chair of mathematics at the University of Pisa. In 1591, his father died, and he was entrusted with the care of his younger siblings. Being Professor of Mathematics at Pisa was not well paid, so Galileo lobbied for a more lucrative post. In 1592, this led to his appointment to the position of Professor of Mathematics at the University of Padua, where he taught Euclid’s geometry, mechanics, and astronomy until 1610.

During this period, Galileo made significant discoveries in both pure fundamental science as well as practical applied science. His multiple interests included the study of astrology, which at the time was a discipline tied to the studies of mathematics and astronomy. It was also while teaching the standard (geocentric) model of the universe that his interest in astronomy and the Copernican theory began to take off.

Telescopes:

In 1609, Galileo received a letter telling him about a spyglass that a Dutchman had shown in Venice. Using his own technical skills as a mathematician and as a craftsman, Galileo began to make a series of telescopes whose optical performance was much better than that of the Dutch instrument.

Galileo Galilei's telescope with his handwritten note specifying the magnifying power of the lens, at an exhibition at The Franklin Institute in Philadelphia. Credit: AP Photo/Matt Rourke — *Galileo Galilei’s telescope with his handwritten note specifying the magnifying power of the lens, at an exhibition at The Franklin Institute in Philadelphia. Credit: AP Photo/Matt Rourke*

As he would later write in his 1610 tract Sidereus Nuncius (“The Starry Messenger”):

“About ten months ago a report reached my ears that a certain Fleming had constructed a spyglass by means of which visible objects, though very distant from the eye of the observer, were distinctly seen as if nearby. Of this truly remarkable effect several experiences were related, to which some persons believed while other denied them. A few days later the report was confirmed by a letter I received from a Frenchman in Paris, Jacques Badovere, which caused me to apply myself wholeheartedly to investigate means by which I might arrive at the invention of a similar instrument. This I did soon afterwards, my basis being the doctrine of refraction.”

His first telescope – which he constructed between June and July of 1609 – was made from available lenses and had a three-powered spyglass. To improve upon this, Galileo learned how to grind and polish his own lenses. By August, he had created an eight-powered telescope, which he presented to the Venetian Senate.

By the following October or November, he managed to improve upon this with the creation a twenty-powered telescope. Galileo saw a great deal of commercial and military applications of his instrument(which he called a perspicillum) for ships at sea. However, in 1610, he began turning his telescope to the heavens and made his most profound discoveries.

Galileo Galilei showing the Doge of Venice how to use the telescope by Giuseppe Bertini (1858). Credit: gabrielevanin.it — *Galileo Galilei showing the Doge of Venice how to use the telescope, by Giuseppe Bertini (1858). Credit: gabrielevanin.it*

Achievements in Astronomy:

Using his telescope, Galileo began his career in astronomy by gazing at the Moon, where he discerned patterns of uneven and waning light. While not the first astronomer to do this, Galileo artistic’s training and knowledge of chiaroscuro – the use of strong contrasts between light and dark – allowed him to correctly deduce that these light patterns were the result of changes in elevation. Hence, Galileo was the first astronomer to discover lunar mountains and craters.

In The Starry Messenger, he also made topographical charts, estimating the heights of these mountains. In so doing, he challenged centuries of Aristotelian dogma that claimed that Moon, like the other planets, was a perfect, translucent sphere. By identifying that it had imperfections, in the forms of surface features, he began advancing the notion that the planets were similar to Earth.

Galileo also recorded his observations about the Milky Way in the Starry Messenger, which was previously believed to be nebulous. Instead, Galileo found that it was a multitude of stars packed so densely together that it appeared from a distance to look like clouds. He also reported that whereas the telescope resolved the planets into discs, the stars appeared as mere blazes of light, essentially unaltered in appearance by the telescope – thus suggesting that they were much farther away than previously thought.

Using his telescopes, Galileo also became one the first European astronomer to observe and study sunspots. Though there are records of previous instances of naked eye observations – such as in China (ca. 28 BCE), Anaxagoras in 467 BCE, and by Kepler in 1607 – they were not identifies as being imperfections on the surface of the Sun. In many cases, such as Kepler’s, it was thought that the spots were transits of Mercury.

In addition, there is also controversy over who was the first to observe sunspots during the 17th century using a telescope. Whereas Galileo is believed to have observed them in 1610, he did not publish about them and only began speaking to astronomers in Rome about them by the following year. In that time, German astronomer Christoph Scheiner had been reportedly observing them using a helioscope of his own design.

At around the same time, the Frisian astronomers Johannes and David Fabricius published a description of sunspots in June 1611. Johannes book, De Maculis in Sole Observatis (“On the Spots Observed in the Sun”) was published in autumn of 1611, thus securing credit for him and his father.

In any case, it was Galileo who properly identified sunspots as imperfections on the surface of the Sun, rather than being satellites of the Sun – an explanation that Scheiner, a Jesuit missionary, advanced in order to preserve his beliefs in the perfection of the Sun.

Using a technique of projecting the Sun’s image through the telescope onto a piece of paper, Galileo deduced that sunspots were, in fact, on the surface of the Sun or in its atmosphere. This presented another challenge to the Aristotelian and Ptolemaic view of the heavens, since it demonstrated that the Sun itself had imperfections.

On January 7th, 1610, Galileo pointed his telescope towards Jupiter and observed what he described in Nuncius as “three fixed stars, totally invisible by their smallness” that were all close to Jupiter and in line with its equator. Observations on subsequent nights showed that the positions of these “stars” had changed relative to Jupiter, and in a way that was not consistent with them being part of the background stars.

Galilean Family Portrait — *The Galilean moons, shown to scale – Io (top right), Europa (upper left), Ganymede (right) and Callisto (bottom left). Credit: NASA/JPL*

By January 10th, he noted that one had disappeared, which he attributed to it being hidden behind Jupiter. From this, he concluded that the stars were in fact orbiting Jupiter, and they were satellites of it. By January 13th, he discovered a fourth, and named them the Medicean stars, in honor of his future patron, Cosimo II de’ Medici, Grand Duke of Tuscany, and his three brothers.

Later astronomers, however, renamed them the Galilean Moons in honour of their discoverer. By the 20th century, these satellites would come to be known by their current names – Io, Europa, Ganymede, and Callisto – which had been suggested by 17th century German astronomer Simon Marius, apparently at the behest of Johannes Kepler.

Galileo’s observations of these satellites proved to be another major controversy. For the first time, a planet other than Earth was shown to have satellites orbiting it, which constituted yet another nail in the coffin of the geocentric model of the universe. His observations were independently confirmed afterwards, and Galileo continued to observe the satellites them and even obtained remarkably accurate estimates for their periods by 1611.

Heliocentrism:

Galileo’s greatest contribution to astronomy came in the form of his advancement of the Copernican model of the universe (i.e. heliocentrism). This began in 1610 with his publication of Sidereus Nuncius, which brought the issue of celestial imperfections before a wider audience. His work on sunspots and his observation of the Galilean Moons furthered this, revealing yet more inconsistencies in the currently accepted view of the heavens.

Cardinal Bellarmine had written in 1615 that the Copernican system could not be defended without "a true physical demonstration that the sun does not circle the earth but the earth circles the sun". Galileo considered his theory of the tides to provide the required physical proof of the motion of the earth. This theory was so important to him that he originally intended to entitle his Dialogue on the Two Chief World Systems the Dialogue on the Ebb and Flow of the Sea. For Galileo, the tides were caused by the sloshing back and forth of water in the seas as a point on the Earth's surface sped up and slowed down because of the Earth's rotation on its axis and revolution around the Sun. He circulated his first account of the tides in 1616, addressed to Cardinal Orsini. His theory gave the first insight into the importance of the shapes of ocean basins in the size and timing of tides; he correctly accounted, for instance, for the negligible tides halfway along the Adriatic Sea compared to those at the ends. As a general account of the cause of tides, however, his theory was a failure. If this theory were correct, there would be only one high tide per day. Galileo and his contemporaries were aware of this inadequacy because there are two daily high tides at Venice instead of one, about twelve hours apart. Galileo dismissed this anomaly as the result of several secondary causes including the shape of the sea, its depth, and other factors. Against the assertion that Galileo was deceptive in making these arguments, Albert Einstein expressed the opinion that Galileo developed his "fascinating arguments" and accepted them uncritically out of a desire for physical proof of the motion of the Earth. Galileo dismissed the idea, held by his contemporary Johannes Kepler, that the moon caused the tides. He also refused to accept Kepler's elliptical orbits of the planets, considering the circle the "perfect" shape for planetary orbits.Cardinal Bellarmine had written in 1615 that the Copernican system could not be defended without "a true physical demonstration that the sun does not circle the earth but the earth circles the sun". Galileo considered his theory of the tides to provide the required physical proof of the motion of the earth. This theory was so important to him that he originally intended to entitle his Dialogue on the Two Chief World Systems the Dialogue on the Ebb and Flow of the Sea. For Galileo, the tides were caused by the sloshing back and forth of water in the seas as a point on the Earth's surface sped up and slowed down because of the Earth's rotation on its axis and revolution around the Sun. He circulated his first account of the tides in 1616, addressed to Cardinal Orsini. His theory gave the first insight into the importance of the shapes of ocean basins in the size and timing of tides; he correctly accounted, for instance, for the negligible tides halfway along the Adriatic Sea compared to those at the ends. As a general account of the cause of tides, however, his theory was a failure. If this theory were correct, there would be only one high tide per day. Galileo and his contemporaries were aware of this inadequacy because there are two daily high tides at Venice instead of one, about twelve hours apart. Galileo dismissed this anomaly as the result of several secondary causes including the shape of the sea, its depth, and other factors. Against the assertion that Galileo was deceptive in making these arguments, Albert Einstein expressed the opinion that Galileo developed his "fascinating arguments" and accepted them uncritically out of a desire for physical proof of the motion of the Earth. Galileo dismissed the idea, held by his contemporary Johannes Kepler, that the moon caused the tides. He also refused to accept Kepler's elliptical orbits of the planets, considering the circle the "perfect" shape for planetary orbits. — Galileo’s Sidereus Nuncius (“Starry Messenger”), published in 1610, laid out his observations of the moon’s surface, which included mountains and impact craters. Credit: brunelleschi.imss.fi.it

Other astronomical observations also led Galileo to champion the Copernican model over the traditional Aristotelian-Ptolemaic (aka. geocentric) view. From September 1610 onward, Galileo began observing Venus, noting that it exhibited a full set of phases similar to that of the Moon. The only explanation for this was that Venus was periodically between the Sun and Earth; while at other times, it was on the opposite side of the Sun.

According to the geocentric model of the universe, this should have been impossible, as Venus’ orbit placed it closer to Earth than the Sun – where it could only exhibit crescent and new phases. However, Galileo’s observations of it going through crescent, gibbous, full and new phases was consistent with the Copernican model, which established that Venus orbited the Sun within the Earth’s orbit.

These and other observations made the Ptolemaic model of the universe untenable. Thus, by the early 17th century, the great majority of astronomers began to convert to one of the various geo-heliocentric planetary models – such as the Tychonic, Capellan and Extended Capellan models. These all had the virtue of explaining problems in the geocentric model without engaging in the “heretical” notion that Earth revolved around the Sun.

In 1632, Galileo addressed the “Great Debate” in his treatise Dialogo sopra i due massimi sistemi del mondo (Dialogue Concerning the Two Chief World Systems), in which he advocated the heliocentric model over the geocentric. Using his own telescopic observations, modern physics and rigorous logic, Galileo’s arguments effectively undermined the basis of Aristotle and Ptolemy’s system for a growing and receptive audience.

*Frontispiece and title page of the Dialogue, 1632. Credit: moro.imss.fi.it*

In the meantime, Johannes Kepler correctly identified the sources of tides on Earth – something which Galileo had become interesting in himself. But whereas Galileo attributed the ebb and flow of tides to the rotation of the Earth, Kepler ascribed this behavior to the influence of the Moon.

Combined with his accurate tables on the elliptical orbits of the planets (something Galileo rejected), the Copernican model was effectively proven. From the middle of the seventeenth century onward, there were few astronomers who were not Copernicans.

The Inquisition and House Arrest:

As a devout Catholic, Galileo often defended the heliocentric model of the universe using Scripture. In 1616, he wrote a letter to the Grand Duchess Christina, in which he argued for a non-literal interpretation of the Bible and espoused his belief in the heliocentric universe as a physical reality:

“I hold that the Sun is located at the center of the revolutions of the heavenly orbs and does not change place, and that the Earth rotates on itself and moves around it. Moreover … I confirm this view not only by refuting Ptolemy’s and Aristotle’s arguments, but also by producing many for the other side, especially some pertaining to physical effects whose causes perhaps cannot be determined in any other way, and other astronomical discoveries; these discoveries clearly confute the Ptolemaic system, and they agree admirably with this other position and confirm it.“

More importantly, he argued that the Bible is written in the language of the common person who is not an expert in astronomy. Scripture, he argued, teaches us how to go to heaven, not how the heavens go.

Galileo facing the Roman Inquisition. by Cristiano Banti's (1857). Credit: law.umkc.edu — *Galileo facing the Roman Inquisition. by Cristiano Banti’s (1857). Credit: law.umkc.ed*u

Initially, the Copernican model of the universe was not seen as an issue by the Roman Catholic Church or it’s most important interpreter of Scripture at the time – Cardinal Robert Bellarmine. However, in the wake of the Counter-Reformation, which began in 1545 in response to the Reformation, a more stringent attitude began to emerge towards anything seen as a challenge to papal authority.

Eventually, matters came to a head in 1615 when Pope Paul V (1552 – 1621) ordered that the Sacred Congregation of the Index (an Inquisition body charged with banning writings deemed “heretical”) make a ruling on Copernicanism. They condemned the teachings of Copernicus, and Galileo (who had not been personally involved in the trial) was forbidden to hold Copernican views.

However, things changed with the election of Cardinal Maffeo Barberini (Pope Urban VIII) in 1623. As a friend and admirer of Galileo’s, Barberini opposed the condemnation of Galileo, and gave formal authorization and papal permission for the publication of Dialogue Concerning the Two Chief World Systems.

However, Barberini stipulated that Galileo provide arguments for and against heliocentrism in the book, that he be careful not to advocate heliocentrism, and that his own views on the matter be included in Galileo’s book. Unfortunately, Galileo’s book proved to be a solid endorsement of heliocentrism and offended the Pope personally.

Portrait, attributed to Murillo, of Galileo gazing at the words "E pur si muove" (not legible in this image) scratched on the wall of his prison cell. Credit: — *Portrait of Galileo gazing at the words “E pur si muove” scratched on the wall of his prison cell, attributed to Bartolomé Esteban Murillo (1618-1682). Credit: Wikipedia Commons*

In it, the character of Simplicio, the defender of the Aristotelian geocentric view, is portrayed as an error-prone simpleton. To make matter worse, Galileo had the character Simplicio enunciate the views of Barberini at the close of the book, making it appear as though Pope Urban VIII himself was a simpleton and hence the subject of ridicule.

As a result, Galileo was brought before the Inquisition in February of 1633 and ordered to renounce his views. Whereas Galileo steadfastly defended his position and insisted on his innocence, he was eventually threatened with torture and declared guilty. The sentence of the Inquisition, delivered on June 22nd, contained three parts – that Galileo renounce Copernicanism, that he be placed under house arrest, and that the Dialogue be banned.

According to popular legend, after recanting his theory publicly that the Earth moved around the Sun, Galileo allegedly muttered the rebellious phrase: “E pur si muove” (“And yet it moves” in Latin). After a period of living with his friend, the Archbishop of Siena, Galileo returned to his villa at Arcetri (near Florence in 1634), where he spent the remainder of his life under house arrest.

Other Accomplishments:

In addition to his revolutionary work in astronomy and optics, Galileo is also credited with the invention of many scientific instruments and theories. Much of the devices he created were for the specific purpose of earning money to pay for his sibling’s expenses. However, they would also prove to have a profound impact in the fields of mechanics, engineering, navigation, surveying, and warfare.

Galileo's La Billancetta, in which he describes a method for hydrostatic balance. Credit: Museo Galileo — Galileo’s La Billancetta, in which he describes a method for hydrostatic balance. Credit: Museo Galileo

In 1586, at the age of 22, Galileo made his first groundbreaking invention. Inspired by the story of Archimedes and his “Eureka” moment, Galileo began looking into how jewelers weighed precious metals in air and then by displacement to determine their specific gravity. Working from this, he eventually theorized of a better method, which he described in a treatise entitled La Bilancetta (“The Little Balance”).

In this tract, he described an accurate balance for weighing things in air and water, in which the part of the arm on which the counter weight was hung was wrapped with metal wire. The amount by which the counterweight had to be moved when weighing in water could then be determined very accurately by counting the number of turns of the wire. In so doing, the proportion of metals like gold to silver in the object could be read off directly.

In 1592, when Galileo was a professor of mathematics at the University of Padua, he made frequent trips to the Arsenal – the inner harbor where Venetian ships were outfitted. The Arsenal had been a place of practical invention and innovation for centuries, and Galileo used the opportunity to study mechanical devices in detail.

In 1593, he was consulted on the placement of oars in galleys and submitted a report in which he treated the oar as a lever and correctly made the water the fulcrum. A year later the Venetian Senate awarded him a patent for a device for raising water that relied on a single horse for the operation. This became the basis of modern pumps.

*A replica of the earliest surviving telescope attributed to Galileo Galilei, on display at the Griffith Observatory. Credit: Wikipedia Commons/Mike Dunn*

To some, Galileo’s Pump was a merely an improvement on the Archimedes Screw, which was first developed in the third century BCE and patented in the Venetian Republic in 1567. However, there is no apparent evidence connecting Galileo’s invention to Archimedes’ earlier and less sophisticated design.

In ca. 1593, Galileo constructed his own version of a thermoscope, a forerunner of the thermometer, that relied on the expansion and contraction of air in a bulb to move water in an attached tube. Over time, he and his colleagues worked to develop a numerical scale that would measure the heat based on the expansion of the water inside the tube.

The cannon, which was first introduced to Europe in 1325, had become a mainstay of war by Galileo’s time. Having become more sophisticated and mobile, gunners needed instruments to help them coordinate and calculate their fire. As such, between 1595 and 1598, Galileo devised an improved geometric and military compass for use by gunners and surveyors.

During the 16th century, Aristotelian physics was still the predominant way of explaining the behavior of bodies near the Earth. For example, it was believed that heavy bodies sought their natural place of rest – i.e at the center of things. As a result, no means existed to explain the behavior of pendulums, where a heavy body suspended from a rope would swing back and forth and not seek rest in the middle.

*The Sector, a military/geometric compass designed by Galileo Galilei. Credit:* *chsi.harvard.edu*

Already, Galileo had conducted experiments that demonstrated that heavier bodies did not fall faster than lighter ones – another belief consistent with Aristotelian theory. In addition, he also demonstrated that objects thrown into the air travel in parabolic arcs. Based on this and his fascination with the back and forth motion of a suspended weight, he began to research pendulums in 1588.

In 1602, he explained his observations in a letter to a friend, in which he described the principle of isochronism. According to Galileo, this principle asserted that the time it takes for the pendulum to swing is not linked to the arc of the pendulum, but rather the pendulum’s length. Comparing two pendulum’s of similar length, Galileo demonstrated that they would swing at the same speed, despite being pulled at different lengths.

According to Vincenzo Vivian, one of Galileo’s contemporaries, it was in 1641 while under house arrest that Galileo created a design for a pendulum clock. Unfortunately, being blind at the time, he was unable to complete it before his death in 1642. As a result, Christiaan Huygens’ publication of Horologrium Oscillatorium in 1657 is recognized as the first recorded proposal for a pendulum clock.

Death and Legacy:

Galileo died on January 8th, 1642, at the age of 77, due to fever and heart palpitations that had taken a toll on his health. The Grand Duke of Tuscany, Ferdinando II, wished to bury him in the main body of the Basilica of Santa Croce, next to the tombs of his father and other ancestors, and to erect a marble mausoleum in his honor.

*Tomb of Galileo Galilei in the Santa Croce Basilica in Florence, Italy. Credit: Wikipedia Commons/stanthejeep*

However, Pope Urban VIII objected on the basis that Galileo had been condemned by the Church, and his body was instead buried in a small room next to the novice’s chapel in the Basilica. However, after his death, the controversy surrounding his works and heliocentricm subsided, and the Inquisitions ban on his writing’s was lifted in 1718.

In 1737, his body was exhumed and reburied in the main body of the Basilica after a monument had been erected in his honor. During the exhumation, three fingers and a tooth were removed from his remains. One of these fingers, the middle finger from Galileo’s right hand, is currently on exhibition at the Museo Galileo in Florence, Italy.

In 1741, Pope Benedict XIV authorized the publication of an edition of Galileo’s complete scientific works which included a mildly censored version of the Dialogue. In 1758, the general prohibition against works advocating heliocentrism was removed from the Index of prohibited books, although the specific ban on uncensored versions of the Dialogue and Copernicus’s De Revolutionibus orbium coelestium (“On the Revolutions of the Heavenly Spheres“) remained.

All traces of official opposition to heliocentrism by the church disappeared in 1835 when works that espoused this view were finally dropped from the Index. And in 1939, Pope Pius XII described Galileo as being among the “most audacious heroes of research… not afraid of the stumbling blocks and the risks on the way, nor fearful of the funereal monuments”.

A bust of Galileo at the Galileo Museum in Florence, Italy. The museum is displaying recovered parts of his body. Credit Kathryn Cook for The New York Times — *A bust of Galileo at the Museo Galileo in Florence, Italy, where recovered parts of his body and many of his possessions are on display. Credit: NYT/Kathryn Cook*

On October 31st, 1992, Pope John Paul II expressed regret for how the Galileo affair was handled, and issued a declaration acknowledging the errors committed by the Catholic Church tribunal. The affair had finally been put to rest and Galileo exonerated, though certain unclear statements issued by Pope Benedict XVI have led to renewed controversy and interest in recent years.

Alas, when it comes to the birth of modern science and those who helped create it, Galileo’s contributions are arguably unmatched. According to Stephen Hawking and Albert Einstein, Galileo was the father of modern science, his discoveries and investigations doing more to dispel the prevailing mood of superstition and dogma than anyone else in his time.

These include the discovery of craters and mountains on the Moon, the discovery of the four largest moons of Jupiter (Io, Europa, Ganymede and Callisto), the existence and nature of Sunspots, and the phases of Venus. These discoveries, combined with his logical and energetic defense of the Copernican model, made a lasting impact on astronomy and forever changed the way people look at the universe.

Galileo’s theoretical and experimental work on the motions of bodies, along with the largely independent work of Kepler and René Descartes, was a precursor of the classical mechanics developed by Sir Isaac Newton. His work with pendulums and time-keeping also previewed the work of Christiaan Huygens and the development of the pendulum clock, the most accurate timepiece of its day.

The 25 Euro coin minted for the 2009 International Year of Astronomy, showing Galileo on the obverse. Credit: coinnews.net — *The* €*25 coin minted for the 2009 International Year of Astronomy, showing Galileo on the obverse. Credit: coinnews.net*

Galileo also put forward the basic principle of relativity, which states that the laws of physics are the same in any system that is moving at a constant speed in a straight line. This remains true, regardless of the system’s particular speed or direction, thus proving that there is no absolute motion or absolute rest. This principle provided the basic framework for Newton’s laws of motion and is central to Einstein’s special theory of relativity.

The United Nations chose 2009 to be the International Year of Astronomy, a global celebration of astronomy and its contributions to society and culture. The year 2009 was selected in part because it was the four-hundredth anniversary of Galileo first viewing the heavens with his a telescope he built himself.

A commemorative €25 coin was minted for the occasion, with the inset on the obverse side showing Galileo’s portrait and telescope, as well as one of his first drawings of the surface of the moon. In the silver circle that surrounds it, pictures of other telescopes – Isaac Newton’s Telescope, the observatory in Kremsmünster Abbey, a modern telescope, a radio telescope and a space telescope – are also shown.

Other scientific endeavors and principles are named after Galileo, including the NASA Galileo spacecraft, which was the first spacecraft to enter orbit around Jupiter. Operating from 1989 to 2003, the mission consisted of an orbiter that observed the Jovian system, and an atmospheric probe that made the first measurements of Jupiter’s atmosphere.

This mission found evidence of subsurface oceans on Europa, Ganymede and Callisto, and revealed the intensity of volcanic activity on Io. In 2003, the spacecraft was crashed into Jupiter’s atmosphere to avoid contamination of any of Jupiter’s moons.

The European Space Agency (ESA) is also developing a global satellite navigation system named Galileo. And in classical mechanics, the transformation between inertial systems is known as “Galilean Transformation“, which is denoted by the non-SI unit of acceleration Gal (sometimes known as the Galileo). Asteroid 697 Galilea is also named in his honor.

Yes, the sciences and humanity as a whole owes a great dept to Galileo. And as time goes on, and space exploration continues, it is likely we will continue to repay that debt by naming future missions – and perhaps even features on the Galilean Moons, should we ever settle there – after him. Seems like a small recompense for ushering in the age of modern science, no?

Universe Today has many interesting articles on Galileo, include the Galilean moons, Galileo’s inventions, and Galileo’s telescope.

For more information, check out the the Galileo Project and Galileo’s biography.

Astronomy Cast has an episode on choosing and using a telescope, and one which deals with the Galileo Spacecraft.

November 3, 2015May 19, 2016 by Matt Williams

Who was Christiaan Huygens?

Portrait of Christiaan Huygens, painted by von Bernard Vaillant in 1686. Credit: Hofwijck Museum, Voorburg/Public Domain

The 17th century was a very auspicious time for the sciences, with advancements being made in the fields of physics, mathematics, chemistry, and the natural sciences. But it was perhaps in the field of astronomy that the greatest achievements were made. In the space of a century, several planets and moons were observed for the first time, accurate models were made to predict the motions of the planets, and the law of universal gravitation was conceived.

In the midst of this, the name of Christiaan Huygens stands out among the rest. As one of the preeminent scientists of his time, he was pivotal in the development of clocks, mechanics and optics. And in the field of astronomy, he discovered Saturn’s Rings and its largest moon – Titan. Thanks to Huygens, subsequent generations of astronomers were inspired to explore the outer Solar System, leading to the discovery of other Cronian moons, Uranus, and Neptune in the following century.

Continue reading “Who was Christiaan Huygens?”

November 1, 2015November 18, 2017 by Matt Williams

Uranus’ “Frankenstein Moon” Miranda

Color composite of the Uranian satellite Miranda, taken by Voyager 2 on Jan. 24, 1986, from a distance of 147,000 km (91,000 mi). Credit: NASA/JPL

Ever since the Voyager space probes ventured into the outer Solar System, scientists and astronomers have come to understand a great deal of this region of space. In addition to the four massive gas giants that call the outer Solar System home, a great deal has been learned about the many moons that circle them. And thanks to photographs and data obtained, human beings as a whole have come to understand just how strange and awe-inspiring our Solar System really is.

This is especially true of Miranda, the smallest and innermost of Uranus’ large moons – and some would say, the oddest-looking! Like the other major Uranian moons, its orbits close to its planet’s equator, is perpendicular to the Solar System’s ecliptic, and therefore has an extreme seasonal cycle. Combined with one of the most extreme and varied topographies in the Solar System, this makes Miranda an understandable source of interest!

Discovery and Naming:

Miranda was discovered on February 16th, 1948, by Gerard Kuiper using the McDonald Observatory‘s Otto Struve Telescope at the University of Texas in Austin. Its motion around Uranus was confirmed on March 1st of the same year, making it the first satellite of Uranus to be discovered in almost a century (the previous ones being Ariel and Umbriel, which were both discovered in 1851 by William Lassell).

A montage of Uranus's moons. Image credit: NASA — *A montage of Uranus’s moons. Image credit: NASA/JPL*

Consistent with the names of the other moons, Kuiper decided to the name the object “Miranda” after the character in Shakespeare’s The Tempest. This continued the tradition set down by John Herschel, who suggested that all the large moons of Uranus – Ariel, Umbriel, Titania and Oberon – be named after characters from either The Tempest or Alexander Pope’s The Rape of the Lock.

Size, Mass and Orbit:

With a mean radius of 235.8 ± 0.7 km and a mass of 6.59 ± 0.75 ×10¹⁹ kg, Miranda is 0.03697 Earths times the size of Earth and roughly 0.000011 as massive. Its modest size also makes it one of the smallest object in the Solar System to have achieved hydrostatic equilibrium, with only Saturn’s moon of Mimas being smaller.

Of Uranus’ five larger moons, Miranda is the closest, orbiting at an average distance (semi-major axis) of 129,390 km. It has a very minor eccentricity of 0.0013 and an inclination of 4.232° to Uranus’ equator. This is unusually high for a body so close to its parent planet – roughly ten times that of the other Uranian satellites.

Since there are no mean-motion resonances to explain this, it has been hypothesized that the moons occasionally pass through secondary resonances. At some point, this would have led Miranda into being locked in a temporary 3:1 resonance with Umbriel, and perhaps a 5:3 resonance with Ariel as well. This resonance would have altered the moon’s inclination, and also led to tidal heating in its interior (see below).

Size comparison of all the Solar Systems moons. Credit: The Planetary Society — *Size comparison of all the Solar Systems moons. Credit: NASA/The Planetary Society*

With an average orbital speed of 6.66 km/s, Miranda takes 1.4 days to complete a single orbit of Uranus. Its orbital period (also 34 hours) is synchronous with its rotational period, meaning that it is tidally-locked with Uranus and maintains one face towards it at all times. Given that it orbits around Uranus’ equator, which means its orbit is perpendicular to the Sun’s ecliptic, Uranus goes through an extreme seasonal cycle where the northern and southern hemispheres experience 42 years of lightness and darkness at a time.

Composition and Surface Structure:

Miranda’s mean density (1.2 g/cm³) makes it the least dense of the Uranian moons. It also suggests that Miranda is largely composed of water ice (at least 60%), with the remainder likely consisting of silicate rock and organic compounds in the interior. The surface of Miranda is also the most diverse and extreme of all moons in the Solar System, with features that appear to be jumbled together in a haphazard fashion.

This consists of huge fault canyons as deep as 20 km (12 mi), terraced layers, and the juxtaposition of old and young surfaces seemingly at random. This patchwork of broken terrain indicates that intense geological activity took place in Miranda’s past, which is believed to have been driven by tidal heating during the time when it was in orbital resonance with Umbriel (and perhaps Ariel).

This resonance would have increased orbital eccentricity, and along with varying tidal forces from Uranus, would have caused warming in Miranda’s interior and led to resurfacing. In addition, the incomplete differentiation of the moon, whereby rock and ice were distributed more uniformly, could have led to an upwelling of lighter material in some areas, thus leading to young and older regions existing side by side.

Another theory is that Miranda was shattered by a massive impact, the fragments of which reassembled to produce a fractured core. In this scenario – which some scientists believe could have happened as many as five times – the denser fragments would have sunk deep into the interior, with water ice and volatiles setting on top of them and mirroring their fractured shape.

Overall, scientists recognize five types of geological features on Miranda, which includes craters, coronae (large grooved features), regiones (geological regions), rupes (scarps or canyons) and sulci (parallel grooves).

Miranda’s cratered regions are differentiated between younger, lightly-cratered regions and older, more-heavily cratered ones. The lightly cratered regions include ridges and valleys, which are separated from the more heavily-cratered areas by sharp boundaries of mismatched features. The largest known craters are about 30 km (20 mi) in diameter, with others lying in the range of 5 to 10 km (3 to 6 mi).

Miranda has the largest known cliff in the Solar System, which is known as Verona Rupes (named after the setting of Shakespeare’s Romeo and Juliet). This rupes has a drop-off of over 5 km (3.1 mi) – making it 12 times as deep as the Grand Canyon. Scientists suspect that Miranda’s ridges and canyons represent extensional tilt blocks – a tectonic event where tectonic plates stretch apart, forming patterns of jagged terrain with steep drops.

*Image taken by the Voyager 2 probe during its close approach on January 24th, 1986, with a resolution of about 700 m (2300 ft). Credit: NASA/JPL*

The most well known coronae exist in the southern hemisphere, with three giant ‘racetrack’-like grooved structures that measure at least 200 km (120 mi) wide and up to 20 km (12 mi) deep. These features, named Arden, Elsinore and Inverness – all locations in Shakespeare’s plays – may have formed via extensional processes at the tops of diapirs (aka. upwellings of warm ice).

Other features may be due to cryovolcanic eruptions of icy magma, which would have been driven by tidal flexing and heating in the past. With an albedo of 0.32, Miranda’s surface is nearly as bright as that of Ariel, the brightest of the larger Uranian moons. It’s slightly darker appearance is likely due to the presence of carbonaceous material within its surface ice.

Exploration:

Miranda’s apparent magnitude makes it invisible to many amateur telescopes. As a result, virtually all known information regarding its geology and geography was obtained during the only flyby of the Uranian system, which was made by Voyager 2 in 1986. During the flyby, Miranda’s southern hemisphere pointed towards the Sun (while the northern was shrouded in darkness), so only the southern hemisphere could be studied.

At this time, no future missions have been planned or are under consideration. But given Miranda’s “Frankenstein”-like appearance and the mysteries that still surround its history and geology, any future missions to study Uranus and its system of moons would be well-advised.

We have many interesting articles on Miranda and Uranus’ moons here at Universe Today. Here’s one about about why they call it the “Frankenstein Moon“, and one about Voyager 2‘s historic flyby. And here’s one that answers the question How Many Moons Does Uranus Have?

For more information, check out NASA’s Solar System Exploration page on Miranda.

Sources:

October 30, 2015November 18, 2017 by Matt Williams

Uranus’ “Sprightly” Moon Ariel

Mosaic of the four highest-resolution images of Ariel taken by the Voyager 2 space probe during its 1986 flyby of Uranus. Credit: NASA/JPL

The outer Solar System has enough mysteries and potential discoveries to keep scientists busy for decades. Case in point, Uranus and it’s system of moons. Since the beginning of the Space Age, only one space probe has ever passed by this planet and its system of moons. And yet, that which has been gleaned from this one mission, and over a century and a half of Earth- (and space-) based observation, has been enough to pique the interest of many generations of scientists.

For instance, just about all detailed knowledge of Uranus’ 27 known moons – including the “sprightly” moon Ariel – has been derived from information obtained by the Voyager 2 probe. Nevertheless, this single flyby revealed that Ariel is composed of equal parts ice and rock, a cratered and geologically active surface, and a seasonal cycle that is both extreme and very unusual (at least by our standards!)

Discovery and Naming:

Ariel was discovered on October 24th, 1851, by English astronomer William Lassel, who also discovered the larger moon of Umbriel on the same day. While William Herschel, who discovered Uranus’ two largest moons of Oberon and Titania in 1787, claimed to have observed four other moons in Uranus’ orbit, those claims have since been concluded to be erroneous.

As with all of Uranus’ moons, Ariel was named after a character from Alexander Pope’s The Rape of the Lock and Shakespeare’s The Tempest. In this case, Ariel refers to a spirit of the air who initiates the great storm in The Tempest and a sylph who protects the female protagonist in The Rape of the Lock. The names of all four then-known satellites of Uranus were suggested by John Herschel in 1852 at the request of Lassell.

Size, Mass and Orbit:

With a mean radius of 578.9 ± 0.6 km and a mass of 1.353 ± 0.120 × 10²¹ kg, Ariel is equivalent in size to 0.0908 Earths and 0.000226 times as massive. Ariel’s orbit of Uranus is almost circular, with an average distance (semi-major axis) of 191,020 km – making it the second closest of Uranus’ five major moons (behind Miranda). It has a very small orbital eccentricity (0.0012) and is inclined very little relative to Uranus’ equator (0.260°).

With an average orbital velocity of 5.51 km/s, Ariel takes 2.52 days to complete a single orbit of Uranus. Like most moons in the outer Solar System, Ariel’s rotation is synchronous with its orbit. This means that the moon is tidally locked with Uranus, with one face constantly pointed towards the planet.

Ariel orbits and rotates within Uranus’ equatorial plane, which means it rotates perpendicular to the Sun. This means that its northern and southern hemispheres face either directly towards the Sun or away from it at the solstices, which results in an extreme seasonal cycle of permanent day or night for a period of 42 years.

Size comparison between Earth, the Moon, and Ariel. Credit: NASA/JPL/USGS/Tom Reding

Ariel’s orbit lies completely inside the Uranian magnetosphere, which means that its trailing hemisphere is regularly struck by magnetospheric plasma co-rotating with the planet. This bombardment is believed to be the cause of the darkening of the trailing hemispheres (see below), which has been observed for all Uranian moons (with the exception of Oberon).

Currently Ariel is not involved in any orbital resonance with other Uranian satellites. In the past, however, it may have been in a 5:3 resonance with Miranda, which could have been partially responsible for the heating of that moon, and 4:1 resonance with Titania, from which it later escaped.

Composition and Surface Features:

Ariel is the fourth largest of Uranus’ moons, but is believed to be the third most-massive. Its average density of 1.66 g/cm³ indicates that it is roughly composed of equal parts water ice and rock/carbonaceous material, including heavy organic compounds. Based on spectrographic analysis of the surface, the leading hemisphere of Ariel has been revealed to be richer in water ice than its trailing hemisphere.

The cause of this is currently unknown, but it may be related to bombardment by charged particles from Uranus’s magnetosphere, which is stronger on the trailing hemisphere. The interaction of energetic particles and water ice causes sublimation and the decomposition of methane trapped in the ice (as clathrate hydrate), darkening the methanogenic and other organic molecules and leaving behind a dark, carbon-rich residue (aka. tholins).

The highest-resolution Voyager 2 color image of Ariel. Canyons with floors covered by smooth plains are visible at lower right. The bright crater Laica is at lower left. Credit: NASA/JPL — *The highest-resolution Voyager 2 color image of Ariel, showing canyons with floors covered by smooth plains (lower right) and the bright Laica crater (lower left). Credit: NASA/JPL*

Based on its size, estimates of its ice/rock distribution, and the possibility of salt or ammonia in its interior, Ariel’s interior is thought to be differentiated between a rocky core and an icy mantle. If true, the radius of the core would account for 64% of the moon’s radius (372 km) and 52% of its mass. And while the presence of water ice and ammonia could mean Ariel harbors an interior ocean at it’s core-mantle boundary, the existence of such an ocean is considered unlikely.

Infrared spectroscopy has also identified concentrations of carbon dioxide (CO²) on Ariel’s surface, particularly on its trailing hemisphere. In fact, Ariel shows the highest concentrations of CO² on of any Uranian satellite, and was the first moon to have this compound discovered on its surface.

Though the precise reason for this is unknown, it is possible that it is produced from carbonates or organic material that have been exposed to Uranus’ magnetosphere or solar ultraviolet radiation – due to the asymmetry between the leading and trailing hemispheres. Another explanation is outgassing, where primordial CO² trapped in Ariel’s interior ice escaped thanks to past geological activity.

The observed surface of Ariel can be divided into three terrain types: cratered terrain, ridged terrain and plains. Other features include chasmata (canyons), fault scarps (cliffs), dorsa (ridges) and graben (troughs or trenches). Impact craters are the most common feature on Ariel, particularly in the south pole, which is the moon’s oldest and most geographically extensive region.

False-color map of Ariel. The prominent noncircular crater below and left of center is Yangoor. Part of it was erased during formation of ridged terrain via extensional tectonics. Credit: NASA/JPL/USGS — *False-color map of Ariel, showing the prominent Yangoor crater (left of center) and patches of ridged terrain (far left). Credit: USGS*

Compared to the other moons of Uranus, Ariel appears to be fairly evenly-cratered. The surface density of the craters, which is significantly lower than those of Oberon and Umbriel, suggest that they do not date to the early history of the Solar System. This means that Ariel must have been completely resurfaced at some point in its history, most likely in the past when the planet had a more eccentric orbit and was therefore more geologically active.

The largest crater observed on Ariel, Yangoor, is only 78 km across, and shows signs of subsequent deformation. All large craters on Ariel have flat floors and central peaks, and few are surrounded by bright ejecta deposits. Many craters are polygonal, indicating that their appearance was influenced by the crust’s preexisting structure. In the cratered plains there are a few large (about 100 km in diameter) light patches that may be degraded impact craters.

The cratered terrain is intersected by a network of scarps, canyons and narrow ridges, most of which occur in Ariel’s mid-southern latitudes. Known as chasmata, these canyons were probably graben that formed due to extensional faulting triggered by global tension stresses – which in turn are believed to have been caused by water and/or liquid ammonia freezing in the interior.

These chasmata are typically 15–50 km wide and are mainly oriented in an east- or northeasterly direction. The widest graben have grooves running along the crests of their convex floors (known as valles). The longest canyon is Kachina Chasma, which is over 620 km long.

was taken Jan. 24, 1986, from a distance of 130,000 km (80,000 mi). The complexity of Ariel's surface indicates that a variety of geologic processes have occurred. Credit: NASA/JPL — *Image of Ariel, taken on Jan. 24, 1986, from a distance of 130,000 km (80,000 mi) showing the complexity of Ariel’s surface. Credit: NASA/JPL*

The ridged terrain on Ariel, which is the second most-common type, consists of bands of ridges and troughs hundreds of kilometers long. These ridges are found bordering cratered terrain and cutting it into polygons. Within each band (25-70 km wide) individual ridges and troughs have been observed that are up to 200 km long and 10-35 km apart. Here too, these features are believed to be a modified form of graben or the result of geological stresses.

The youngest type of terrain observed on Ariel are its plains, which consists of relatively low-lying smooth areas. Due to the varying levels of cratering found in these areas, the plains are believed to have formed over a long period of time. They are found on the floors of canyons and in a few irregular depressions in the middle of the cratered terrain.

The most likely origin for the plains is through cryovolcanism, since their geometry resembles that of shield volcanoes on Earth, and their topographic margins suggests the eruption of viscous liquid – possibly liquid ammonia. The canyons must therefore have formed at a time when endogenic resurfacing was still taking place on Ariel.

Uranus and Ariel — Ariel’s transit of Uranus, which was captured by the Hubble Space Telescope on July 26th, 2008. Credit: NASA, ESA, L. Sromovsky (University of Wisconsin, Madison), H. Hammel (Space Science Institute), and K. Rages (SETI)

Ariel is the most reflective of Uranus’s moons, with a Bond albedo of about 23%. The surface of Ariel is generally neutral in color, but there appears to be an asymmetry where the trailing hemisphere is slightly redder. The cause of this, is believed to be interaction between Ariel’s trailing hemisphere and radiation from Uranus’ magnetosphere and Solar ultraviolet radiation, which converts organic compounds in the ice into tholins.

Like all of Uranus’ major moons, Ariel is thought to have formed in the Uranunian accretion disc; which existed around Uranus for some time after its formation, or resulted from a large impact suffered by Uranus early in its history.

Exploration:

Due to its proximity to Uranus’ glare, Ariel is difficult to view by amateur astronomers. However, since the 19th century, Ariel has been observed many times by ground-based on space-based instruments. For example, on July 26th, 2006, the Hubble Space Telescope captured a rare transit made by Ariel of Uranus, which cast a shadow that could be seen on the Uranian cloud tops. Another transit, in 2008, was recorded by the European Southern Observatory.

It was not until the 1980s that images were obtained by the first and only orbiter to ever pass through the Uranus’ system. This was the Voyager 2 space probe, which photographed the moon during its January 1986 flyby. The probe’s closest approach was at a distance of 127,000 km (79,000 mi) – significantly less than the distances to all other Uranian moons except Miranda.

Voyager 2. Credit: NASA — *Artist’s impression of the Voyager 2 space probe. Credit: NASA*

The images acquired covered only about 40% of the surface, but only 35% was captured with the quality required for geological mapping and crater counting. This was partly due to the fact that the flyby coincided with the southern summer solstice, where the southern hemisphere was pointed towards the Sun and the northern hemisphere was completely concealed by darkness.

No missions have taken place to study Uranus’ system of moons since and none are currently planned. However, the possibility of sending the Cassini spacecraft to Uranus was evaluated during its mission extension planning phase in April of 2008. It was determined that it would take about twenty years for Cassini to get to the Uranian system after departing Saturn. However, this proposal and the ultimate fate of the mission remain undecided at this time.

All in all, Uranus’ moon Ariel is in good company. Like it’s fellow Uranians, its axial tilt is almost the exact same as Uranus’, it is composed of almost equal parts ice and rock, it is geologically active, and its orbit leads to an extreme seasonal cycle. However, Ariel stands alone when its to its brightness and its youthful surface. Unfortunately, this bright and youthful appearance has not made it an easier to observe.

Alas, as with all Uranian moons, exploration of this moon is still in its infancy and there is much we do not know about it. One can only hope another deep-space mission, like a modified Cassini flyby, takes place in the coming years and finishes the job started by Voyager 2!

We have many interesting articles on Ariel and Uranus’ moons here at Universe Today. Here’s one about Ariel’s 2006 transit of Uranus, its 2008 transit, and one which answers the all-important question How Many Moons Does Uranus Have?

For more information, check out NASA’s Solar System Exploration page on Ariel, and The Planetary Society’s Voyager 2 Ariel image catalog.

Sources:

October 30, 2015December 16, 2016 by Matt Williams

Saturn’s Moon Dione

Ringside With Dione — Saturn's moon Dione, with Saturn's rings visible in the background. Credit: NASA/JPL

Thanks to the Cassini mission, a great deal has been learned about Saturn’s system of moons (aka. the Cronian system) in the past decade. Thanks to the presence of an orbiter in the system, astronomers and space exploration enthusiasts have been treated to a seemingly endless stream of images and data, which in turn has enabled us to learn many interesting things about these moons’ appearances, surface features, composition, and history of formation.

This is certainly true of Saturn’s bright moon of Dione. In addition to being the 15th largest moon in the Solar System, and more massive than all known moons smaller than itself combined, it has much in common with other Cronian satellites – like Tethys, Iapetus and Rhea. This includes being mainly composed of ice, having a synchronous rotation with Saturn, and an unusual coloration between its leading and trailing hemispheres.

Discovery and Naming:

Dione was first observed by Italian astronomer Giovanni Domenico Cassini on in 1684 using a large aerial telescope he set up on the grounds of the Paris Observatory. Along with the moons of Iapetus, Rhea and Tethys – which he had discovered in 1671, 1672 and 1684, respectively – he named these moons Sidera Lodoicea (“Stars of Louis”, after his patron, King Louis XIV of France).

These names, however, did not catch on outside of France. By the end of the 17th century, astronomers instead fell into the habit of naming Saturn’s then-known moons as Titan and Saturn I through V, in order of their observed distance from the planet. Being the second most-distant (behind Tethys) Dione came to be known as Saturn II for over a century.

An engraving of the Paris Observatory during Cassini's time. Credit: Public Domain — *An engraving of the Paris Observatory during Cassini’s time. Credit: Public Domain*

The modern names were suggested in 1847 by John Herschel (the son of famed astronomer William Herschel), who suggested all the moons of Saturn be named after Titans – the sons and daughters of Cronos in the Greek mythology (the equivalent of the Roman Saturn).

In his 1847 publication, Results of Astronomical Observations made at the Cape of Good Hope, he suggested the name Dione, an ancient oracular Titaness who was the wife of Zeus and the mother of Aphrodite. Dione is featured in Homer’s The Iliad, and geological features – such as craters and cliffs – take their names from people and places in Virgil’s Aeneid.

Size, Mass and Orbit:

With a mean radius of 561.4 ± 0.4 km and a mass of about 1.0954 × 10²¹ kg, Dione is equivalent in size to 0.088 Earths and 0.000328 times as massive. It orbits Saturn at an average distance (semi-major axis) of 377,396 km, with a minor eccentricity of 0.0022 – ranging from 376,566 km at periapsis and 378,226 km at apoapsis.

Dione’s semi-major axis is about 2% less than that of the Moon. However, reflecting Saturn’s greater mass, Dione’s orbital period is one tenth that of the Moon (2.736915 days compared to 28). Dione is currently in a 1:2 mean-motion orbital resonance with Saturn’s moon Enceladus, completing one orbit of Saturn for every two orbits completed by Enceladus.

Size comparison between Earth, the Moon, and Saturn's moon Dione. Credit: NASA/JPL/Space Science Institute — Size comparison between Earth, the Moon, and Saturn’s moon Dione. Credit: NASA/JPL/Space Science Institute

This resonance maintains Enceladus’s orbital eccentricity (0.0047) and provides tidal flexing that powers Enceladus’ extensive geological activity (which in turn powers its cryovolcanic jets). Dione has two co-orbital (aka. trojan) moons: Helene and Polydeuces. They are located within Dione’s Lagrangian points, 60 degrees ahead of and behind it, respectively.

Composition and Surface Features:

With a mean density of 1.478 ± 0.003 g/cm³, Dione is composed mainly of water, with a small remainder likely consisting of a silicate rock core. Though somewhat smaller and denser than Rhea, Dione is otherwise very similar in terms of its varied terrain, albedo features, and the different between its leading and trailing hemisphere.

Overall, scientists recognize five classes of geological features on Dione – Chasmata (chasms), dorsa (ridges), fossae (long, narrow depressions), craters, and catenae (crater chains). Craters are the most common feature, as with many Cronian moons, and can be distinguished in terms of heavily cratered terrain, moderately cratered plains, and lightly cratered plains.

The heavily cratered terrain has numerous craters greater than 100 km (62 mi) in diameter, whereas the plains areas tend to have craters less than 30 km (19 mi) in diameter (with some areas being more heavily cratered than others).

This global map of Dione, a moon of Saturn, shows dark red in the trailing hemisphere, which is due to radiation and charged particles from Saturn's intense magnetic environment. Credit: NASA/JPL/Space Science Institute — *Global map of Dione, showing dark red in the trailing hemisphere (left), which is due to radiation and charged particles from Saturn’s. Credit: NASA/JPL/Space Science Institute*

Much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains areas present on the leading hemisphere. This is the opposite of what many scientists expected, and suggests that during the period of Heavy Bombardment, Dione was tidally locked to Saturn in the opposite orientation.

Because Dione is relatively small, it is theorized that an impact large enough to cause a 35 km crater would have been sufficient to spin the satellite in the opposite direction. Because there are many craters larger than 35 km (22 mi), Dione could have been repeatedly spun during its early history. The pattern of cratering since then and the leading hemisphere’s bright albedo suggests that Dione has remained in its current orientation for several billion years.

Dione is also known for its differently colored leading and trailing hemispheres, which are similar to Tethys and Rhea. Whereas its leading hemisphere is bright, its trailing hemisphere is darker and redder in appearance. This is due to the leading hemisphere picking up material from Saturn’s E-Ring, which is fed by Enceladus’ cryovolcanic emissions.

Meanwhile, the trailing hemisphere interacts with radiation from Saturn’s magnetosphere, which causes organic elements contained within its surface ice to become dark and redder in appearance.

Dione's trailing hemisphere, showing the patches of "whispy terrain". Credit: NASA/JPL — *Dione’s trailing hemisphere, pictured by the Cassini orbiter, which shows its patches of “wispy terrain”. Credit: NASA/JPL*

Another prominent feature is Dione’s “wispy terrain“, which covers its trailing hemisphere and is composed entirely of high albedo material that is also thin enough as to not obscure the surface features beneath. The origin of these features are unknown, but an earlier hypothesis suggested that that Dione was geologically active shortly after its formation, a process which has since ceased.

During this time of geological activity, endogenic resurfacing could have pushed material from the interior to the surface, with streaks forming from eruptions along cracks that fell back to the surface as snow or ash. Later, after the internal activity and resurfacing ceased, cratering continued primarily on the leading hemisphere and wiped out the streak patterns there.

This hypothesis was proven wrong by the Cassini probe flyby of December 13th, 2004, which produced close-up images. These revealed that the ‘wisps’ were, in fact, not ice deposits at all, but rather bright ice cliffs created by tectonic fractures (chasmata). During this flyby, Cassini also captured oblique images of the cliffs which showed that some of them are several hundred meters high.

Atmosphere:

Dione also has a very thin atmosphere of oxygen ions (O+²), which was first detected by the Cassini space probe in 2010. This atmosphere is so thin that scientists prefer to call it an exosphere rather than a tenuous atmosphere. The density of molecular oxygen ions determined from the Cassini plasma spectrometer data ranges from 0.01 to 0.09 per cm³.

Crescent Dione from Cassini, October 11, 2005. The crater near the limb at top is Alcander, with larger crater Prytanis adjacent to its left. At lower right, several of the Palatine Chasmata fractures are visible, one of which can be seen bisecting the smaller craters Euryalus (right) and Nisus. NASA / Jet Propulsion Laboratory / Space Science Institute — *Dione viewed by Cassini on October 11th, 2005, showing the Alcander crater (top) and the larger Prytanis crater to its left. Credit: NASA/JPL/SSI*

Unfortunately, the prevalence of water molecules in the background (from Saturn’s E-Ring) obscured detection of water ice on the surface, so the source of oxygen remains unknown. However, photolysis is a possible cause (similar to what happens on Europa), where charged particles from Saturn’s radiation belt interact with water ice on the surface to create hydrogen and oxygen, the hydrogen being lost to space and the oxygen retained.

Exploration:

Dione was first imaged by the Voyager 1 and 2 space probes as they passed by Saturn on their way to the Outer Solar System in 1980 and 1981, respectively. Since that time, the only probe to conduct a flyby or close-up imaging of Dione has been the Cassini orbiter, which conducted five flybys of the moon between 2005 and 2015.

The first close flyby took place on October 11th, 2005, at a distance of 500 km (310 mi), followed by another on April 7th, 2010, (again at a distance of 500 km). A third flyby was performed on December 12th, 2011, and was the closest, at an distance of 99 km (62 mi). The fourth and fifth flybys took place on June 16th and August 17th, 2015, at a distance of 516 km (321 mi) and 474 km (295 mi), respectively.

In addition to obtaining images of Cassini’s cratered and differently-colored surface, the Cassini mission was also responsible for detecting the moon’s tenuous atmosphere (exosphere). Beyond that, Cassini also provided scientists with new evidence that Dione could be more geologically active than previously predicted.

Based on models constructed by NASA scientists, it is now believed that Dione’s core experiences tidal heating, which increases the closer it gets to Saturn. Because of this, scientists also believe that Dione may also have a liquid water ocean at its core-mantle boundary, thus joining moons like Enceladus, Europa and others in being potential environments where extra-terrestrial life could exist.

This, as well as Dione’s geological history and the nature of its surface (which could be what gives rise to its atmosphere) make Dione a suitable target for future research. Though no missions to study the moon are currently being planned, any mission to the Saturn system in the coming years would likely include a flyby or two!

We have many great articles on Dione and Saturn’s moons here at Universe Today. Here is one about Cassini’s first flyby, its closest flyby, it’s possible geological activity, its canyons, and its wispy terrain.

Universe Today also has an interview with Dr. Kevin Grazier, a member of the Cassini-Huygens mission.