Posted on by Matt Williams

What is the Color of Pluto?

Pluto was re-classified as a dwarf planet based on our growing understanding of its nature. Will Schlaufman's new study help us more accurately classify gas giants and brown dwarfs? NASA's New Horizons spacecraft captured this high-resolution enhanced color view of Pluto on July 14, 2015. Credit: NASA/JHUAPL/SwRI
Pluto was re-classified as a dwarf planet based on our growing understanding of its nature. Will Schlaufman's new study help us more accurately classify gas giants and brown dwarfs? NASA's New Horizons spacecraft captured this high-resolution enhanced color view of Pluto on July 14, 2015. Credit: NASA/JHUAPL/SwRI

When Pluto was first discovered by Clybe Tombaugh in 1930, astronomers believed that they had found the ninth and outermost planet of the Solar System. In the decades that followed, what little we were able to learn about this distant world was the product of surveys conducted using Earth-based telescopes. Throughout this period, astronomers believed that Pluto was a dirty brown color.

In recent years, thanks to improved observations and the New Horizons mission, we have finally managed to obtain a clear picture of what Pluto looks like. In addition to information about its surface features, composition and tenuous atmosphere, much has been learned about Pluto’s appearance. Because of this, we now know that the one-time “ninth planet” of the Solar System is rich and varied in color.

Composition:

With a mean density of 1.87 g/cm3, Pluto’s composition is differentiated between an icy mantle and a rocky core. The surface is composed of more than 98% nitrogen ice, with traces of methane and carbon monoxide. Scientists also suspect that Pluto’s internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of water ice.

The Theoretical structure of Pluto, consisting of 1. Frozen nitrogen 2. Water ice 3. Rock. Credit: NASA/Pat Rawlings

The diameter of the core is believed to be approximately 1700 km, which accounts for 70% of Pluto’s total diameter. Thanks to the decay of radioactive elements, it is possible that Pluto contains a subsurface ocean layer that is 100 to 180 km thick at the core–mantle boundary.

Pluto has a thin atmosphere consisting of nitrogen (N2), methane (CH4), and carbon monoxide (CO), which are in equilibrium with their ices on Pluto’s surface. However, the planet is so cold that during part of its orbit, the atmosphere congeals and falls to the surface. The average surface temperature is 44 K (-229 °C), ranging from 33 K (-240 °C) at aphelion to 55 K (-218 °C) at perihelion.

Appearance:

Pluto’s surface is very varied, with large differences in both brightness and color. Pluto’s surface also shows signs of heavy cratering, with ones on the dayside measuring 260 km (162 mi) in diameter. Tectonic features including scarps and troughs has also been seen in some areas, some as long as 600 km (370 miles).

Mountains have also been seen that are between 2 to 3 kilometers (6500 – 9800 ft) in elevation above their surroundings. Like much of the surface, these features are believed to be composed primarily of frozen nitrogen, carbon monoxide, and methane, which are believed to sit atop a “bedrock” of frozen water ice.

Color mosaic map of Pluto’s surface, created from the New Horizons many photographs. Credit: NASA/JHUAPL/SwRI

The surface also has many dark, reddish patches due to the presence of tholins, which are created by charged particles from the Sun interacting with mixtures of methane and nitrogen. Pluto’s visual apparent magnitude averages 15.1, brightening to 13.65 at perihelion. In other words, the planet has a range of colors, including pale sections of off-white and light blue, to streaks of yellow and subtle orange, to large patches of deep red.

Overall, its appearance could be described as “ruddy”, given that the combination can lend it a somewhat brown and earthy appearance from a distance. In fact, prior to the New Horizon‘s mission, which provided the first high-resolution, close-up images of the planet, this is precisely what astronomers believed Pluto looked like.

Major Surface Features:

Several different regions (“regio”) have been characterized based on the notable features they possess. Perhaps the best known is the large, pale area nicknamed the “Heart” – aka. Tombaugh Regio (named after Pluto’s founder). This large bright area is located on the side of Pluto that lies opposite the side that faces Charon, and is named because of its distinctive shape.

Tombaugh Regio is about 1,590 km (990 mi) across and contains 3,400 m (11,000 ft) mountains made of water ice along its southwestern edge. The lack of craters suggests that its surface is relatively young (about 100 million years old) and hints at Pluto being geologically active. The Heart can be subdivided into two lobes, which are distinct geological features that are both bright in appearance.

This new global mosaic view of Pluto was created from the latest high-resolution images to be downlinked from NASA’s New Horizons spacecraft and released on Sept. 11, 2015. Credits: NASA/Johns Hopkins APL/SwRI/Marco Di Lorenzo/Ken Kremer

The western lobe, Sputnik Planitia, is vast plain of nitrogen and carbon monoxide ices measuring 1000 km in width. It is divided into polygonal sections that are believed to be convection cells, which carry blocks of water ice and sublimation pits along towards the edge of the plain. This region is especially young (less than 10 million years old), which is indicated by its lack of cratering.

Then there is the large, dark area on the trailing hemisphere known as Cthulhu Regio (aka.the “Whale”). Named for its distinctive shape, this elongated, dark region along the equator is the largest dark feature on Pluto – measuring 2,990 km (1,860 mi) in length. The dark color is believed to be the result methane and nitrogen in the atmosphere interacting with ultraviolet light and cosmic rays, creating the dark particles (“tholins”) common to Pluto.

And then there are the “Brass Knuckles”, a series of equatorial dark areas on the leading hemisphere. These features average around 480 km (300 mi) in diameter, and are located along the equator between the Heart and the tail of the Whale.

New Horizons Mission:

The NH mission launched from Cape Canaveral Air Force Station in Florida on January 19th, 2006. After swinging by Jupiter for a gravity boost and to conduct some scientific studies in February of 2007, it reached Pluto in the summer of 2015. Once there, it conducted a six month-long reconnaissance flyby of Pluto and its system of moons, culminating with a closest approach that occurred on July 14th, 2015.

A portrait from the final approach of the New Horizons spacecraft to the Pluto system on July 11th, 2015. Pluto and Charon display striking color and brightness contrast in this composite image. Credit: NASA-JHUAPL-SWRI.

The first images of Pluto acquired by NH were taken on September 21st to 24th, 2006, during a test of the Long Range Reconnaissance Imager (LORRI). At the time, the probe was still at a distance of approximately 4.2 billion km (2.6 billion mi) or 28 AU, and the photos were released on November 28th, 2006. Between July 1st and 3rd, the first images were taken that were able to resolved Pluto and its largest moon, Charon, as separate objects.

Between July 19th–24th, 2014, the probe snapped 12 images of Charon revolving around Pluto, covering almost one full rotation at distances ranging from 429 to 422 million kilometers (267,000,000 to 262,000,000 mi). After a brief hibernation during its final approach, New Horizons “woke up” on Dec. 7th, 2014. Distant-encounter operations began on January 4th, 2015, and NH began taking images of Pluto as it grew closer.

During its closest approach (July 14th, 2015, at at 11:50 UTC), the NH probe passed within 12,500 km (7,800 mi) of Pluto. About 3 days before making its closest approach, long-range imaging of Pluto and Charon took place that were 40 km (25 mi) in resolution, which allowed for all sides of both bodies to be mapped out.

Close-range imaging also took place twice a day during this time to search for any indication of surface changes. The NH probe also analyzed Pluto’s atmosphere using its suite of scientific instruments. This included it’s ultraviolet imaging spectrometer (aka. Alice) and the Radio Science EXperiment (REX), which analyzed the composition and structure of Pluto’s atmosphere.

Haze with multiple layers in the atmosphere of Pluto. Part of the plain Sputnik Planitia with nearby mountains is seen below. Photo by New Horizons, taken 15 min after the closest approach to Pluto. Credit: NASA/JHUAPL/SwRI

It’s Solar Wind Around Pluto (SWAP) and Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) examined the interaction of Pluto’s high atmosphere with solar wind. Pluto’s diameter was also resolved by measuring the disappearance and reappearance of the radio occultation signal as the probe flew by behind Pluto. And the gravitational tug on the probe were used to determine Pluto’s mass and mass distribution.

All of this information has helped astronomers to make the first detailed maps of Pluto, and led to numerous discoveries about Pluto’s structure, composition, and the kinds of forces that actively shape its surface. The mission also led to the first true images of what Pluto looks like up close, revealing its true colors, it’s famous “Heart” region, and the many other now-famous features.

We have written many interesting articles about the colors of astronomical bodies here at Universe Today. Here’s What Color is the Sun?, What are the Colors of the Planets?, What Color is Mercury?, What Color is Venus?, What Color is the Moon?, Why is Mars Red?, What Color is Jupiter?, What Color is Saturn?, What Color is Uranus?, and What Color is Neptune?

Sources:

Posted on by Matt Williams

What is an Astronomical Unit?

Apsis
The Earth revolves around the Sun like this.

When it comes to dealing with the cosmos, we humans like to couch things in familiar terms. When examining exoplanets, we classify them based on their similarities to the planets in our own Solar System – i.e. terrestrial, gas giant, Earth-size, Jupiter-sized, Neptune-sized, etc. And when measuring astronomical distances, we do much the same.

For instance, one of the most commonly used means of measuring distances across space is known as an Astronomical Unit (AU). Based on the distance between the Earth and the Sun, this unit allows astronomers to characterize the vast distances between the Solar planets and the Sun, and between extra-solar planets and their stars.

Definition:

According to the current astronomical convention, a single Astronomical Unit is equivalent to 149,597,870.7 kilometers (or 92,955,807 miles). However, this is the average distance between the Earth and the Sun, as that distance is subject to variation during Earth’s orbital period. In other words, the distance between the Earth and the Sun varies in the course of a single year.

Earth’s orbit around the Sun, showing its average distance (or 1 AU). Credit: Huritisho/Wikipedia Commons

During the course of a year, the Earth goes from distance of 147,095,000 km (91,401,000 mi) from the Sun at perihelion (its closest point) to 152,100,000 km (94,500,000 mi) at aphelion (its farthest point) – or from a distance of 0.983 AUs to 1.016 AUs.

History of Development:

The earliest recorded example of astronomers estimating the distance between the Earth and the Sun dates back to Classical Antiquity. In the 3rd century BCE work, On the Sizes and Distances of the Sun and Moon – which is attributed to Greek mathematician Aristarchus of Samos – the distance was estimated to be between 18 and 20 times the distance between the Earth and the Moon.

However, his contemporary Archimedes, in his 3rd century BCE work Sandreckoner, also claimed that Aristarchus of Samos placed the distance of 10,000 times the Earth’s radius. Depending on the values for either set of estimates, Aristarchus was off by a factor of about 2 (in the case of Earth’s radius) to 20 (the distance between the Earth and the Moon).

The oldest Chinese mathematical text – the 1st century BCE treatise known as Zhoubi Suanjing – also contains an estimate of the distance between the Earth and Sun. According to the anonymous treatise, the distance could be calculated by conducting geometric measurements of the length of noontime shadows created by objects spaced at specific distances. However, the calculations were based on the idea that the Earth was flat.

Illustration of the Ptolemaic geocentric conception of the Universe, by Bartolomeu Velho (?-1568), from his work Cosmographia, made in France, 1568. Credit: Bibilotèque nationale de France, Paris

Famed 2nd century CE mathematician and astronomer Ptolemy relied on trigonometric calculations to come up with a distance estimate that was equivalent to 1210 times the radius of the Earth. Using records of lunar eclipses, he estimated the Moon’s apparent diameter, as well as the apparent diameter of the shadow cone of Earth traversed by the Moon during a lunar eclipse.

Using the Moon’s parallax, he also calculated the apparent sizes of the Sun and the Moon and concluded that the diameter of the Sun was equal to the diameter of the Moon when the latter was at it’s greatest distance from Earth. From this, Ptolemy arrived at a ratio of solar to lunar distance of approximately 19 to 1, the same figure derived by Aristarchus.

For the next thousand years, Ptolemy’s estimates of the Earth-Sun distance (much like most of his astronomical teachings) would remain canon among Medieval European and Islamic astronomers. It was not until the 17th century that astronomers began to reconsider and revise his calculations.

This was made possible thanks to the invention of the telescope, as well as Kepler’s Three Laws of Planetary Motion, which helped astronomers calculate the relative distances between the planets and the Sun with greater accuracy. By measuring the distance between Earth and the other Solar planets, astronomers were able to conduct parallax measurements to obtain more accurate values.

With parallax technique, astronomers observe object at opposite ends of Earth’s orbit around the Sun to precisely measure its distance. Credit: Alexandra Angelich, NRAO/AUI/NSF.

By the 19th century, determinations of about the speed of light and the constant of the aberration of light resulted in the first direct measurement of the Earth-Sun distance in kilometers.  By 1903, the term “astronomical unit” came to be used for the first time. And throughout the 20th century, measurements became increasingly precise and sophisticated, thanks in part to accurate observations of the effects of Einstein’s Theory of Relativity.

Modern Usage:

By the 1960s, the development of direct radar measurements, telemetry, and the exploration of the Solar System with space probes led to precise measurements of the positions of the inner planets and other objects. In 1976, the International Astronomical Union (IAU) adopted a new definition during their 16th General Assembly. As part of their System of Astronomical Constants, the new definition stated:

“The astronomical unit of length is that length (A) for which the Gaussian gravitational constant (k) takes the value 0.01720209895 when the units of measurement are the astronomical units of length, mass and time. The dimensions of k² are those of the constant of gravitation (G), i.e., L³M-1T–2. The term “unit distance” is also used for the length A.”

In response to the development of hyper-precise measurements, the International Committee for Weights and Measures (CIPM) decided to modify the the International System of Units (SI) in 1983. Consistent with this, they redefined the meter to be measured in terms of the speed of light in vacuum.

Infographic comparing the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Credit: ESO

However, by 2012, the IAU determined that the equalization of relativity made the measurement of AUs too complex, and redefined the astronomical unit in terms of meters. In accordance with this, a single AU is equal to 149597870.7 km exactly (92.955807 million miles), 499 light-seconds, 4.8481368×10-6 of a parsec, or 15.812507×10-6 of a light-year.

Today, the AU is used commonly to measure distances and create numerical models for the Solar System. It is also used when measuring extra-solar systems, calculating the extent of protoplanetary clouds or the distance between extra-solar planets and their parent star. When measuring interstellar distances, AUs are too small to offer convenient measurements. As such, other units – such as the parsec and the light year – are relied upon.

The Universe is a huge place, and measuring even our small corner of it producing some staggering results. But as always, we prefer to express them in ways that are as relatable and familiar.

We’ve written many interesting articles about distances in the Solar System here at Universe Today. Here’s How Far are the Planets from the Sun?, How Far is Mercury from the Sun?, How Far is Venus from the Sun?, How Far is Earth from the Sun?, How Far is Mars from the Sun?, How Far is Jupiter from the Sun?, How Far is Saturn from the Sun?, How Far is Uranus from the Sun?, How Far is Neptune from the Sun?, How Far is Pluto from the Sun?

If you’d like more information about the Earth’s orbit, check out NASA’s Solar System Exploration page.

We’ve also recorded an episode of Astronomy Cast dedicated to the measurement of distances in astronomy. Listen here, Episode 10: Measuring Distance in the Universe.

Sources:

Posted on by Tammy Plotner

Messier 38 – The Starfish Cluster

The open star cluster Messier 38, in proximity to Messier 36 and Messier 37. Credit: Wikisky

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the Starfish Cluster, otherwise known as Messier 38. Enjoy!

During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

One of these objects it the Starfish Cluster, also known as Messier 38 (or M38). This open star cluster is located in the direction of the northern Auriga constellation, along with the open star clusters M36 and M37. While not the brightest of the three, the location of the Starfish within the polygon formed by the brightest stars of Auriga makes it very easy to find.

Description:

Cruising around our Milky Way some 4200 light years from our solar system, this 220 million year old group of stars spreads itself across about 25 light years of space. If you’re using a telescope, you may have noticed its not alone… Messier 38 might very well be a binary star cluster! As Anil K. Pandey (et al) explained in a 2006 study:

“We present CCD photometry in a wide field around two open clusters, NGC 1912 and NGC 1907. The stellar surface density profiles indicate that the radii of the clusters NGC 1912 and NGC 1907 are 14′ and 6′ respectively. The core of the cluster NGC 1907 is found to be 1′.6±0′.3, whereas the core of the cluster NGC 1912 could not be defined due to its significant variation with the limiting magnitude. The clusters are situated at distances of 1400±100 pc (NGC 1912) and 1760±100 pc (NGC 1907), indicating that in spite of their close locations on the sky they may be formed in different parts of the Galaxy.”

The Starfish Cluster also known as Messier 38. Credit: Wikisky

So what’s happening here? Chances are, when you’re looking at M38, you’re looking at a star cluster that’s currently undergoing a real close encounter! Said M.R. de Oliveira (et al) said in their 2002 study:

“The possible physical relation between the closely projected open clusters NGC 1912 (M 38) and NGC 1907 is investigated. Previous studies suggested a physical pair based on similar distances, and the present study explores in more detail the possible interaction. Spatial velocities are derived from available radial velocities and proper motions, and the past orbital motions of the clusters are retrieved in a Galactic potential model. Detailed N-body simulations of their approach suggest that the clusters were born in different regions of the Galaxy and presently experience a fly-by.”

However, it was Sang Hyun Lee and See-Woo Lee who gave us the estimates of M38’s distance and age. As they wrote in their 1996 study, “UBV CCD Photometry of Open Cluster NGC 1907 and NGC 1912“: “The distance difference of the two clusters is 300pc and the age difference is 150 Myr. These results imply that the two clusters are not physically connected.”

So, how do we know they are two clusters passing in the night? The credit for that goes to de Oliveira and colleagues, who also asserted in their 2002 study:

“These simulations also shows that the faster the clusters approach the weaker the tidal debris in the bridge region, which explain why there is, apparently, no evidence of a material link between the clusters and why it should not be expected. It would be necessary to analyse deep wide field CCD photometry for a more conclusive result about the apparent absence of tidal link between the clusters.”

Atlas image mosaic of the Starfish Cluster (Messier 38), obtained as part of the Two Micron All Sky Survey (2MASS). Credit: NASA/NSF/Caltech/UofMass/IPAC

History of Observation:

This wonderful star cluster was originally discovered by Giovanni Batista Hodierna before 1654 and independently rediscovered by Le Gentil in 1749. However, it was Charles Messier’s catalog which brought it to attention:

“In the night of September 25 to 26, 1764, I have discovered a cluster of small stars in Auriga, near the star Sigma of that constellation, little distant from the two preceding clusters: this one is of square shape, and doesn’t contain any nebulosity, if one examines it with a good instrument: its extension may be 15 minutes of arc. I have determined its position: its right ascension was 78d 10′ 12″, and its declination 36d 11′ 51″ north.”

By correcting cataloging its position, M38 could later be studied by other astronomers who would also add their own notes. Caroline, then William Herschel would observe it, where the good Sir William would add to his private notes: “A cluster of scattered, pretty large [bright] stars of various magnitudes, of an irregular figure. It is in the Milky Way.”

Messier Object 38 would then later be added to the New General Catalog by John Herschel, who wasn’t particularly descriptive, either. However, there was an historic astronomer who was determined to examine this star cluster and it was Admiral Symth:

“A rich cluster of minute stars, on the Waggoner’s left thigh, of which a remarkable pair in the following are here estimated. A [mag] 7, yellow; and B 9, pale yellow; having a little companion about 25″ off in the sf [south following, SE] quarter. Messier discovered this in 1764, and described it as ‘a mass of stars of a square form without any nebulosity, extending to about 15′ of a degree;’ but it is singular that the palpable cruciform shape of the most clustering part did not attract his notice. It is an oblique cross, with a pair of large [bright] stars in each arm, and a conspicuous single one at the centre; the whole followed by a bright individual of the 7th magnitude. The very unusual shape of this cluster, recalls the sagacity of Sir William Herschel’s speculations upon the subject, and very much favours the idea of an attractive power lodged in the brightest part. For although the form be not globular, it is plainly to be seen that there is a tendency toward sphericity, by the swell of the dimensions as they draw near the most luminous place, denoting, as it were, a stream, or tide, of stars, setting toward the centre. As the stars in the same nebula must be very merely all at the same relative distance from us, and they appear to be about the same size [brightness], Sir William infers that their real magnitudes must be nearly equal. Granting, therefore, that these nebulae and clusters of stars are formed by their mutual attraction, he concludes that we may judge of their relative age, by the disposition of their component parts, those being the oldest which are the most compressed.”

Open Cluster M38, photographed on Feb 19, 2015. Credit: Wikipedia Commons/Miguel Garcia

Perhaps by taking his time and really observing, Smyth gained some insight into the true nature of M38! Observe it yourself, and see if you can also locate NGC 1907. It’s quite a pair!

Locating Messier 38:

Locating Messier 38 is relatively easy once you understand the constellation of Auriga. Looking roughly like a pentagon in shape, start by identifying the brightest of these stars – Capella. Due south of it is the second brightest star which shares its border with Beta Tauri, El Nath. By aiming binoculars at El Nath, go north about 1/3 the distance between the two and enjoy all the stars!

You will note two very conspicuous clusters of stars in this area, and so did Le Gentil in 1749. Binoculars will reveal the pair in the same field, as will telescopes using lowest power. The dimmest of these is the M38, and will appear vaguely cruciform in shape. At roughly 4200 light years away, larger aperture will be needed to resolve the 100 or so fainter members. About 2 1/2 degrees to the southeast (about a finger width) you will see the much brighter M36.

More easily resolved in binoculars and small scopes, this “jewel box” galactic cluster is quite young and about 100 light years closer. If you continue roughly on the same trajectory about another 4 degrees southeast you will find open cluster M37. This galactic cluster will appear almost nebula-like to binoculars and very small telescopes – but comes to perfect resolution with larger instruments.

The location of Messier 38 open star cluster in the Auriga constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

While all three open star clusters make fine choices for moonlit or light polluted skies, remember that high sky light means less faint stars which can be resolved – robbing each cluster of some of its beauty. Messier 38 is faintest and northernmost of the trio and located almost in the center of the Auriga pentagon. Binoculars and small telescopes will easily spot its cross-shaped pattern.

And here are the quick facts on the Starfish Nebula to help you get started:

Object Name: Messier 38
Alternative Designations: M38, NGC 1912
Object Type: Galactic Open Star Cluster
Constellation: Auriga
Right Ascension: 05 : 28.4 (h:m)
Declination: +35 : 50 (deg:m)
Distance: 4.2 (kly)
Visual Brightness: 7.4 (mag)
Apparent Dimension: 21.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Posted on by Bob King

See Mercury At Dusk, New Comet Lovejoy At Dawn

Stellarium
Mercury requests the company of your gaze now through the beginning of April, when it shines near Mars low in the west after sunset. Created with Stellarium

March has been a busy month for planet and comet watchers. Lots of action. Venus, the planet that’s captured our attention at dusk in the west for months, is in inferior conjunction with the Sun today. Watch for it to rise before the Sun in the eastern sky at dawn in about a week.

Mercury like Venus and the Moon shows phases when viewed through a telescope. Right now, the planet is in waning gibbous phase. Stellarium

As Venus flees the evening scene, steadfast Mars and a new planet, Mercury keep things lively. For northern hemisphere skywatchers, this is Mercury’s best dusk apparition of the year. If you’d like to make its acquaintance, this week and next are best. And it’s so easy! Just find a spot with a wide open view of the western horizon, bring a pair of binoculars for backup and wait for a clear evening.

Plan to watch starting about 40 minutes after sundown. From most locations, Mercury will appear about 10° or one fist held at arm’s length above the horizon a little bit north of due west. Shining around magnitude +0, it will be the only “star” in that part of the sky. Mars is nearby but much fainter at magnitude +1.5. You’ll have to wait at least an hour after sunset to spot it.

Have a telescope? Check out the planet using a magnification around 50x or higher. You’ll see that it looks like a Mini-Me version of the Moon. Mercury is brightest when closest to full. Over the next few weeks, it will wane to a crescent while increasing in apparent size.

If you have any difficulty finding brilliant Jupiter and its current pal, Spica, just start with the Big Dipper, now high in the northeastern sky at nightfall. Use the Dipper’s handle to “arc to Arcturus” and then “jump to Jupiter.” Credit: Bob King

If you like planets, don’t forget the combo of Jupiter and Spica at the opposite end of the sky. Jupiter climbs out of bed and over the southeastern horizon about 9 p.m. local time in late March, but to see it and Spica, Virgo’s brightest star, give it an hour and look again at 10 p.m. or later. Quite the duo!

You’re not afraid of getting up with the first robins are you? If you set your alarm to a half hour or so before the first hint of dawn’s light and find a location with an open view of the southeastern horizon, you might be first in your neighborhood to spot Terry Lovejoy’s brand new comet. His sixth, the Australian amateur discovered C/2017 E4 Lovejoy on the morning of March 10th in the constellation Sagittarius at about 12th magnitude.

C/2017 E4 Lovejoy glows blue-green this morning March 26. Structure around the nucleus including a small jet is visible. The comet is currently in Aquarius and quickly moving north and will reach perihelion on April 23. Credit: Terry Lovejoy

The comet has rapidly brightened since then and is now a small, moderately condensed fuzzball of magnitude +9, bright enough to spot in a 6-inch or larger telescope. Some observers have even picked it up in large binoculars. Lovejoy’s comet should brighten by at least another magnitude in the coming weeks, putting it within 10 x 50 binocular range.

This map shows the sky tomorrow morning before dawn from the central U.S. (latitude about 41° north). Created with Stellarium

Good news. E4 Lovejoy is moving north rapidly and is now visible about a dozen degrees high in Aquarius just before the start of dawn. I’ll be out the next clear morning, eyepiece to eye, to welcome this new fuzzball from beyond Neptune to my front yard. The map above shows the eastern sky near dawn and a general location of the comet. Use the more detailed map below to pinpoint it in your binoculars and telescope.

This chart shows the comet’s position nightly (5:30 a.m. CDT) through April 9. On the morning of April 1 it passes just a few degrees below the bright globular cluster M15. Click to enlarge, save and then print out for use at the telescope. Map: Bob King, Source: Chris Marriott’s SkyMap

Spring brings with it a new spirit and the opportunity to get out at night free of the bite of mosquitos or cold. Clear skies!

Posted on by Bob King

Watch Stars Orbit The Milky Way’s Supermassive Black Hole

Stars circle 'round the Milky Way central supermassive black hole. Credit: ESO
The Milky Way’s supermassive black hole, called Sagittarius A* (or Sgr A*), is arrowed in the image made of the innermost galactic center in X-ray light by NASA’s Chandra Observatory. To the left or east of Sgr A* is Sgr A East, a large cloud that may be the remnant of a supernova. Centered on Sgr A* is a spiral shaped group of gas streamers that might be falling onto the hole. Credit: NASA/CXC/MIT/Frederick K. Baganoff et al.

When your ordinary citizen learns there’s a supermassive black hole with a mass of 4 million suns sucking on its teeth in the center of the Milky Way galaxy, they might kindly ask exactly how astronomers know this. A perfectly legitimate question. You can tell them that the laws of physics guarantee their existence or that people have been thinking about black holes since 1783. That year, English clergyman John Michell proposed the idea of “dark stars” so massive and gravitationally powerful they could imprison their own light.

This time-lapse movie in infrared light shows how stars in the central light-year of the Milky Way have moved over a period of 14 years. The yellow mark at the image center represents the location of Sgr A*, site of an unseen supermassive black hole.
Credit: A. Eckart (U. Koeln) & R. Genzel (MPE-Garching), SHARP I, NTT, La Silla Obs., ESO

Michell wasn’t making wild assumptions but taking the idea of gravity to a logical conclusion. Of course, he had no way to prove his assertion. But we do. Astronomers  now routinely find bot stellar mass black holes — remnants of the collapse of gas-guzzling supergiant stars — and the supermassive variety in the cores of galaxies that result from multiple black hole mergers over grand intervals of time.

Some of the galactic variety contain hundreds of thousands to billions of solar masses, all of it so to speak “flushed down the toilet” and unavailable to fashion new planets and stars. Famed physicist Stephen Hawking has shown that black holes evaporate over time, returning their energy to the knowable universe from whence they came, though no evidence of the process has yet been found.

On September 14, 2013, astronomers caught the largest X-ray flare ever detected from Sgr A*, the supermassive black hole at the center of the Milky Way, using NASA’s Chandra X-ray Observatory.  This event was 400 times brighter than the usual X-ray output from the source and was possibly caused when Sgr A*’s strong gravity tore apart an asteroid in its neighborhood, heating the debris to X-ray-emitting temperatures before slurping down the remains.The inset shows the giant flare. Credit: NASA

So how do we really know a massive, dark object broods at the center of our sparkling Milky Way? Astronomers use radio, X-ray and infrared telescopes to peer into its starry heart and see gas clouds and stars whirling about the center at high rates of speed. Based on those speeds they can calculate the mass of what’s doing the pulling.

The Hubble Space Telescope took this photo of the  5000-light-year-long jet of radiation ejected from the active galaxy M87’s supermassive black hole, which is aboutt 1,000 times more massive than the Milky Way’s black hole. Although black holes are dark, matter whirling into their maws at high speed is heated to high temperature, creating a bright disk of material and jets of radiation. Credit: NASA/The Hubble Heritage Team (STScI/AURA)

In the case of the galaxy M87 located 53.5 million light years away in the Virgo Cluster, those speeds tell us that something with a mass of 3.6 billion suns is concentrated in a space smaller than our Solar System. Oh, and it emits no light! Nothing fits the evidence better than a black hole because nothing that massive can exist in so small a space without collapsing in upon itself to form a black hole. It’s just physics, something that Mr. Scott on Star Trek regularly reminded a panicky Captain Kirk.

So it is with the Milky Way, only our black hole amounts to a piddling 4 million-solar-mass light thief confined within a spherical volume of space some 27 million miles in diameter or just shy of Mercury’s perihelion distance from the Sun. This monster hole resides at the location of Sagittarius A* (pronounced A- star), a bright, compact radio source at galactic center about 26,000 light years away.


Video showing a 14-year-long time lapse of stars orbiting Sgr A*

The time-lapse movie, compiled over 14 years, shows the orbits of several dozen stars within the light year of space centered on Sgr A*. We can clearly see the star moving under the influence of a massive unseen body — the putative supermassive black hole. No observations of Sgr A* in visible light are possible because of multiple veils of interstellar dust that lie across our line of sight. They quench its light to the tune of 25 magnitudes.


Merging black holes (the process look oddly biological!). Credit: SXS

How do these things grow so big in the first place? There are a couple of ideas, but astronomers don’t honestly know for sure. Massive gas clouds around early in the galaxy’s history could have collapsed to form multiple supergiants that evolved into black holes which later then coalesced into one big hole. Or collisions among stars in massive, compact star clusters could have built up stellar giants that evolved into black holes. Later, the clusters sank to the center of the galaxy and merged into a single supermassive black hole.

Whichever you chose, merging of smaller holes may explain its origin.

On a clear spring morning before dawn, you can step out to face the constellation Sagittarius low in the southern sky. When you do, you’re also facing in the direction of our galaxy’s supermassive black hole. Although you cannot see it, does it not still exert a certain tug on your imagination?

Posted on by Matt Williams

Process Behind Martian Streaks Continues To Puzzle

These dark streaks, called recurring slope lineae, are on a sloped wall on a crater on Mars. A new study says they may have been formed by boiling water. Image: NASA/JPL-Caltech/Univ. of Arizona
These dark streaks, called recurring slope lineae (RSL), are on the sloped wall of a crater on Mars. A new study says they may have been formed by boiling water. Credit: NASA/JPL-Caltech/Univ. of Arizona

It’s a well-documented fact that roughly 4 billion years ago, Mars had liquid water flowing on its surface. However, there have also been recent findings that suggest that Mars might periodically have liquid water on its surface today. One of the strongest bits of evidence comes in the form of Recurring Slope Lineae, which are ventured to be seasonal flows of salty water which occur during Mars’ warmest months.

However, a new study produced by an international team of scientists has casts doubt on this theory and offered another possible explanation. Using numerical simulations, they show how a “dry” process – where rarefied gas is pumped up through the soil (due to temperature variations) – could lead to the formation of the dark streaks that have been observed on Martian slopes.

Their study, titled “Formation of recurring slope lineae on Mars by rarefied gas-triggered granular flows“, appeared recently in the journal Nature Geoscience. In it, the research team – which hails from the Géosciences Paris Sud (GEOPS) laboratory in Orsay, France, and the Slovak Academy of Sciences in Bratislava- explain how the current theories about what creates RSLs fall short.

Reprojected view of warm-season flows in Newton Crater. Credit: NASA/JPL-Caltech/Univ. of Arizona

As Frédéric Schmidt, a professor from GEOPS and the lead author of the study told Universe Today via email, the current theory about RSLs is based on the morphology, composition and seasonality of lineae which in the past, seemed to suggest that liquid salt water played a role in their formation:

“They attributed the appearance to liquid water mainly because of seasonality and salt detection. The activity occurs at the maximum temperature season only, in the most favorable condition for water to be liquid. The salt permits to decrease the freezing temperature of liquid water.”

This theory has met with its share of excitement, considering that the presence of water on the Martian surface would mean that the chances of finding present-day life there would be significantly greater. Unfortunately, recent studies have cast doubt on this by showing how there is insufficient water on Mars to account for the lineae that have been observed on various slopes.

“[T]here is not enough atmospheric water to fill all the dark flows and internal subsurface sources are very unlikely (Chojnacki et al., 2016),” said Dr. Schmidt. “Also, because there is no signature in the thermal range as one may have in the case of abundant liquid water. From the data, the maximum allowed water is too little (Edwards et al., 2016).”

Evolution of RSL at Garni Crater, Valles Marineris, Mars. Credit: MRO, HiRISE, NASA/JPL/University of Arizona

However, Mars does have sufficient air pressure to allow for another process known as thermal creep. Also known as thermal transpiration, this process involves gas molecules drifting from the cold end of a narrow channel to the warm end. This occurs as a result of the walls of the channel experiencing temperature changes, which triggers a gas flow.

According to their study, sections of the Martian surface could be heated by solar radiation while others remained cooler because they were covered by a source of shade.  When this happens, rarefied gas beneath the surface (i.e. gas with lower pressure than the atmosphere) could be pumped up through the Martian soil. Once it reached the surface, this gas would disturb patches of small particles, triggering tiny avalanches along Martian slopes.

To test this “dry” process of RSL formation, the team ran numerical simulations that took into account various locations on Mars and seasonal changes. “We tested our theory by modeling it and estimating its efficiency for different facet orientation and different seasons,” said Dr. Schmidt. “We find that the observed activity is coherent with our prediction. Also we simulated it in the lab in order validate the principle.

Basically, they found that in rough and boulder-strewn terrain on Mars (where shadows are cast that can cause temperature differences in small sections of soil) this process could result in the formation of dark streaks along slopes. Not only were their results consistent with observered RSLs in some areas, but they also explained how they could form without the need for liquid water or CO² frost (dry ice) activity.

Simulation of the 100 meter-long recurring slope lineae detected on the Hale crater, produced by the High Resolution Imaging Science Experiment (University of Arizona). Credits: NASA/JPL/University of Arizona

This may sound like bad news, and it certainly is if you’re planning on establishing a settlement on Mars anytime soon (Elon Musk and Bas Lansdorp might want to take heed!). And as Dr. Schmidt explained, it doesn’t bode well for those who are looking to confirm that there could be present-day life on Mars either:

“Since RSL are the main features to argue about the presence of liquid water at present time on Mars, it was also the argument for possible habitability and life on Mars. If the new theory is correct, the present Mars is not as habitable as we previously thought. Liquid water was most probably present billions of years ago, but not today. These findings paint the portrait of an inhospitable world for human exploration.”
 So it seems that the prospect of water-procurement on Mars might be trickier than we thought. Perhaps future missions to the surface that rely on in-situ resource utilization (ISRU) will either have to drill for water, or harvest it directly from the ice caps. And as for full-blown colonization plans… well, let’s hope they don’t mind drilling wells or chopping ice either!

Further Reading: Nature Geoscience

Posted on by Matt Williams

Curiosity’s Battered Wheels Show First Breaks

Image taken by the Mars Hand Lens Imager (MAHLI) of Curiosity's wheels on March 19, 2017. Credit: NASA

Since it landed on August 6th, 2012, the Curiosity rover has spent a total of 1644 Sols (or 1689 Earth days) on Mars. And as of March 2017, it has traveled almost 16 km (~10 mi) across the planet and climbed almost a fifth of a kilometer (0.124 mi) uphill. Spending that kind of time on another planet, and traveling that kind of distance, can certainly lead to its share of wear of tear on a vehicle.

That was the conclusion when the Curiosity science team conducted a routine check of the rover’s wheels on Sunday, March 19th, 2017. After examining images taken by the Mars Hand Lens Imager (MAHLI), they noticed two small breaks in the raised treads on the rover’s left middle wheel. These breaks appeared to have happened since late January, when the last routine check of the wheels took place.

To get around, the Curiosity rover relies on six solid aluminum wheels that are 40 cm (16 in) wide. The skin of the wheels is thinner than a US dime, but each contains 19 zigzag-shaped treads that are about 0.75 cm (three-quarters of an inch) thick. These “grousers”, as they are called, bear most of the rover’s weight and provide most of the wheel’s traction.

Close-up image of the broken grousers on Curiosity’s left-middle wheel. Credit: NASA/JPL-Caltech/MSSS

Ever since the rover was forced to cross a stretch of terrain that was studded with sharp rocks in 2013, the Curiosity team has made regular checks on the rover’s wheels using the MAHLI camera. At the time, the rover was moving from the Bradbury Landing site (where it landed in 2012) to the base of Mount Sharp, and traversing this terrain caused holes and dents in the wheels to grow significantly.

However, members of Curiosity’s science team emphasized that this is nothing to be worried about, as it will not affect the rover’s performance or lifespan. As Jim Erickson, the Curiosity Project Manager at NASA’s Jet Propulsion Laboratory, said in a recent NASA press statement:

“All six wheels have more than enough working lifespan remaining to get the vehicle to all destinations planned for the mission. While not unexpected, this damage is the first sign that the left middle wheel is nearing a wheel-wear milestone.”

In addition to regular monitoring, a wheel-longevity testing program was started on Earth in 2013 using identical aluminum wheels. These tests showed that once a wheel got to the point where three of its grousers were broken, it had passed about 60% of its lifespan. However, Curiosity has already driven more than 60% of the total distance needed for it to make it to all of its scientific destinations.

Graphic depicting aspects of the driving distance, elevation, geological units and time intervals of NASA’s Curiosity Mars rover mission, as of late 2016. Credit: NASA/JPL-Caltech

Curiosity’s Project Scientist – Ashwin Vasavada, also at JPL – was similarly stoic in his appraisal of this latest wheel check:

“This is an expected part of the life cycle of the wheels and at this point does not change our current science plans or diminish our chances of studying key transitions in mineralogy higher on Mount Sharp.”

At present, Curiosity is examining sand dunes in the geographical region known as the Murray Buttes formation, which is located on the slope of Mount Sharp. Once finished, it will proceed up higher to a feature known as “Vera Rubin Ridge”, inspecting a layer that is rich in the mineral hematite. From there, it will proceeded to even higher elevations to inspect layers that contain clays and sulfates.

Getting to the farthest destination (the sulfate unit) will require another 6 km (3.7 mi) of uphill driving. However, this is a short distance compared to the kind of driving the rover has already performed. Moreover, the science team has spent the past four years implementing various methods designed to avoid embedded rocks and other potentially hazardous terrain features.

MRO image of Gale Crater illustrating the landing location and trek of the Rover Curiosity. Credits: NASA/JPL, illustration, T.Reyes

It is expected that this drive up Mount Sharp will yield some impressive scientific finds. During its first year on Mars, Curiosity succeeded in gathering evidence in the Gale Crater that showed how Mars once had conditions favorable to life. This included ample evidence of liquid water, all the chemical elements needed for life, and even a chemical source of energy.

By scaling Mount Sharp and examining the layers that were deposited over the course of billions of years, Curiosity is able to examine a living geological record of how the planet has evolved since then. Luckily, the rover’s wheels seem to have more than enough life to make these and (most likely) other scientific finds.

Further Reading: NASA – Mars Exploration

Posted on by Matt Williams

Rosetta Images Show Comet’s Changing Surface Close Up

Rosetta mission poster showing the deployment of the Philae lander to comet 67P/Churyumov-Gerasimenko.. Credit: ESA/ATG medialab (Rosetta/Philae); ESA/Rosetta/NavCam (comet)

The Rosetta spacecraft learned a great deal during the two years that it spent monitoring Comet 67P/Churyumov-Gerasimenko – from August 6th, 2014 to September 30th, 2016. As the first spacecraft to orbit the nucleus of a comet, Rosetta was the first space probe to directly image the surface of a comet, and observed some fascinating things in the process.

For instance, the probe was able to document some remarkable changes that took place during the mission with its OSIRIS camera. According to a study published today (March. 21st) in Science, these included growing fractures, collapsing cliffs, rolling boulders and moving material on the comet’s surface that buried some features and exhumed others.

These changes were noticed by comparing images from before and after the comet reached perihelion on August 13th, 2015 – the closets point in its orbit around the Sun. Like all comets, it is during this point in 67P/Churyumov-Gerasimenko’s orbit that the surface experiences its highest levels of activity, since perihelion results in greater levels of surface heating, as well as increased tidal stresses.

Images taken by Rosetta’s OSIRIS camera show changes in the surface between 2015 and 2016. Credit: ESA/Rosetta/NAVCAM (top center images); ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA (all others)

Basically, as comets gets closer to the Sun, they experience a combination of in-situ weathering and erosion, sublimation of water-ice, and mechanical stresses arising from an increased spin rate. These processes can be either unique and transient, or they can place over longer periods of time.

As Ramy El-Maarry, a scientist from the Max-Planck Institute for Solar System Research and the lead author of the study, said in an ESA press statement:

“Monitoring the comet continuously as it traversed the inner Solar System gave us an unprecedented insight not only into how comets change when they travel close to the Sun, but also how fast these changes take place.”

For instance, in-situ weathering occurs all over the comet and is the result of heating and cooling cycles that happen on both a daily and a seasonal basis. In the case of 67P/Churyumov-Gerasimenko’s (6.44 Earth years), temperatures range from 180 K (-93 °C; -135 °F) to 230 K (-43 °C; -45 °F) during the course of its orbit. When the comet’s volatile ices warm, they cause consolidated material to weaken, which can cause fragmentation.

Combined with the heating of subsurface ices – which leads to outgassing – this process can result in the sudden collapse of cliff walls. As other photographic evidence that was recently released by the Rosetta science team can attest, this sort of process appears to have taken place in several locations across the comet’s surface.

Images showing a new fracture and boulder movement in Anuket. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/ID

Similarly, comets experience increased stress because their spin rates speed up as they gets closer to the Sun. This is believed to be what caused the 500 meter-long (1640 ft) fracture that has been observed in the Anuket region. Originally discovered in August of 2014, this fracture appeared to have grown by 30 meters (~100 ft) when it was observed again in December of 2014.

This same process is believed to be responsible for a new fracture that was identified from OSIRIS images taken in June 2016. This 150-300 meter-long (492 – 984 ft) fracture appears to have formed parallel to the original. In addition, photographs taken in February of 2015 and June of 2016 (shown above) revealed how a 4 meter-wide (13 ft) boulder that was sitting close to the fractures appeared to have moved by about 15 meters (49 ft).

Whether or not the two phenomena are related is unclear. But it is clear that something very similar appears to have taken place in the Khonsu region. In this section of the comet (which corresponds to one of its larger lobes), images taken between May of 2015 and June 2016 (shown below) revealed how a much larger boulder appeared to have moved even farther between the two time periods.

Images showing a moving boulder in the Khonsu region. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

This boulder – which measures some 30 meters (98 ft) across and weighs an estimated 12,800 metric tonnes (~14,100 US tons) – moved a distance of about 140 meters (~460 ft). In this case, outgassing during perihelion is believed to be the culprit. On the one hand, it could have caused the surface material to erode beneath it (thus causing it to roll downslope) or by forcibly pushing it.

For some time, it has been known that comets undergo changes during the course of their orbits. Thanks to the Rosetta mission, scientists have been able to see these processes in action for the first time. Much like all space probes, vital information continues to be discovered long after the Rosetta mission officially came to an end. Who knows what else the probe managed to witness during its historic mission, and which we will be privy to?

Further Reading: ESA

Posted on by Matt Williams

Eye Opening Numbers On Space Debris

Still image taken from a movie, Space debris ? a journey to Earth, to be released April 18th, 2017. Credit: ESA

Orbital debris, otherwise known as “space junk”, is a major concern. This massive cloud that orbits the Earth is the result of the many satellites, platforms and spent launchers that have been sent into space over the years. And as time went on, collisions between these objects (as well as disintegrations and erosion) has created even more in the way of debris.

Aside from threatening satellites and posing a danger to long-term orbital missions – like the International Space Station – this situation could pose serious problems for future space launches. And based on the latest numbers released by the Space Debris Office at the European Space Operations Center (ESOC), the problem has been getting getting worse. Continue reading “Eye Opening Numbers On Space Debris”

Posted on by Matt Williams

What is Uranus Named After?

Uranus as seen by NASA's Voyager 2. Credit: NASA/JPL

The period known as the Scientific Revolution (ca. 16th to the 18th century) was a time of major scientific upheaval. In addition to advances made in mathematics, chemistry, and the natural sciences, several major discoveries were made in the field of astronomy. Because of this, our understanding of the size and structure of the Solar System was forever revolutionized.

Consider the discovery of Uranus. While this planet had been viewed on many occasions by astronomers in the past, it was only with the birth of modern astronomy that its true nature came to be understood. And with William Herschel‘s discovery in the 18th century, the planet would come to be officially named and added to the list of known Solar Planets.

Past Observations:

The first recorded instance of Uranus being spotted in the night sky is believed to date back to the 2nd century BCE. At this time, Hipparchos – the Greek astronomer, mathematician and founder of trigonometry – apparently recorded the planet as a star in his star catalogue (completed in 129 BCE).

Large floor mosaic from a Roman villa in Sassoferrato, Italy (ca. 200–250 CE). Aion (Uranus), the god of eternity, stands above Tellus (Gaia) and her four children (the seasons). Credit: Wikipedia Commons/Bibi Saint-Poi

This catalog was later incorporated into Ptolemy’s Almagest, which became the definitive source for Islamic astronomers and for scholars in Medieval Europe for over one-thousand years. During the 17th and 18th centuries, multiple recorded sightings were made by astronomers who catalogued it as being a star.

This included English astronomer John Flamsteed, who in 1690 observed the star on six occasions and catalogued it as a star in the Taurus constellation (34 Tauri). During the mid-18th century, French astronomer Pierre Lemonnier made twelve recorded sightings, and also recorded it as being a star. It was not until March 13th, 1781, when William Herschel observed it from his garden house in Bath, that Uranus’ true nature began to be revealed.

Discovery:

Herschel’s first report on the object was recorded on April 26th, 1781. Initially, he described it as being a “Nebulous star or perhaps a comet”, but later settled on it being a comet since it appeared to have changed its position in the sky. When he presented his discovery to the Royal Society, he maintained this theory, but also likened it to a planet.

Replica of Herschel’s Seven-foot Reflecting Telescope, located at the Herschel Museum of Astronomy. Credit: herschelmuseum.org.uk

As was recorded in the Journal of the Royal Society and Royal Astronomical Society on the occasion of his presentation:

“The power I had on when I first saw the comet was 227. From experience I know that the diameters of the fixed stars are not proportionally magnified with higher powers, as planets are; therefore I now put the powers at 460 and 932, and found that the diameter of the comet increased in proportion to the power, as it ought to be, on the supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well-founded, this proving to be the Comet we have lately observed.”

While Herschel would continue to maintain that what he observed was a comet, his “discovery” stimulated debate in the astronomical community about what Uranus was. In time, astronomers like Johann Elert Bode would conclude that it was a planet, based on its nearly-circular orbit. By 1783, Herschel himself acknowledged that it was a planet to the Royal Society.

Name and Meaning:

As he lived in England, Herschel originally wanted to name Uranus after his patron, King George III. Specifically, he wanted to call it Georgium Sidus (Latin for “George’s Star”), or the Georgian Planet. Although this was a popular name in Britain, the international astronomy community didn’t think much of it, and wanted to follow the historical precedent of naming the planets after ancient Greek and Roman gods.

These two pictures of Uranus — one in true color (left) and the other in false color — were compiled from images returned Jan. 17, 1986, by the narrow-angle camera of Voyager 2. Credit: NASA/JPL

Consistent with this, Bode proposed the name Uranus in a 1782 treatise. The Latin form of Ouranos, Uranus was the grandfather of Zeus (Jupiter in the Roman pantheon), the father of Cronos (Saturn), and the king of the Titans in Greek mythology. As it was discovered beyond the orbits of Jupiter and Saturn, the name seemed highly appropriate. As he would later write in his 1784 book, “From the Newly Discovered Planet“:

“Already in the pre-read at the local Natural History Society on 12th March 1782 treatise, I have the father’s name from Saturn, namely Uranus, or as it is usually with the Latin suffix, proposed Uranus, and have since had the pleasure that various astronomers and mathematicians, cited in their writings or letters to me approving this designation. In my view, it is necessary to follow the mythology in this election, which had been borrowed from the ancient name of the other planets; because in the series of previously known, perceived by a strange person or event of modern times name of a planet would very noticeable. Diodorus of Cilicia tells the story of Atlas, an ancient people that inhabited one of the most fertile areas in Africa, and looked at the sea shores of his country as the homeland of the gods. Uranus was her first king, founder of their civilized life and inventor of many useful arts. At the same time he is also described as a diligent and skilful astronomers of antiquity … even more: Uranus was the father of Saturn and the Atlas, as the former is the father of Jupiter.”

There were some holdouts to this new name, largely in Britain, where the name Georgium Sidus remained popular. Nevertheless, Herschel’s proposal would become universally accepted by 1850. Uranus was the only planet in the Solar System named after a god from Greek mythology, rather than using the Roman counterpart’s name. 

Other Names:

While Uranus remains the widely-recognized name for the Solar System’s seventh planet (and third gas giant), other cultures have recognized it by various other names. For example in traditional Chinese astronomy, it is known as Tianwángxing, which means literally “Sky King Star”.

Uranus. Image credit: Hubble
Uranus, as imaged by the Hubble Space Telescope. Credit: NASA/Hubble

The same name is recognized in the Korean, Japanese and Vietnamese astronomical traditions. To the Aztecs (and other Nahuatl-speaking peoples), Uranus was known as “Ilhuicateocitlalli” – named after the word for “sky” (“ilhuicatl”) – and also as “Xiuhteuccitlalli”, the Aztec god of fire, day, and heat. Many other cultures recognized Uranus in their mythological traditions and assigned various names.

The discovery of Uranus was one of several that would follow from the 18th century onward. In time, Neptune, the Asteroid Belt, Ceres, Vesta, Pluto and the Kuiper Belt would be added to the mix, thus creating a model of the Solar System that would endure until the early 21st century – when new bodies were discovered beyond the orbit that Neptune that would lead to the nomenclature debate.

We have written many interesting articles on Uranus here at Universe Today. Here’s The Planet Uranus, Ten Interesting Facts About Uranus, Why is Uranus on its Side?, Tilt of Saturn, and Who Discovered Uranus?

For more information, here’s an article from the Hubble educational site about the discovery of Uranus, and here’s NASA’s Solar System Exploration page on Uranus.

We have recorded an episode of Astronomy Cast just about Uranus. You can access it here: Episode 62: Uranus.

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