Five of the planets in the night sky are easy to see with the unaided eye, and have been known since ancient times. Uranus is just bright enough that you can see it in a perfectly dark place if you know where to look. But Neptune can only be seen in a telescope. And since telescopes have only been around for a few hundred years, Neptune was discovered recently. So, who discovered Neptune?
The mathematician Alexis Bouvard published a series of astronomical tables detailing the orbit of Uranus. Over time, several astronomers realized that there had to be some additional planet deeper out in the Solar System that was influencing the motion of Uranus with its gravity. They set to work calculating where this additional planet might be located in the Solar System.
Two astronomers, Britain’s John Couch Adams and France’s Urbain Le Verrier had worked out the position of the hypothetical 8th planet independently from each other. And both had a difficult time convincing their colleagues to spend any time actually looking where they suggested the planet might be.
The Berlin Observatory astronomer Johann Gottfried Galle used the calculations by Le Verrier to find Neptune within just 1° of its predicted location, and just 12° of Adams’ predictions. Both astronomers claimed that they were the first to discover the planet, and it led to an international dispute.
After the discovery, there was rivalry between England and France about who should get credit for finding Neptune, Adams or Le Verrier. The international astronomy community agreed that the two astronomers should share credit for the discovery. Eventually both Le Verrier and Adams were given credit for discovering Neptune in 1846.
The planet was named after the Roman god of the oceans; the same as the Greek God Poseidon.
Of course, this is just a shortened version of the discovery of Neptune. If you’d like to read more, check out this article that talks about the mathematical discovery of planets. And here’s more information on Le Verrier.
The "surface" of Neptune, its uppermost layer, is one of the most turbulent and active places in the Solar System. Credit: NASA/JPL
How did Neptune get its name? Shortly after its discovery, Neptune was only referred to as “the planet exterior to Uranus” or as “Le Verrier’s planet”. The first suggestion for a name came from Johann Galle, who proposed the name Janus. Another proposal was Oceanus. Urbain Le Verrier, who discovered the planet, claimed the right to name his discovery: Neptune. Soon Neptune became the internationally accepted name.
In roman mythology, Neptune was the god of the sea. The demand for a mythological name seemed to be in keeping with the nomenclature of the other planets, all of which, except for Earth, were named for Greek and Roman mythology. Most languages today use some variant of the name “Neptune” for the planet.
Now that you know how the planet was named, how about some facts about the planet itself. Size wise, the planet has an equatorial radius 24,764 km, a polar radius of 24,341 km, and a surface area of 7.6408×10,sup>9km2. It has a volume of 6.254×1013km3, a mass of 1.0243×1026kg, and a mean density of 1.638 g/cm3.
Its atmosphere is composed primarily of hydrogen and helium along with traces of hydrocarbons and nitrogen. It also contains a high proportion of ices like: water, ammonia, and methane. Astronomers occasionally categorize Neptune as an ice giant. The interior of Neptune is primarily composed of ices and rock. Traces of methane in the outermost regions account for the planet’s blue appearance. Neptune’s atmosphere is notable for its active and visible weather patterns. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 km/h.Because of its great distance from the Sun, Neptune’s outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching ?218°C. Temperatures at the planet’s center are approximately 5,000°C. Neptune is one of the most interesting planets in our solar system. There are plenty of other articles about the planet here on Universe Today.
We have written many articles about Neptune for Universe Today. Here’s an article about the size of Neptune, and here’s an article about the atmosphere of Neptune.
[/caption] Mass: 1.98892 x 1030 kg Diameter: 1,391,000 kilometers Radius: 695,500 km Surface gravity of the Sun: 27.94 g Volume of the Sun: 1.412 x 1018 km3 Density of the Sun: 1.622 x 105 kg/m3
How Big is the Sun?
The Sun is the largest object in the Solar System, accounting for 99.86% of the mass.
As stars go, the Sun is actually a medium-sized, and even smallish star. Stars with much more mass can be much larger than the Sun. For example, the red giant Betelgeuse, in the constellation of Orion is thought to be 1,000 times larger than the Sun. And the largest known star is VY Canis Majoris, measuring approximately 2,000 times larger than the Sun. If you could put VY Canis Majoris into our Solar System, it would stretch out past the orbit of Saturn.
The size of the Sun is changing. In the future when it runs out of usable hydrogen fuel in the core, it will become a red giant as well. It will engulf the orbits of Mercury and Venus, and possibly even the orbit of the Earth. For a few million years, the Sun will be about 200 times bigger than its current size.
After the Sun becomes a red giant, it will shrink down to become a white dwarf star. Then the size of the Sun will only be roughly the size of the Earth.
Mass of the Sun
The mass of the Sun is 1.98892 x 1030 kilograms. That’s a really big number, and it’s really hard to put it into context, so let’s write out the mass of the Sun, with all the zeros.
Still need to wrap your head around this? Let’s give you some comparisons. The mass of the Sun is 333,000 times the mass of the Earth. It’s 1,048 times the mass of Jupiter, and 3,498 times the mass of Saturn.
In fact, the Sun accounts for 99.8% of all the mass in the entire Solar System; and most of that non-Sun mass is Jupiter and Saturn. To say that the Earth is an insignificant speck is an understatement.
When astronomers try to gauge the mass of another star-like object, they use the mass of the Sun for comparison. This is known as a “solar mass”. So the mass of objects, like black holes, will be measured in solar masses. A massive star might have 5-10 solar masses. A supermassive black hole could have hundreds of millions of solar masses.
Astronomers will refer to this with an M beside a symbol that looks like a circle with a dot in the middle – M⊙. To show a star that has 5 times the mass of the Sun, or 5 solar masses, it would be 5 M⊙. Eta Carinae, one of the most massive stars known. Image credit: NASA
The Sun is massive, but it’s not the most massive star out there. In fact, the most massive star we know of is Eta Carinae, which has a mass of 150 times the mass of the Sun.
The Sun’s mass is actually slowly decreasing over time. There are two processes at work here. The first is the fusion reactions in the core of the Sun, converting atoms of hydrogen into helium. Some of the Sun’s mass is lost through the fusion process, as atoms of hydrogen are converted into energy. The warmth we feel from the Sun, is the Sun’s lost mass. The second way is the solar wind, which is constantly blowing protons and electrons into outer space.
Mass of the Sun in kilograms: 1.98892 x 1030 kg
Mass of the Sun in pounds: 4.38481 x 1030 pounds
Mass of the Sun in tons: 2.1924 x 1027 tons
Diameter of the Sun
The diameter of the Sun is 1.391 million kilometers or 870,000 miles.
Again, let’s put this number into perspective. The diameter of the Sun is 109 times the diameter of the Earth. It’s 9.7 times the diameter of Jupiter. Really, really big.
Pardon the pun, but the Sun doesn’t hold a candle to some of the largest stars in the Universe. The biggest star we know of is called VY Canis Majoris, and astronomers think it could be 2,100 times the diameter of the Sun. The Earth compared to Sunspot 1312 on 10-10-11. Credit: Ron Cottrell.
Diameter of the Sun in kilometers: 1,391,000 km
Diameter of the Sun in miles: 864,000 miles
Diameter of the Sun in meters: 1,391,000,000 meters
Diameter of the Sun compared to Earth: 109 Earths
Radius of the Sun
The radius of the Sun, the measurement from the exact center of the Sun out to its surface, is 695,500 kilometers.
This radius is essentially the same however you measure it, from the center to the equator, or the from the center to the Sun’s poles. But you need to be careful with other objects, however, because the speed of their rotation affects the radius.
The Sun takes about 25 days to turn once on its axis. Because it rotates relatively slowly, the Sun doesn’t flatten out at all. The distance from the center to the poles is almost exactly the same as the distance from the center to the equator.
There are stars out there which are dramatically different, though. For example, the star Achernar, located in the constellation Eridanus, is flattened by 50%. In other words, the distance from the poles is half the distance across the equator. In this situation, the star actually looks like spinning-top toy.
So, relative to out stars out there, the Sun is almost a perfect sphere.
Astronomers use the Sun’s radius, or “solar radius” to compare the sizes of stars and other celestial bodies. For example, a star with 2 solar radii is twice as large as the Sun. A star with 10 solar radii is 10x as large as the Sun, and so on. VY Canis Majoris. The biggest known star.
Polaris, the North Star, is the brightest star in the constellation Ursa Minor (Little Dipper) and, because of its proximity to the north celestial pole, is considered the current northern pole star. Polaris is primarily used for navigation and has a solar radius of 30. That means, it is 30 times bigger than the Sun.
Sirius which is the brightest star in the night sky. In terms of apparent magnitude, the second brightest star, Canopus, has only half that of Sirius’. No wonder it really stands out. Sirius is actually a binary star system, with Sirius A having a solar radius of 1.711 and B, which is much smaller, at about 0.0084.
Radius of the Sun in kilometers: 695,500 km
Radius of the Sun in miles: 432,200 miles
Radius of the Sun in meters: 695,500,000 meters
Radius of the Sun compared to Earth: 109 Earths
Gravity of the Sun
The Sun has an enormous amount of mass, and so it has a lot of gravity. In fact, the mass of the Sun is 333,000 times more than the mass of the Earth. Forget that the surface temperature of the Sun is 5,800 Kelvin and made of hydrogen – what would you feel if you could walk on the surface of the Sun? Think about this, the gravity of the Sun at the surface is 28 times the gravity of the Earth.
In other words, if your scale says 100 kg on Earth, it would measure 2,800 kg if you tried to walk on the surface of the Sun. Needless to say, you would die pretty quickly just from the pull of gravity, not to mention the heat, etc.
The Sun’s gravity pulls all of its mass (mostly hydrogen and helium) into an almost perfect sphere. Down at the core of the Sun, the temperatures and pressures are so high that fusion reactions are possible. The tremendous amount of light and energy pouring out of the Sun counteracts the pull of gravity trying to collapse it down. The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA
Astronomers define the Solar System as the distance under the influence of gravity from the Sun. We know that the Sun holds distant Pluto in orbit (5.9 billion km away on average). But astronomers think that the Oort Cloud extends out to a distance of 50,000 astronomical units (1 AU is the distance from the Earth to the Sun), or 1 light-year. In fact, the influence of the Sun’s gravity could extend out to 2 light-years away, the point at which the pull from other stars is stronger.
Surface gravity of the Sun: 27.94 g
Density of the Sun
The density of the Sun is 1.4 grams per cubic centimeter. Just to give you a comparison, the density of water is 1 g/cm3. In other words, if you could find a pool large enough, the Sun would sink down and not float. And this seems kind of counter-intuitive. Isn’t the Sun made of hydrogen and helium, the two lightest elements in the Universe? So how can the density of the Sun be so high?
Well, it all comes down to gravity. But first, let’s calculate the density of the Sun for ourselves.
Formula for density is to divide mass by volume. The mass of the Sun is 2 x 1033 grams, and the volume is 1.41 x 1033 cm3. And so, if you do the math, the density of the Sun works out to be 1.4 g/cm3. Interior of the Sun. Image credit: NASA
The Sun holds itself together with gravity. Although the outermost layers of the Sun might be less dense, the intense gravity crushes the inner regions to enormous pressures. At the core of the Sun, the pressure is more than 1 million metric tons/cm sq – that’s equivalent to more than 10 billion times the atmosphere of the Earth. And once you get those kinds of pressures, fusion can ignite.
Density of the Sun: 1.622 x 105 kg/m3
Volume of the Sun
The volume of the Sun is 1.412 x 1018 km3. That’s a lot of cubic kilometers. Do you need something to compare this with? The volume of the Sun is so great that it would take 1.3 million planets the size of the Earth to fill it up. Or you could fill it with almost 1000 planets the size of Jupiter.
Volume of the Sun in cubic kilometers: 1.412 x 1018 km3
Volume of the Sun compared to Earth: 1,300,000
Circumference of the Sun
The circumference of the Sun is 4,379,000 km.
Just for comparison, the equatorial circumference of the Earth is 40,075 km. So, the circumference of the Sun is 109 times larger than the circumference of the Earth. And the circumference of the Sun is 9.7 times bigger than the circumference of Jupiter.
This article was originally written in 2008, but we created a cool video to go along with it yesterday
Let’s find out why Pluto is no longer considered a planet.
Pluto was first discovered in 1930 by Clyde W. Tombaugh at the Lowell Observatory in Flagstaff Arizona. Astronomers had long predicted that there would be a ninth planet in the Solar System, which they called Planet X. Only 22 at the time, Tombaugh was given the laborious task of comparing photographic plates. These were two images of a region of the sky, taken two weeks apart. Any moving object, like an asteroid, comet or planet, would appear to jump from one photograph to the next.
After a year of observations, Tombaugh finally discovered an object in the right orbit, and declared that he had discovered Planet X. Because they had discovered it, the Lowell team were allowed to name it. They settled on Pluto, a name suggested by an 11-year old school girl in Oxford, England (no, it wasn’t named after the Disney character, but the Roman god of the underworld).
The Solar System now had 9 planets.
Astronomers weren’t sure about Pluto’s mass until the discovery of its largest Moon, Charon, in 1978. And by knowing its mass (0.0021 Earths), they could more accurately gauge its size. The most accurate measurement currently gives the size of Pluto at 2,400 km (1,500 miles) across. Although this is small, Mercury is only 4,880 km (3,032 miles) across. Pluto is tiny, but it was considered larger than anything else past the orbit of Neptune.
Over the last few decades, powerful new ground and space-based observatories have completely changed previous understanding of the outer Solar System. Instead of being the only planet in its region, like the rest of the Solar System, Pluto and its moons are now known to be just a large example of a collection of objects called the Kuiper Belt. This region extends from the orbit of Neptune out to 55 astronomical units (55 times the distance of the Earth to the Sun).
Astronomers estimate that there are at least 70,000 icy objects, with the same composition as Pluto, that measure 100 km across or more in the Kuiper Belt. And according to the new rules, Pluto is not a planet. It’s just another Kuiper Belt object.
Here’s the problem. Astronomers had been turning up larger and larger objects in the Kuiper Belt. 2005 FY9, discovered by Caltech astronomer Mike Brown and his team is only a little smaller than Pluto. And there are several other Kuiper Belt objects in that same classification.
Astronomers realized that it was only a matter of time before an object larger than Pluto was discovered in the Kuiper Belt.
And in 2005, Mike Brown and his team dropped the bombshell. They had discovered an object, further out than the orbit of Pluto that was probably the same size, or even larger. Officially named 2003 UB313, the object was later designated as Eris. Since its discovery, astronomers have determined that Eris’ size is approximately 2,600 km (1,600 miles) across. It also has approximately 25% more mass than Pluto.
With Eris being larger, made of the same ice/rock mixture, and more massive than Pluto, the concept that we have nine planets in the Solar System began to fall apart. What is Eris, planet or Kuiper Belt Object; what is Pluto, for that matter? Astronomers decided they would make a final decision about the definition of a planet at the XXVIth General Assembly of the International Astronomical Union, which was held from August 14 to August 25, 2006 in Prague, Czech Republic.
Astronomers from the association were given the opportunity to vote on the definition of planets. One version of the definition would have actually boosted the number of planets to 12; Pluto was still a planet, and so were Eris and even Ceres, which had been thought of as the largest asteroid. A different proposal kept the total at 9, defining the planets as just the familiar ones we know without any scientific rationale, and a third would drop the number of planets down to 8, and Pluto would be out of the planet club. But, then… what is Pluto?
In the end, astronomers voted for the controversial decision of demoting Pluto (and Eris) down to the newly created classification of “dwarf planet”.
Is Pluto a planet? Does it qualify? For an object to be a planet, it needs to meet these three requirements defined by the IAU:
It needs to be in orbit around the Sun – Yes, so maybe Pluto is a planet.
It needs to have enough gravity to pull itself into a spherical shape – Pluto…check
It needs to have “cleared the neighborhood” of its orbit – Uh oh. Here’s the rule breaker. According to this, Pluto is not a planet.
What does “cleared its neighborhood” mean? As planets form, they become the dominant gravitational body in their orbit in the Solar System. As they interact with other, smaller objects, they either consume them, or sling them away with their gravity. Pluto is only 0.07 times the mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit.
Any object that doesn’t meet this 3rd criteria is considered a dwarf planet. And so, Pluto is a dwarf planet. There are still many objects with similar size and mass to Pluto jostling around in its orbit. And until Pluto crashes into many of them and gains mass, it will remain a dwarf planet. Eris suffers from the same problem.
It’s not impossible to imagine a future, though, where astronomers discover a large enough object in the distant Solar System that could qualify for planethood status. Then our Solar System would have 9 planets again.
Even though Pluto is a dwarf planet, and no longer officially a planet, it’ll still be a fascinating target for study. And that’s why NASA has sent their New Horizons spacecraft off to visit it. New Horizons will reach Pluto in July 2015, and capture the first close-up images of the (dwarf) planet’s surface.
Space enthusiasts will marvel at the beauty and remoteness of Pluto, and the painful deplaneting memories will fade. We’ll just be able to appreciate it as Pluto, and not worry how to categorize it. At least now you know why Pluto was demoted.
Before you consider buying expensive equipment for viewing the wonders of the night sky, binoculars are one piece of equipment every amateur astronomer should have.
Many beginners to astronomy (especially around the holiday period) are sometimes dead-set on getting a telescope, but many aren’t aware that a good pair of binoculars can outperform many entry level telescopes for a similar cost, or much less.
Binoculars are simplicity in themselves — maintenance free, instantly available for use and very versatile, as they can be used for daytime, or “terrestrial viewing” just as well. It is difficult to say the same for with most telescopes.
Go into any photographic store, or website that sells binoculars and you will be met with literally hundreds of different makes, types and sizes – confusing for the beginner, but with a few pointers it can be easy to choose.
Credit: astronomybinoculars.com
So how do you choose a pair of binoculars that will give good results with astronomy?
When choosing binoculars for astronomy, the only variables you need to think about are size of the optics and weight.
Too small and they won’t be powerful enough or let enough light in; too big and heavy means they are almost impossible to use without a support or tripod. Beginners need to find a pair of binoculars which are just right.
The key is to get as much light into the binoculars as possible without making them too heavy. This will give sharp views and comfort when used.
Size and weight come hand in hand, the more light gathered, the heavier the binoculars will be.
All binoculars are measured or rated by two numbers, for example: 10 X 25 or 15 X 70. The first number is the magnification and the second number is the “objective diameter” which is the diameter of the objective lens and this determines how much light can be gathered to form an image.
Credit: Halfblue Wikipedia
The second number or objective diameter is the most important one to consider when buying binoculars for astronomy, as you need to gather as much light as possible.
As a rule of thumb, binoculars with an objective diameter of 50mm or more are more suited to astronomy than smaller “terrestrial” binoculars. In many cases a larger objective also gives better eye relief (larger exit pupil) making the binoculars much more comfortable to use.
For the beginner or general user, don’t go too big with the objective diameter as you are also making the binoculars physically larger and heavier. Large binoculars are fantastic, but — again — almost impossible to keep steady without a support or tripod.
Celestron Skymaster 15 X 70 Binoculars
Good sizes of binoculars for astronomy start at around or just under 10 X 50 and can go up to 20 X 80, but any larger and they will need to be supported when using them. Some very good supported binoculars have objective diameters of more than 100mm. Theses are fantastic, but not as portable as their smaller counterparts.
Binoculars are one of the most important items a new or seasoned astronomer can buy. They are inexpensive, easy to choose, use and will last a very long time.
A daytime Moon over New Zealand in August 2010. Credit: NASA/Phil Davis
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Did you know that you can see the Moon during the day?
Many people only notice our Moon at night, when there is considerably more contrast between the Moon and the night sky. Being the second brightest object in the sky (after the Sun, of course) and with Venus visible during the day to trained eyes, it’s no real surprise that the Moon is visible during the day.
Why then, do so many people act surprised when they notice the Moon during the day? What makes it possible for the Moon to be visible during the day?
Understanding how and when you can spot the Moon is a matter of knowing the different lunar phases, specifically the relationship between the Sun, Earth and the Moon during each phase. The image below shows the simple geometry responsible for each of the Moon’s distinct phases.
In the diagram it’s pretty easy to see that when Earth is between the Sun and the Moon, we see a full moon. When the Moon is between Earth and the Sun, we see a new Moon. The other phases are simply transitions from new to full and from full back to new.
Schedule of Moon Phases. Times shown indicate when the moon is overhead.
Based on the orbital geometry of the Moon, there will certainly be times where the Sun will partially illuminate the Moon, during the day and at night. What makes the lunar cycle even more interesting is that the moon rises about an hour later each day, and yet invariably, a full moon rises near dusk and sets near sunrise. The reverse is true in that a new moon rises near sunrise and sets near dusk.
Looking at the above diagram though, a question comes to mind…
Why don’t we have a lunar eclipse during each full moon, or a solar eclipse each new moon?
I’ll explain the conditions needed for a solar or lunar eclipse in an upcoming article.
In the meantime enjoy the transition from waning gibbous to waning crescent over the next week and get your telescopes out during the weekend of the 25th. The Moon will almost be at its new phase.
If you’d like to learn more about moon phases and when the moon will be visible in your area, the US Naval observatory has a great calculator at: http://aa.usno.navy.mil/data/docs/RS_OneYear.php
Launch of Atlantis's final mission, STS-135. Credit: NASA. 3-D by Nathanial Burton-Bradford.
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When it comes to measuring motion, that is the relative passage of an object through space at a certain rate of time, several different things need to be taken into account. For example, it is not enough to know the rate of change (i.e. the speed) of the object. Scientists must also be able to assign a vector quantity; or in other words, to know the direction as well as the rate of change of that object. In the end, this is major difference between Speed and Velocity. Though both are calculated using the same units (km/h, m/s, mph, etc.), the two are different in that one is described using numerical values alone (i.e. a scalar quantity) whereas the other describes both magnitude and direction (a vector quantity).
By definition, the speed of an object is the magnitude of its velocity, or the rate of change of its position. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval. Represented mathematically, it looks like this: ν=[v]=[?] = [dr/dt]•, where speed ν is defined as the magnitude of the velocity v, that is the derivative of the position r with respect to time. The fastest possible speed at which energy or information can travel, according to special relativity, is the speed of light in vacuum (a.k.a. c = 299,792,458 meters per second, which is approximately 1079 million kilometers per hour or 671,000,000 mph).
Velocity, on the other hand, is the measurement of the rate and direction of change in the position of an object. Since it is a vector physical quantity, both magnitude and direction are required to define it. The scalar absolute value (magnitude) of velocity is speed, a quantity that is measured in metres per second (m/s) when using the SI (metric) system. Mathematically, this is represented as: v = Δx/Δt, where v is the average velocity of an object, (Δx) is the displacement and (Δt) is the time interval. Add to this a vector (i.e. Δx/Δt→, ←, or what have you), and you’ve got velocity!
As an example, consider the case of a bullet being fired from a gun. If we divide the overall distance it travels within a set period of time (say, one minute), than we have successfully calculated its speed. On the other hand, if we want to determine its velocity, we must consider the direction of the bullet after it’s been fired. Whereas the average speed of the object would be rendered as simple meters per second, the velocity would be meters per second east, north, or at a specific angle.
We have written many articles about speed and velocity for Universe Today. Here’s an article about formula for velocity, and here’s an article about escape velocity.
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Look up into the rainy sky! What do you see? Well, if its just rained and the sun is once again shining, chances are you see a rainbow. Always a lovely sight isn’t it? But why is it that after a rainstorm, the air seems to catch the light in just the right way to produce this magnificent natural phenomenon? Much like stars, galaxies, and the flight of a bumblebee, some complicated physics underlie this beautiful act of nature. For starters, this effect, where light is broken into the visible spectrum of colors, is known as the Dispersion of Light. Another name for it is the prismatic effect, since the effect is the same as if one looked at light through a prism.
To put it simply, light is transmitted on several different frequencies or wavelengths. What we know as “color” is in reality the visible wavelengths of light, all of which travel at different speeds through different media. In other words, light moves at different speed through the vacuum of space than it does through air, water, glass or crystal. And when it comes into contact with a different medium, the different color wavelengths are refracted at different angles. Those frequencies which travel faster are refracted at a lower angle while those that travel slower are refracted at a sharper angle. In other words, they are dispersed based on their frequency and wavelength, as well as the materials Index of Refraction (i.e. how sharply it refracts light).
The overall effect of this – different frequencies of light being refracted at different angles as they pass through a medium – is that they appear as a spectrum of color to the naked eye. In the case of the rainbow, this occurs as a result of light passing through air that is saturated with water. Sunlight is often referred to as “white light” since it is a combination of all the visible colors. However, when the light strikes the water molecules, which have a stronger index of refraction than air, it disperses into the visible spectrum, thus creating the illusion of a colored arc in the sky.
Now consider a window pane and a prism. When light passes through glass that has parallel sides, the light will return in the same direction that it entered the material. But if the material is shaped like a prism, the angles for each color will be exaggerated, and the colors will be displayed as a spectrum of light. Red, since it has the longest wavelength (700 nanometers) appears at the top of the spectrum, being refracted the least. It is followed shortly thereafter by Orange, Yellow, Green, Blue, Indigo and Violet (or ROY G. GIV, as some like to say). These colors, it should be noted, do not appear as perfectly distinct, but blend at the edges. It is only through ongoing experimentation and measurement that scientists were able to determine the distinct colors and their particular frequencies/wavelengths.
We have written many articles about dispersion of light for Universe Today. Here’s an article about the refractor telescope, and here’s an article about visible light.
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For some time, the behavior of light has baffled scientists. Initially, and in accordance with classic physics, light was thought to be a wave, an indefinable form of energy that simply flowed from a heated source. However, with the advent of quantum physics, scientists came to realize that photons, a tiny elementary particle responsible for all forms of electromagnetic radiation, was in fact the source. So you can imagine how confounded they were when, in the course of performing experiments, they discovered that it exhibited the behavior of both a particle and a wave! This rather unique behavior, the ability of light to behave as a wave, even though it is made up of tiny particles, is known as the Diffraction of Light.
By definition, diffraction refers to the apparent bending of waves around small obstacles and the spreading out of waves past small openings. It had long been understood that this is what happens when a wave encounters an obstacle, and by the 17th and 18th centuries, this behavior was observed through experiments involving light. One such physicist who observed this at work was Thomas Young (1773 – 1829), an English polymath who is credited devised the double-slit experiment. In this experiment, Young shone a monochromatic light source (i.e. light of a single color) through an aperture (in this case, a wall with a horizontal slits cut in it) and measured the results on a screen located on the other side. The results were interesting, to say the least. Instead of appearing in the same relative shape as the aperture, the light appeared to be diffracting, implying that it was made up of waves. The experiment was even more interesting when a second slit was cut into the screen (hence the name double-slit). Young, and those who repeated the experiment, found that interference waves resulted, meaning that two propagation waves occurred which then began to interfere with one another.
A more common example comes to us in the form of shadows. Ever notice how the outer edges do not appear solid, but slightly fuzzy instead? This occurs as a result of light bending slightly as it passes around the edge of an object, again, consistent with the behavior of a wave. Similar effects occur when light waves travel through a medium with a varying refractive index, resulting in a spectrum of color or a distorted image. Since all physical objects have wave-like properties at the atomic level, diffraction can be studied in accordance with the principles of quantum mechanics.
We have written many articles about diffraction of light for Universe Today. Here’s an article about visible light, and here’s an article about telescope resolution.
If you’d like more info on diffraction of light, check out these articles:
The Physics of Light: Diffraction Experiments on Diffraction of Light
This view of Earth comes from NASA's Moderate Resolution Imaging Spectroradiometer aboard the Terra satellite
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The planet Earth has three motions: it rotates about its axis, which gives us day and night; it revolves around the sun, giving us the seasons of the year, and through the Milky Way along with the rest of the Solar System. In each case, scientists have striven to calculate not only the time it takes, but the relative velocities involved. When it comes to the Earth rotating on its axis, a process which takes 23 hours, 56 minutes and 4.09 seconds, the process is known as a sidereal day, and the speed at which it moves is known as the Earth’s Angular Velocity. This applies equally to the Earth rotating around the axis of the Sun and the center of the Milky Way Galaxy.
In physics, the angular velocity is a vector quantity which specifies the angular speed of an object and the axis about which the object is rotating. The SI unit of angular velocity is radians per second, although it may be measured in other units such as degrees per second, revolutions per second, etc. and is usually represented by the symbol omega (ω, rarely Ω). A radian, by definition, is a unit which connects the radius of an arc, the length of the arc and the angle subtended by the arc. A full radian is 360 degrees, hence we know that the Earth performs two radians when performing a full rotation around an axis. However, it is sometimes also called the rotational velocity and its magnitude – the rotational speed – is typically measured in cycles or rotations per unit time (e.g. revolutions per minute). In addition, when an object rotating about an axis, every point on the object has the same angular velocity.
Mathematically, the average angular velocity of an object can be represented by the following equation: ωaverage= Δθ/Δt, where ω is the radians/revolutions per second (on average), Δ is the change in quantity, θ is the velocity, and t is time. When calculating the angular velocity of the Earth as it completes a full rotation on its own axis (a solar day), this equation is represented as: ωavg = 2πrad/1day (86400 seconds), which works out to a moderate angular velocity of 7.2921159 × 10-5 radians/second. In the case of a Solar Year, where ωavg = 2πrad/1year (3.2×107 seconds), we see that the angular velocity works out to 2.0×10-7 rad/s.
We have written many articles about the angular velocity of Earth for Universe Today. Here’s an article about angular velocity, and here’s an article about why the Earth rotates.
