URANUS
Uranus (pronounced "yer-AYN-us", or "YOOR-e-nus" to avoid embaressment) is the seventh planet from the Sun. It is a gas giant, the third largest by diameter and fourth largest by mass. It was named after the Greek god Ouranos. Uranus is frequently known as "the sideways planet" because its rings are horizontal instead of vertical. Many astronomers are still unsure as to how this happened.
The Facts
Mass (kg): 86.832x1024
Equatorial Radius (km): 25,559
Mean Density (kg/m3): 1250
Length of Day (hours): 17.24
Period of Revolution about Sun (days): 30,685.4
Acceleration due to Gravity (m/s2): 8.69
Mean Orbital Velocity (km/s): 6.81
Inclination of Axis (degrees): 97.77
Mean Distance from the Sun (AU): 19.19
The Discovery of Uranus
William Herschel discovered Uranus on March 13, 1781. He called it Georgium Sidus, meaning 'star of George', after George III, his patron. After that it was known as Herschel in his honour, before Johann Elert Bode suggested the name Uranus, after the Greek god of the heavens, husband of Gaea and father of the Titans.
The Planet
Uranus seems to have been pushed over on to its side. Its axial tilt is 97.77°, which means that it rotates on its side. This means that at the poles, days last for 42 Earth years, half of the Uranian year.
Uranus is a blue-green colour, because of methane gas in the mainly hydrogen and helium atmosphere. Uranus looks very bland, but actually has fast-moving clouds in its upper atmosphere, which are very faint. It's atmosphere is subject to seasonal variations; different things happen as the Sun is in different parts of the Uranian sky. The discovery of 'weather' on Uranus is very recent, made by pictures from the Hubble Space telescope; the images from Voyager 2 made Uranus look very calm and peaceful, with no real weather features at all.
Uranus, like Neptune, is mainly made up of rocks and ice beneath its atmosphere, but unlike Jupiter and Saturn, it doesn't have large amounts of liquid hydrogen, which makes it much less massive than those two planets. Uranus probably doesn't have a rocky core - the ice and rock are probably quite well mixed.
Around and About
Uranus has an interesting ring system and a vast number of satellites. Uranus is very cool, both literally and figuratively.
The Rings
James L Eliot, an American astronomer, first observed Uranus's rings in 1977. He was observing the planet passing before a star - an event called an occultation - when he saw them. They are named Alpha, Beta, Gamma, Delta and Epsilon, starting at the inner ring, and Voyager 2 found a further four rings. The rings are quite dark, like those of Jupiter, with the brightest being the Epsilon ring.
The Moons
Uranus may have up to 21 satellites. The five biggest are Miranda, Ariel, Umbriel and Titania, and there are twelve minor satellites that have been given names. There is also S/1986 U10, which is awaiting certification, and potentially three more satellites observed in 1999. Titania has the largest radius at 788.9km, but the sizes of the satellites range down to 13km for Cordelia, one of the ten satellites discovered in the Voyager 2 flyby of 1986.
At one point, astronomers 'lost' two of Uranus's satellites, Cordelia and Ophelia. They shepherd the Epsilon ring, and were discovered by Voyager 2. They are both very small, and were being tracked prior to observations by the Hubble Space telescope, when they were lost, although happily they have been found again now. If you ever loose a satellite, please report it to lost and found.
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