Uranus
Uranus, the third of the giant gas planets, is also the third largest
planet. Like Jupiter and Saturn it does not have a solid surface and is
covered by thick cloud. It was the first major planet to be discovered
- that is to say, it was not known to the ancient astronomers. In good
viewing conditions, Uranus is just barely visible to the naked eye. Probably
it had been mistaken for a star until 1781, when William Herschel reported
its discovery as a planet.
Uranus is so far away that it cannot be observed in any detail from Earth.
It was not until Voyager 2 flew past it in 1986 that we learned what it is
really like. We now know that Uranus is mainly made of hydrogen and helium,
with thick methane clouds surrounding it. The methane absorbs the red out of
the sunlight and makes Uranus look blue-green. There has been no evidence of an
internal heat source (unlike the other gaseous planets), and Uranus' distance
from the Sun makes it a very cold planet.
In 1977 the ring system of Uranus was discovered by an indirect technique
called "stellar occultation". In conjunction with NASA'S airborne observatory,
Perth Observatory astronomers detected the rings when Uranus passed in front
of a faint star. The star seemed to wink off and on as Uranus' rings interrupted
the beam of starlight that was being monitored by the observers' instruments.
No actual picture of the planet and the rings was obtained; that had to wait
until Voyager 2's flyby in 1986. The rings are very narrow and are at radii
of between 39,000 km and 51,000 km. The 17 known moons are outside of the
ring system.
Uranus is very different from the other planets in that it lies nearly on
its side; the planet's axis is tilted 98o from the orbital plane instead of
being nearly upright like the other planets (Earth is tilted at 23.5 degrees).
This means that if you could live near one of the poles of Uranus, the Sun would
be visible for 42 years, and then you would be in darkness for the next 42 years
of the Uranian orbit around the Sun.
| Diameter | 51,118 km |
| Distance from Earth | 2,721 million km |
| Distance from Sun | 2,870 million km |
| Rotation period | 17hr 14min |
| Period of orbit | 84 years |
| Number of moons | 21 (+ a ring system) |
