A COULD GAS GIANT, Uranus is the third largest planet in the Solar System. Its most unusual characteristic is that its axis of rotation lies almost in the plane of its orbit, and so it seems to orbit the Sun on its side. Although it can be seen with the naked eye when at its brightest, Uranus was not discovered until 1781, when it was found by a British astronomer, William Herschel.
ATOMSPHERE AND CLIMATE
Uranus is covered in methane clouds that absorb red light, giving the planet a greenish appearance. Its disk is nearly featureless, but some bright clouds were seen by the Voyager 2 space Telescope. The planet’s extreme axial tilt means that in the course of each orbit the Sun appears overhead at the equator and both poles; each pole experiences 42 years of daylight followed by 42 years of darkness. The interior is probably significantly different from those of Jupiter and Saturn, consisting mostly of water, methane, ammonia, and rock, rather than liquid hydrogen.
RINGS AND MOONS
Uranus has 11 thin rings and 21 known moons (six only recently identified and whose orbits are not yet accurately known). The rings are too faint to be seen with the size of telescope used by amateur observers. The outermost ring, called the Epsilon ring, is 60 miles (100 km) wide and has “shepherd” moons, Cordelier and Ophelia, one on each side. The moons, too, faint; even the biggest and brightest, Tatiana, is one of 14th magnitude and hence cannot be seen without a large telescope.
Under good conditions, and when its position is known, Uranus can be located with the naked eye, resembling a star of 6th magnitude. It is easy to find with binoculars, even from urban areas. Its identity can be confirmed by watching its movements from night to night. At oppositions, which occur a year and four days apart, a magnification of 500 times will show Uranus about the same size as the full moon.