Uranus, the third-largest planet in our solar system, is bright enough to see with the unaided eye -- when the conditions are just right. Yet it is one of the great surprises of ancient astronomy that no one realized that it was a planet until 1781, when William Herschel "discovered" Uranus by plotting its position against the stars over the course of many nights. The planet was named after the father of Saturn in Roman mythology.

Like Jupiter and Saturn, Uranus is a gas-giant world. Its cloudy atmosphere, which is made mostly of hydrogen and helium, appears featureless.

Unlike the other planets in our solar system, Uranus spins sideways. It may be that, long ago, Uranus was hit by some large object that knocked the huge planet on its side.

When the Voyager 2 spacecraft flew past Uranus in 1986, it found an almost featureless blue-green disk. Methane high in the planet's atmosphere gives it that color, and thick clouds hide everything below them. Hubble Space Telescope (HST) has seen faint cloud bands blowing around the planet at high speed, but no other features are visible.

Uranus is encircled by very thin, dark rings, similar to those around Jupiter. HST has seen them through long-exposure photographs, but the only way to see them from Earth's surface is to watch as stars pass behind Uranus; as the star passes behind the rings, it fades in and out of view. Voyager 2 photographed the rings of Uranus and confirmed a ring system around Neptune, making it clear that all the gas giants have rings around them -- not just Saturn.

Day and night on Uranus would be very different from what we experience on Earth. At the north or south pole, "winter" would last for 42 years, and the Sun would not be visible at all during that time. The Sun becomes visible again at the "spring" equinox and remains in the sky for another 42 years.

You could not breathe on Uranus because its atmosphere is poisonous. And you couldn't stand on the surface, because there isn't one. Instead, the atmosphere simply gets thicker and thicker, until it changes from gas to liquid.

Uranus has a solid core that's several times as massive as Earth, but it lies far below a deep "ocean" of ammonia, methane, hydrogen, and helium.

Five large moons and at least a dozen smaller ones orbit Uranus (two new moons were discovered in late 1997, bringing the total to 17 -- second only to Saturn). Their orbital paths lie in Uranus' equatorial plane, but appear "sideways" compared to other planets and their moons because of Uranus' sideways spin.

The largest moon, Titania, measures 1,000 miles (1,610 km) in diameter.