Uranus should in theory be visible to the naked eye; at its brightest is of magnitude 5.5 But it is so insignificant that it was never noticed by ancient astronomers, for whom the Solar System stopped at Saturn. Uranus was not discovered until March 13, 1781, when William Herschel spotted it through his telescope during a systematic survey of the skies. Once its position is known, Uranus can easily be followed in binoculars as it moves against the background stars. There is a certain fascination in seeing the blue-green disk of this distant planet through a telescope, but even with large apertures Uranus displays no detail. Uranus is one of the four 'gas giants' of the outer Solar System, the others being Jupiter, Saturn, and Neptune. Uranus has an equatorial diameter of 51,100 km, less than half that of Saturn but four times larger than Earth. Seasons would pass slowly on Uranus, for it takes 84 years to complete one orbit, but the seasons would be extreme, for Uranus appears to have been knocked over onto its side: the planet's axial tilt is 98°, meaning that its axis of rotation lies almost in the plane of its orbit. Every 42 years, therefore, one of the poles of Uranus is pointing towards the Sun, while its opposite pole is in darkness for decades. In between times its equatorial region faces sunwards. During each 84-year orbit of Uranus the Sun can appear overhead at every latitude, something that never happens on any other planet. No one knows why Uranus should be lying on its side in this unique way; perhaps it suffered a collision with another large body long ago. Uranus is celebrated for another reason: it was the second planet discovered to have rings. In 1977, astronomers watched as Uranus passed in front of a star. Unexpectedly, they noticed that the star winked on and off a number of times before and after it was obscured by the disk of Uranus, from which they deduced that Uranus was encircled by nine faint rings. The existence of these rings was confirmed, and their number increased to eleven, by the Voyager 2 space probe which flew past the planet in January 1986. Since then the rings have been photographed from Earth at infrared wavelengths. Our Galaxy has two small companion galaxies called the Magellanic Clouds. To the naked eye they appear like detached portions of the The rings are narrow, mostly only a few kilometers wide, separated by gaps many times greater than their own width. They lie between 13,000 and 26,000 km above the cloud tops of Uranus, and are composed of dust and other debris from tiny moonlets that orbit among them. In addition to the rings, Uranus has five moons visible from Earth: Miranda, Ariel, Umbriel, Titania and Oberon, ranging in size from 500 to 1500 km in diameter. Eleven smaller moons were discovered by the Voyager 2 space probe and other, more distant moons have since been discovered from Earth, giving Uranus a larger family than any other planet. The moons, like the rings, move in nearly circular orbits around Uranus's crazily tilted equator, with the exception of the outer moons discovered from Earth, which have highly elliptical and inclined orbits, suggesting they have been captured since the planet's formation. Disappointingly, even to the close-up eye of Voyager, the gaseous surface of the planet itself was bland, with scarcely any cloud features. In structure, Uranus is believed to have a rocky core surrounded by a mantle of ice, topped by an atmosphere of hydrogen and helium mixed with a fair proportion of methane, which gives the planet its greenish color.