General Information

Uranus is the seventh planet of the Solar System. It has a diameter of about 52400 km, a mass 14.6 times that of the Earth and orbits the Sun every 84 years.  Its distance from the Sun varies between 18.3 and 20.1 astronomical units.  In January 1986, four and a half years after visiting Saturn, Voyager 2 completed the first close-up survey of the Uranian system.  The brief flyby revealed more information about Uranus and its retinue of icy moons than had been gleaned from ground observations since the planet's discovery over two centuries ago by the English astronomer William Herschel. Uranus, third largest of the planets, is an oddball of the solar system. Unlike the other planets (with the exception of Pluto), this giant lies tipped on its side with its north and south poles alternately facing the sun during an 84-year swing around the solar system. During Voyager 2's flyby, the south pole faced the Sun. Uranus might have been knocked over when an Earth sized object collided with it early in the life of the solar system.

From the Earth Uranus can be seen with a telescope. But even with large telescopes it can only be seen as a small greenish disk with no surface details visible.  Our knowledge of the outer planets in the solar system has been revolutionised by the amazing results from the close fly-bys of each by Voyager 2 which was at its closest to Uranus on January 24 1986. The most striking observation was that the planet Uranus shows almost no features! It appears as a large green-blue ball from Earth and from the Voyager pictures.  Astronomers already knew that Uranus rotated about an axis that is tilted at 98 to the pole of its orbital plane. This means that Uranus is `lying on its side' and rotating in the opposite sense to that of the other planets.  The effect on the surface is that the seasons on Uranus are linked to its orbital period about the Sun, so near the poles winter will last 42 earth-years!

Uranus was the first planet to be discovered that was not known to the ancients. William Herschel announced its discovery in 1781 although initially he thought that it was a comet.  After he had realised that it was a planet he wanted to call it Georgium Sidus, after his patron King George III.  It was not until 1850 when John Couch Adams suggested a change of name that Uranus was universally adopted.  Herschel was not the first astronomer to record Uranus but he was the first to recognise that it was not a star.  The earliest records of Uranus are by Flamsteed in 1690 (he called it a star, 34 Tauri), 1712 and four times in 1715. There are at least 15 other known sightings by three other astronomers before Herschel's discovery.

Nearly 3 billion kilometres (1.8 billion miles) from Earth, Uranus is so far away that scientists knew comparatively little about it before Voyager 2 undertook its historic first-ever encounter with the planet. Indeed, since its discovery by William Herschel in 1781, Uranus had remained largely a mystery throughout the ensuing two centuries. Five moons — the first discovered in 1787, the last in 1948 — were visible only as tiny points of light. A system of nine narrow rings went undetected until 1977. The planet's rate of rotation could be estimated only roughly and was believed to be anywhere from 16 to 24 hours. Before Voyager, there were indirect indications of a magnetic field at Uranus, although the evidence was not conclusive.