English astronomer William Herschel discovered Uranus in 1781. It was the first planet discovered that is not easily visible with the naked eye. It lies twice as far from the Sun as Saturn. Uranus is unusual because it spins on an axis tilted at 98° and so appears to spin on its side. This may be because Uranus collided with another large object as it was forming.
Table 16. ESSENTIAL DATA
|Diameter at equator||51,118 km (31,764 miles)|
|Average distance from Sun||2,871 million km (1,784 million miles)|
|Orbital period||84 years|
|Rotation period||17.24 hours|
|Cloud-top temperature||-197°C (-323°F)|
|Number of moons||22|
The rings of Uranus were discovered in 1977. As the planet passed in front of a star the rings could be seen against the bright background. In 1986, Voyager 2 imaged the rings and 11 were identified.
The atmosphere of Uranus is a greenish-blue colour. It is almost completely featureless in ordinary light. There are no signs of the cloud bands visible on Jupiter and Saturn.
In all, Uranus has more than 20 moons, but only five are of substantial size. Of these, Titania with a diameter of 1,578 km (981 miles) is the largest and Miranda with a diameter of 470 km (290 miles) the smallest. The smaller moons include asteroids captured by the planet’s gravity.
Herschel moved to England in 1757 where he worked as a musician, but also began to build superb reflecting telescopes. In 1781, he discovered an object he first thought was a comet, but was a new planet, Uranus. He later built the largest telescope in the world at that time, and discovered hundreds of nebulas.