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 |
| Mass (Earth=1) | 14.5 |
| Gravity (Earth=1) | 0.89 |
| 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.
BIOGRAPHY: WILLIAM HERSCHEL German, 1738-1822
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.
