Uranus

Uranus

Uranus was discovered by William Herschel on March 13, 1781, in the course of a systematic survey of the stars with a comparatively small telescope. Some astronomers had seen the planet before, but had mistaken it for a faint, fixed star. Herschel at first mistook it for a distant comet, but further observations showed that it was a giant planet moving outside the orbit of Saturn and taking just over 84 years for one revolution. Its average distance from the sun is 1,783 million miles, or roughly twice the corresponding distance for Saturn. Its diameter, measured along its equator, is about 29,300 miles. There is a slight polar flattening and the period of rotation, about 10 hours 49 minutes, is comparatively short.

Uranus, like Jupiter and Saturn, is encased in clouds. The cloud layer has an estimated temperature of less than -185 degrees centigrade, and is very rich in methane. At this low temperature free hydrogen would be gaseous but the ammonia must be completely frozen. The overall physical nature of the planet is thought to be similar of Jupiter and Saturn.

Uranus

The five satellites of Uranus

Uranus has five satellites. Two of them, Titania and Oberon, the brightest and most distant from the planet, were discovered by William Herschel. Two more, Ariel and Umbriel, were detected by William Lassell in 1851. Miranda, the fifth and nearest satellite to Uranus, was discovered by the American astronomer Gerard Kuiper in 1948. The estimated diameters of the satellites range from 200 to 700 and their orbits are nearly circular.

A curious feature of the Uranian system is that the planes of the orbits of the satellites, and also the planes of the planet’s equator, are tilted almost at right angles to the plane of the planet’s orbit. As a result the satellites sometimes appear to move in straight lines (when their orbits are presented edgewise) and sometimes in nearly perfect circles (when their orbits are presented in plan). Actually, the planet rotates and its satellites revolve in the retrograde sense, or opposite to the direction of the earth’s rotation and revolution. This situation, unique in the solar system, suggests that Uranus and its satellites once conformed to the general rule but have been turned over at some time in the remote past.