URANUS, in astronomy, the seventh major planet in the order of distance from the sun, and denoted by the symbol 1g. It was discovered by Sir William Herschel on March 13, 1781. He saw it as a round nebulous disc, slowly moving among the stars, and at first supposed it to be a comet, and announced it as such to the Royal Society. A few weeks' observation, however, showed it to be moving in a nearly circular orbit at a distance from the sun about nineteen times that of the earth. Its planetary charac ter was thus established, and Herschel named it the Georgium Sidus in honour of his royal patron. "The Georgian" was used in the Nautical Almanac up to 1850; but it was disliked outside Eng land as was Herschel's name, proposed by Lalande. The name Uranus was proposed by Bode.
The mean distance of Uranus from the sun is 1,782,800,00o miles, and the time required for a revolution in its orbit just over 84 years. The diameter has been found to be about 30,900 miles, and, since observations of the satellites show its mass to be 14.6 times that of the earth, its density is very low viz., 0.25 that of the earth, and 1.36 times that of water. The spectrum of the planet contains a number of strong absorption bands of as yet unknown origin, similar to those in the spectra of Jupiter, Saturn and Neptune. They are much more intense than those of Jupiter and Saturn, but less strong than those of Neptune. It is evident that Uranus is in the- same physical state as the other planets of the outer group. Despite its distance from the sun Uranus, in consequence of its high albedo, shines as a star slightly brighter than the 6th magnitude. It is therefore, under favour able conditions, within the reach of naked eye vision. Indeed, long before its discovery by Herschel, Flamsteed had taken it for a star. Lemonnier during the opposition of 1768-69 observed it eight times, but did not detect its planetary character.
Its disc, which is of a pale sea-green colour, has a diameter of rather less than 4", and measurements by several observers have shown it to be definitely elliptical. A white streak has also been recorded crossing the centre and some faint dusky bands or belts have been noted, but it has not been possible to determine the period of its rotation in the usual way by observation of definite surface features. Spectroscopic observations, based on
the Doppler principle (see LIGHT), by the late Professor Lowell and Dr. Slipher in 1912, however, indicated a rotation period of oh 45m, and photometric observations by Leon Campbell at Harvard have revealed variations of brightness, which might well be due to an outbreak of spots combined with axial rotation, in excellent accord with this value.
Satellites of Uranus.—In Jan. 1787 Herschel detected two satellites of Uranus of which the inner one, now known as Titania, had a period of 9 days, the outer, Oberon, of 131 days.
He also on other occasions saw what he supposed to be two ad ditional satellites, but these objects could not have been of this character. In 1851-52 William Lassell at Malta, in conjunction with his assistant A. Marth, observed two satellites yet nearer the planet than those of Herschel. These are now known as Ariel and Umbriel. Their periodic times are about 21 and 4 days respectively. Lassell's telescopes, which were reflectors, were superior in light-power to others of his time, and these inner satellites were not seen by other astronomers for more than twenty years after their discovery. Indeed, doubts of their reality were only resolved in 1373, when they were observed with the Washington 26-in. telescope.
The most remarkable feature about these satellites is the very high inclination of their orbit planes. This amounts to 98° to the plane of the planet's orbit, and 97.8° to the ecliptic, so that the motion of the satellites is really retrograde. The result of the high inclination is that, as Uranus revolves in its orbit, there are two opposite points at which the planes are presented edgewise to view, and the satellites will be seen to travel nearly north and south in the telescopic field. At intermediate points their orbits will appear circular. The former appearance last occurred in 1924, and the latter will next be seen in 1945.
For the irregularity in the motion of Uranus which led to the discovery of Neptune, see NEPTUNE.