[Illustration: Fig. 301.] 276. _Discovery of Uranus._--This planet was discovered by Sir William Herschel in March, 1781. He was engaged at the time in examining the small stars of the constellation _Gemini_, or the Twins. He noticed that this object which had attracted his attention had an appreciable disk, and therefore could not be a star. He also perceived by its motion that it could not be a nebula; he therefore concluded that it was a comet, and announced his discovery as such. On attempting to compute its orbit, it was soon found that its motions could be accounted for only on the supposition that it was moving in a circular orbit at about twice the distance of Saturn from the sun. It was therefore recognized as a new planet, whose discovery nearly doubled the dimensions of the solar system as it was then known. 277. _The Name of the Planet._--Herschel, out of compliment to his patron, George III., proposed to call the new planet _Georgium Sidus_ (the Georgian Star); but this name found little favor. The name of _Herschel_ was proposed, and continued in use in England for a time, but did not meet with general approval. Various other names were suggested, and finally that of _Uranus_ was adopted. [Illustration: Fig. 302.] 278. _The Satellites of Uranus._--Uranus is accompanied by four satellites, whose orbits are shown in Fig. 302. These satellites are remarkable for the great inclination of their orbits to the plane of the planet's orbit, amounting to about eighty degrees, and for their _retrograde_ motion; that is, they move _from east to west_, instead of from west to east, as in the case of all the planets and of all the satellites previously discovered. Neptune. 279. _Orbit and Dimensions of Neptune._--So far as known, Neptune is the most remote member of the solar system, its mean distance from the sun being twenty-seven hundred and seventy-five million miles. This distance is considerably less than twice that of Uranus. Neptune revolves around the sun in a period of a little less than a hundred and sixty-five years. Its orbit has but slight eccentricity, and is inclined less than two degrees to the ecliptic. This planet is considerably larger than Uranus, its diameter being nearly thirty-five thousand miles. It is somewhat less dense than Uranus. Neptune is invisible to the naked eye, and no telescope has revealed any markings on its disk: hence nothing is certainly known as to its rotation. Fig. 303 shows the comparative size of Neptune and the earth. [Illustration: Fig. 303.] 280. _The Discovery of Neptune._--The discovery of Neptune was made in 1846, and is justly regarded as one of the grandest triumphs of astronomy.