On the 13th of March, 1781, between 10 and 11 P. M., as Sir William Herschel was sweeping the constellation Gemini with one of his great reflecting telescopes, one star among all that passed through the field of view attracted his attention. Removing the eyepiece and applying another with a higher magnifying power, he found that, unlike all the other stars, this one had a small disk and was not a mere point of light, as all the fixed stars seem to be.

A few nights' observation showed that the stranger was moving among the stars, so he thought it must be a comet; but a week's observation following showed that he had discovered a new member of the planetary system, far out beyond Saturn, which from time immemorial had been assumed to be the outermost planet of all. This, then, was the first real discovery of a planet, as the finding of the satellites of Jupiter had been the first of all astronomical discoveries. Herschel's discovery occasioned great excitement, and he named the new planet Georgium Sidus or the Georgian, after his King. The King created him a knight and gave him a pension, besides providing the means for building a huge telescope, 40 feet long, with which he subsequently made many other astronomical discoveries. The planet that Herschel discovered is now called Uranus. Uranus is an object not wholly impossible to see with the naked eye, if the sky background is clear and black, and one knows exactly where to look for it. Its brightness is about that of a sixth magnitude star or a little fainter. Its average distance from the sun is about 1,800 million miles and it takes eighty-four years to complete its journey round the sun, traveling only a little more than four miles a second. When we examine Uranus closely with a large telescope, we find a small disk slightly greenish in tint, very slightly flattened, and at times faint bands or belts are apparently seen. Uranus is about 30,000 miles in diameter, and is probably surrounded by a dense atmosphere. Its rotation time is 10 h. 50 m.

Uranus is attended by four moons or satellites, named Ariel, Umbriel, Titania, and Oberon, the last being the most remote from the planet. This system of satellites has a remarkable peculiarity: the plane of the orbits in which they travel round Uranus is inclined about 80 degrees to the plane of the ecliptic, so that the satellites travel backward, or in a retrograde direction; or we might regard their motion as forward, or direct, if we considered the planes of their orbits inclined at 100 degrees.

For many years after the discovery of Uranus it was thought that all the great bodies of the solar system had surely been found. Least of all was any planet suspected beyond Uranus until the mathematical tables of the motion of Uranus, although built up and revised with the greatest care and thoroughness, began to show that some outside influence was disturbing it in accordance with Newton's law of gravitation. The attraction of a still more distant planet would account for the disturbance, and since no such planet was visible anywhere a mathematical search for it was begun.

NEPTUNE

Wholly independently of each other, two young astronomers, Adams of England and Le Verrier of France, undertook to solve the unique problem of finding out the position in the sky where a planet might be found that would exactly account for the irregular motion of Uranus. Both reached practically identical results. Adams was first in point of time, and his announcement led to the earliest observation, without recognition of the new planet (July 30, 1846), although it was Le Verrier's work that led directly to the new planet's being first seen and recognized as such (September 23, 1846). Figuring backward, it was found that the planet had been accidentally observed in Paris in 1795, but its planetary character had been overlooked.

Neptune is the name finally assigned to this historical planet. It is thirty times farther from the sun than the earth, or 2,800 million miles; its velocity in orbit is a little over three miles per second, and it consumes 164 years in going once completely round the sun. So faint is it that a telescope of large size is necessary to show it plainly. The brightness equals that of a star of the eighth magnitude, and with a telescope of sufficient magnifying power, the tiny disk can be seen and measured. The planet is about 30,000 miles in diameter, and is not known to possess more than one moon or satellite. If there are others, they are probably too faint to be seen by any telescope at present in existence.