Ancient peoples saw five objects in the sky that had a special significance and appeared to wander around in relationship to the "fixed" stars; the word planet is derived from the greek word for wanderer.

For centuries, astronomers believed our solar system consisted of Mercury, Venus, Earth, Mars, Jupiter, and Saturn. In 1781, however, a new planet was discovered so far from the Sun that it doubled the diameter of the solar system.

William Herschel, a German musician who had moved to England, developed a love for astronomy and made his own high-quality telescopes. He began a review of all the stars listed in a current catalog, hoping to determine how far they were from the Earth. In March 1781 he was searching the constellation Gemini when he spotted a faint object that, unlike the stars nearby, showed as a clear disk. The next night he found it had moved...