The planet Uranus was discovered by British astronomer William Herschel in 1781. Herschel was engaged in charting the faint stars in the sky when he came across an odd-looking object that he described as "a curious either nebulous star or perhaps a comet." Repeated observations showed that it was neither. The object appeared as a disk in Herschel's 6-inch telescope and moved relative to the stars, but it traveled too slowly to be a comet. Herschel soon realized that he had found the seventh planet in the solar system. Since this was the first new planet discovered in well over 2000 years, the event caused quite a stir at the time. The story goes that Herschel's first instinct was to name the new planet "Sidus Georgium" (Latin for "George's star") after his king, George III of England. The world was saved from a planet named George by the wise advice of another astronomer, Johann Bode. He suggested instead that the tradition of using names from Greco—Roman mythology be continued and that the planet be named after Uranus, the father of Saturn.

Uranus is in fact just barely visible to the naked eye, if you know exactly where to look. At opposition, it has a maximum angular diameter of 4.1" and shines just above the unaided eye's threshold of visibility. It looks like a faint, undistinguished star. No wonder it went unnoticed by the ancients. Even today, few astronomers have seen it without a telescope.

Through a large Earth-based optical telescope (see Figure 13.1), Uranus appears hardly more than a tiny pale greenish disk. With the flyby of Voyager 2 in 1986, our detailed knowledge of Uranus increased dramatically. Figure 13.2 is a combination of close-up visible-light images of the planet and one of its moons. The apparently featureless atmosphere of Uranus contrasts sharply with the bands and spots visible on all the other jovian worlds. Some orbital and physical properties of Uranus are presented in Uranus Data box.

Figure 13.1 Details are barely visible on photographs of Uranus made with large Earth-based telescopes. (Arrows point to three of its moons.)

Figure 13.2 A montage of Uranus and one of its satellites, composed of photos sent back to Earth by the Voyager 2 spacecraft as it whizzed past this giant planet at 10 times the speed of a rifle bullet. The image of Uranus, taken from a distance of about 100,000 km, shows the planet's blue yet featureless upper atmosphere. The image in the foreground is Miranda, one of Uranus's moons. (Uranus's rings, not visible in the photograph, have been added by an artist.)