The British astronomer William Herschel was systematically scanning the skies of Aries when on March 13,1781 he happened on a star he did not recall as being there. Since he was searching for comets,his first assumption was that he had found one. But after a few weeks of observing his new comet he came to realize that it neither looked like a comet, nor did it move like one. The object seemed to display a planetary disk, albeit a tiny one and it appeared to be moving in a circular orbit well beyond that of Saturn. It gradually dawned on Herschel that he had discovered a new planet, the first new planet since antiquity. Herschel actually was not the first to see it. Nearly a century before, John Flamsteed, who gained fame cataloguing stars listed Uranus as "34 Tauri" as far back as 1690. The new world was found to circle the Sun at a distance of 2,891,000 kilometers and required a full 84 years to complete one circle of the Sun. Over time the new planet's diameter was measured to be 51,118 kilometers. Suggestions abounded as to what to call the new world. Many simply named it "Herschel" for its discoverer. Herschel himself favored calling it the Georgium Sidnus or "Georgian Planet." The moniker honored Britain's King George III32. The name that eventually stuck was "Uranus." The German astronomer Johann Bode suggested the name and though it did not really become commonly accepted until about 1850, it did finally become the official name. This kept the tradition of naming the planets for Roman deities. Uranus was the Roman god of the heavens, father of Saturn and grandfather of Jupiter. For his historic find, George III named Herschel the Astronomer Royal. Herschel continued to study his new planet and in 1787 he found that Uranus had companions. Two faint moons circled the planet. The moons were given the names Titania and Oberon. Unlike other bodies in the solar system, Herschel's new finds were named for characters in
32 The same King George III that the American colonies were rebelling against as Herschel made his discovery. One of Britain's longest reigning monarchs (and certifiably insane), he ruled from 1760 to 1820.
Shakespearean dramas. That tradition continued as more moons were found. The British astronomer William Lassell found Ariel and Umbriel in 1851. Uranus' fifth moon, Miranda, was found in 1948 by the Dutch-born American astronomer Gerard Kuiper. All of the moons are relatively small. Miranda orbits closest to the planet at a distance of 130,000 kilometers and measures 572 km across. Ariel is at 191,000 km distance and measures 1,158km in diameter. Umbriel circles Uranus at a distance of 266,000km and is 1,170km in diameter. Titania is the fourth moon out and measures 1,578 km in diameter. Titania orbits at a distance of 436,000 km. The fifth moon out is Oberon at a distance of 583,000km and is slightly smaller than Titania with a diameter of 1,522 kilometers.
The moons helped give away an amazing aspect of Uranus' behavior. As is normal, all five of the planet's large moons orbit near the plane of Uranus' equator. When Herschel discovered Titania and Oberon in 1787, he realized the moons were traveling in almost perfect circles around the planet rather than shuttling back and forth in lines like Jupiter's moons, almost as though Uranus' pole was pointed straight at Earth. As the planet circled the Sun over the next twenty years, the path of the moons became more typically straight as the equator pointed at Earth. Uranus revolves around the Sun lying on its side. In fact, one of the more heated semantic arguments that astronomers have is over exactly how to define the planet's axial inclination. If it is assumed that the planet rotates in the normal (prograde) direction, then the planet's axis is tilted beyond sideways at 98 degrees. If the planet is assumed to rotate retrograde, then the inclination is 82 degrees. Either way, Uranus' axial inclination makes it one of the true oddities in the solar system and no one clearly understands why or how this happened.
In 1978, astronomers were preparing to observe Uranus as it passed in front of a star, a rare occultation for the tiny four arc-second planet. The occultation would provide an opportunity to make precision observations of Uranus' diameter and motion. But no one was prepared for what they saw as Uranus neared the star. The brightness of the star suddenly dipped and just as quickly recovered. Then it dipped again. And again. For a total of nine times, the star suddenly faded and brightened. The star then disappeared behind Uranus and reappeared on the other side. Astronomers then noticed that once again, the star dipped in brightness nine times in exactly the reverse order that it did on the way in. The conclusion was that Uranus had a ring system. Further observations using numerous wavelengths revealed that Uranus did indeed have rings, a total of nine of them. The rings of Uranus were unlike the rings of Saturn in two important ways. First they were very narrow, not more than a few hundred kilometers wide. Secondly they were as black as coal.
Eight years later, Uranus got its one and only visitor from planet Earth. On January 28, 1986 within a span of five hours, Voyager 2 flew past Uranus and observed it rings and moons. Because the planet's south pole was pointed straight at Earth, the probe flew across the plane of the moons perpendicularly rather than parallel as it did at Jupiter and Saturn. The encounter was therefore over very quickly. Tragically the Voyager 2 flyby of Uranus was almost completely ignored by the media because just a few hours before encounter, the Space Shuttle Challenger erupted into flame shortly after launch from the Kennedy Space Center, plunging the world into shock and mourning.
In the five-hour encounter period, Voyager 2 took thousands of images of the planet and its moons that are still yielding discoveries to this day. Voyager 2 added ten new moons to Uranus' ledger. Five more were later discovered telescopically. The twenty-one moons of Uranus can be clearly divided into three families. The innermost eleven moons orbit in amongst the rings and play critical roles in keeping the rings organized. The moons are nearly as black in color as the rings themselves so the moons may not just be shepherding the rings, but supplying the source material for the rings as well. The outermost five moons circle well beyond Oberon in far-flung orbits. These moons range in size from the 160-kilometer diameter Sycorax to the ten-kilometer diameter Trinculo.
While Voyager 2 logged plenty of new moons during the flyby, it found very little of visual interest in Uranus itself. At visible wavelengths, the planet was a bland uniform blue and green with no sign of the vibrant cloud bands of Jupiter or even the more subtle ones of Saturn. Some years later, the Hubble Space Telescope would image Uranus at wavelengths that were not available to Voyager and image processing tools that could only have been dreamed of in 1986. Uranus grudgingly began to reveal signs of circulation in its atmosphere. Also in 1986, Uranus' south pole was pointing almost directly at the Sun. In the early part of this decade, the Sun was moving much more towards the equator in the Uranian sky. This probably causes more diurnal variations in weather in the atmosphere and thus Uranus appears to be a much more vibrant place than it was when Voyager flew by it 20 years ago. Voyager discovered that Uranus is a very different kind of world than are Jupiter and Saturn. Uranus is only 15% hydrogen, while the inner gas giants are mostly hydrogen. The planet lacks the massive liquid metallic hydrogen envelope that surrounds the cores of Jupiter and Saturn. Uranus seems to be mostly rock and ices with most of the atmosphere being dominated by the planet's relatively sparse amount of hydrogen. Most of the planet's other material seems to be evenly distributed throughout Uranus with no sign of an obvious rocky core. The icy nature of the planet and lack of hydrogen has led astronomers to call Uranus an "ice giant."
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