Jeremy Colin Newell
April 5, 2001
Astronomy 145 ¡V Stars, Galaxies, and the Universe
2,870,990,000 km from the Sun, Uranus hangs on the wall of space as a mysterious blue green planet. With a mass of 8.683e25 kg and a diameter of 51,118 km at the equator, Uranus is the third largest planet in our solar system. It has been described as a planet that was slugged a few billion years ago by a large onrushing object, knocked down (never to get up), and now proceeds to roll around an 84-year orbit on its belly. As the strangest of the Jovian planets, the description is accurate. Uranus has a 17 hour and 14 minute day and takes 84 years to make its way about the sun with an axis tilted at around 90¢X with retrograde rotation. Stranger still is the fact that Uranus' axis is almost parallel to the ecliptic, hence the expression "on its belly.¡¨ Uranus is so far away that scientists knew comparatively little about it before NASA's Voyager 2 undertook its historic first encounter with the planet. The spacecraft flew closely past distant Uranus, and came within 50,600 miles of Uranus's cloud tops on Jan. 24, 1986. Voyager 2 radioed thousands of images and mass amounts of other scientific data about Uranus, its moons, rings, atmosphere, interior and magnetic environment. However, while Voyager has revealed much about the gas giant, many questions remain to be answered. The history of the discovery of the planet is the first we have of its kind; Uranus was the first planet to be discovered with a telescope. The circumstances surrounding the discovery of the object are befitting of the odd planet. The earliest recorded sighting of Uranus was in 1690 by John Flamsteed, but the object was catalogued as another star. On March 13, 1781 Uranus was sighted again by amateur astronomer William Herschel and thought to be a comet or nebulous star. In 1784, Jean-Dominique Cassini, director of the Paris Observatory and prominent professional astronomer, made the following comment: "A discovery so unexpected could only have singular circumstances, for it was not due to an astronomer and the marvelous telescope¡Kwas not the work of an optician; it is Mr. Herschel, a [German] musician, to whom we owe the knowledge of this seventh principal planet.¡¨ (Hunt, 35) Four years passed before Uranus was recognized as a new planet, the first to be discovered in 'modern' times. The discovery poses an interesting question however. Why was it Herschel and not someone like Cassini - a director of a prominent Observatory? It was by no accident that he discovered the first new planet. William Herschel had more than a passing fancy for the telescope. By purchasing the materials and even grinding the lenses himself, he built telescopes (namely reflectors) of exceptional quality for the period. That same quality afforded Herschel better observational conditions than his contemporaries did, and the result was a changed view of astronomy. A new planet had been discovered, and our view of the solar system was never to be the same again. The atmosphere and geology of the first new planet is fascinating. Uranus is primarily composed of rock and various ices; with only about 15% hydrogen and a little helium - in contrast to the compositions of Jupiter and Saturn, which are mostly hydrogen. Uranus' average temperature is around 350a Fahrenheit and the atmosphere is made of 83% hydrogen, 15% helium, and 2% methane. The blue color we often see is the result of absorption of red light absorbed by methane in the upper atmosphere. There may be colored bands like Jupiter's but they are hidden from view by the overlaying methane layer. Just below the clouds visible to earthbound observers are enormous quantities of ammonia, hydrogen sulfide, and water. Still deeper inside Uranus, under the crushing weight of the overlying atmosphere, is an invisible rocky surface - discovered only by...