Uranus
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 in relation to the stars and decided it might be a comet. He sent a letter to the Astronomer Royal and another leading astronomer in England mentioning his discovery.
Herschel's telescope was so superior that neither of the other two men could verify his findings because the object was too faint in their telescopes' eyepieces. A Swedish astronomer, Anders Johan Lexell (1740-1784), plotted the orbit of the new object and found that it was not a comet heading for the Sun; it was a new planet moving in a near circular orbit 19 astronomical units from the Sun (the distance from the Earth to the Sun equals one a.u.). Herschel wanted to name the planet after the king of England, but it became known as Uranus, the mythological father of Saturn.
Since Uranus is large enough to be visible with the naked eye, astronomers were curious whether anyone had listed it earlier as a star. Records indicated that the first Astronomer Royal for England, John Flamsteed (1646-1719), had cataloged it as far back as 1690. Herschel continued observing the new planet with his telescope, which was by then proclaimed the finest made in all England. He found two satellites revolving around Uranus: Titania and Oberon. It wasn't until 1851 that William Lassell (1799-1880) found two more moons, Ariel and Umbria. Gerard Kuiper (1905-1973) discovered a fifth moon, Miranda, in 1948.
Uranus had a surprise in store for observers of these moons. In most cases the satellites of planets orbit in the same plane as the planets do when they revolve around the Sun. However, it soon became clear that the satellites of Uranus had orbital planes nearly vertical to the orbital plane of Uranus. This puzzled astronomers until someone suggested Uranus might be tipped over on its side. If this were true, then the planet was going around the Sun with its axis in the plane of its orbit, unlike every other planet, whose axes are generally upright on their orbits. Thus, the satellites of Uranus were orbiting around the planet's equator like all other planets, but the planet itself was traveling around the Sun on its side. Much serious thought went into trying to account for this configuration. The best current theory says Uranus was hit by a huge moon-size body long ago, and the resulting debris eventually formed the satellites circling the planet.
Uranus was frustrating for observers to study. It did not have the clear markings of its two neighbors, Jupiter and Saturn, and did not have the spectacular rings of Saturn. It had a bland, pale green face that hinted at a lighter band at the equator, seen only under exceptional conditions. Because of this featureless disk, it was very hard to determine a rotation period for the planet. It wasn't until 1911 that V. M. Slipher (1875-1969) at the Lowell Observatory in Arizona used spectrograms to find a rotation period of 10.7 hours, a speed that accounted for the planet's flattened appearance.
The unmanned spacecraft Voyager 2 flew by Uranus in 1986, producing a wealth of information on the planet. The face of Uranus was so featureless that Voyager's cameras revealed little to the curious astronomers on earth, but other instruments were used successfully. Uranus's rotation was established at 17.24 hours and winds were clocked at over 400 miles per hour (643 kph). Voyager's data also indicated that beneath Uranus's hydrogen and helium atmosphere is a vast "ocean" of water, methane, and ammonia that surrounds a molten core of heavy materials.
Voyager looked closely at the thin rings which had been first discovered in 1977 when Uranus moved in front of a star, causing the star's light to flicker as the rings passed in front of it. These rings are extremely dark and narrow. The spacecraft discovered ten new moons apparently made of frozen water and rock. The moons photographed by Voyager revealed grooved and cracked surfaces probably caused by tidal forces. Scientists assert that the Voyager exploration yielded more information about Uranus than all the knowledge that had been accumulated since the planet's discovery. Voyager also discovered ten new moons, bringing the size of the Uranian satellite system to 15.
In 1997, astronomers discovered two new moons orbiting Uranus. The new bodies are quite small (less than 100 miles in diameter) and are irregularly-shaped, as is characteristic of small satellites. This brought the total number of moons around Uranus to 17, and there may well be more yet to be found.
This is the complete article, containing 888 words
(approx. 3 pages at 300 words per page).
