Uranus
From The Book of THoTH (Leaves of Wisdom)
Uranus (IPA: /ˈjurənəs/ or /jəˈreɪnəs/) is the seventh planet from the Sun. It is a gas giant, the third largest by diameter and fourth largest by mass. It is named after Uranus, the Greek god of the sky and progenitor of the other gods. Its symbol is either ♅; (Unicode U+2645, mostly astrological) or (mostly astronomical).
NASA's Voyager 2 is the only spacecraft to have visited the planet and no other visits are currently planned. Launched in 1977, Voyager 2 made its closest approach to Uranus on January 24, 1986, before continuing its journey to Neptune.
Discovery and naming Uranus was the first planet to be discovered that was not known in ancient times; although it had been observed on many previous occasions, it was always mistakenly identified as a star. The earliest recorded sighting was in 1690 when John Flamsteed catalogued it as 34 Tauri. Flamsteed observed Uranus twice again. Sir William Herschel formally discovered the planet on March 13, 1781, but reported it on April 26, 1781, as a "comet."
Herschel originally named it Georgium Sidus (George's Star) in honour of King George III of Great Britain. When it was pointed out that sidus means star and not planet, he rebaptised it the Georgian Planet. This name was not acceptable outside of Britain. Lalande proposed in 1784 to name it Herschel, at the same time that he created the planet's symbol ("a globe surmounted by your initial"); his proposal was readily adopted by French astronomers. Prosperin, of Uppsala, proposed the names Astraea, Cybele, and Neptune (now borne by two asteroids and a planet). Lexell, of St. Petersburg, compromised with George III's Neptune and Great-Britain's Neptune. Bernoulli, from Berlin, suggested Hypercronius and Transaturnis. Lichtenberg, from Göttingen, chimed in with Austräa, a goddess mentioned by Ovid (but who is traditionally associated with Virgo). The name Minerva was also proposed.[2] Finally, Bode, as editor of the Berliner Astronomisches Jahrbuch, opted for Uranus, after Latinized version of the Greek god of the sky, Ouranos; Maximilian Hell followed suit by using it in the first ephemeris, published in Vienna. Examination of earliest issues of Monthly Notices of the Royal Astronomical Society from 1827 shows that the name Uranus was already the most common name used even by British astronomers by then, and probably earlier. The name Georgium Sidus or "the Georgian" were still used infrequently (by the British alone) thereafter. The final holdout was HM Nautical Almanac Office, which did not switch to Uranus until 1850.
In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star.
Physical characteristics
Composition
Uranus is composed primarily of rock and various ices, with about 85% hydrogen, 15% helium and traces of methane (in contrast to Jupiter and Saturn which are almost entirely hydrogen). Uranus (like Neptune) is very much similar to the cores of Jupiter and Saturn without the massive liquid metallic hydrogen envelope. It appears that Uranus does not have a rocky core like Jupiter and Saturn but rather that its material is more or less uniformly distributed. Uranus' cyan color is due to the absorption of red light by atmospheric methane. Surface temperature on Uranus's cloud cover is approximately 55 K (−218 °C or −360 °F).
Axial tilt
One of the most distinctive features of Uranus is its axial tilt of ninety-eight degrees. Consequently, for part of its orbit one pole faces the Sun continually while the other pole faces away. At the other side of Uranus' orbit the orientation of the poles towards the Sun is reversed. Between these two extremes of its orbit the Sun rises and sets around the equator normally.
At the time of Voyager 2's passage in 1986, Uranus' south pole was pointed almost directly at the Sun. Note that the labelling of this pole as "south" is actually in some dispute. Uranus can either be described as having an axial tilt of slightly more than 90°, or it can be described as having an axial tilt of slightly less than 90° and rotating in a retrograde direction; these two descriptions are exactly equivalent as physical descriptions of the planet but result in different definitions of which pole is the North Pole and which is the South Pole.
One result of this odd orientation is that the polar regions of Uranus receive a greater energy input from the Sun than its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles, although the underlying mechanism which causes this is unknown. The reason for Uranus' extreme axial tilt is also not known. It is speculated that perhaps during the formation of the planet it collided with an enormous protoplanet, resulting in the skewed orientation.
It appears that Uranus' extreme axial tilt also results in extreme seasonal variations in its weather. During the Voyager 2 flyby, Uranus' banded cloud patterns were extremely bland and faint. Recent Hubble Space Telescope observations, however, show a more strongly banded appearance now that the Sun is approaching Uranus' equator. By 2007 the Sun will be directly over Uranus's equator.
Magnetic field
Uranus' magnetic field is odd for it is not centered on the center of the planet and is tilted almost 60° from the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus. Neptune has a similarly displaced magnetic field, which suggests the magnetic field is not necessarily a consequence of Uranus' axial tilt. The magnetotail is twisted by the planet's rotation into a long corkscrew shape behind the planet. The magnetic field's source is unknown; the electrically conductive, super-pressurized ocean of water and ammonia once thought to lie between the core and the atmosphere now appears to be nonexistent.
Planetary rings
Uranus has a faint planetary ring system, composed of dark particulate matter up to ten meters in diameter. This ring system was discovered in March 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink using the Kuiper Airborne Observatory. The discovery was serendipitous; they planned to use the occultation of a star by Uranus to study the planet's atmosphere. However, when their observations were analyzed, they found that the star had disappeared briefly from view five times both before and after it disappeared behind the planet. They concluded that there must be a ring system around the planet; it was directly detected when Voyager 2 passed Uranus in 1986. As of 2005, 13 rings had been identified. In December 2005, the Hubble Space Telescope photographed a pair of previously unknown rings. The largest is twice the diameter of the planet's previously known rings. The new rings are so far from the planet that they are being called Uranus's "second ring system." Hubble also spotted two small satellites. One shares its orbit with one of the newly discovered rings. The new data reveals that the orbits of Uranus's family of inner moons have changed significantly in the last decade.
In April 2006, information about two more rings was published, one of them appearing blue and the other red. The rest of the planet's rings appear grey. The blue ring is thought to get its color from being swept by a moon, which may draw away all large debris, leaving only fine dust which refracts light in much the same way the Earth's atmosphere does.
