Uranus

For other uses, see Uranus (disambiguation).

Uranus

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Discovery
Discovered by	William Herschel
Discovered on	March 13, 1781
Orbital characteristics (Epoch J2000)
Semi-major axis	2,870,972,220 km 19.191 263 93 AU
Orbital circumference	18.029 Tm 120.515 AU
Eccentricity	0.047 167 71
Perihelion	2,735,555,035 km 18.286 055 96 AU
Aphelion	3,006,389,405 km 20.096 471 90 AU
Orbital period	30,707.4896 d (84.07 a)
Synodic period	369.65 d
Orbital speed	6.795 km/s
Max. orbital speed	7.128 km/s
Min. orbital speed	6.486 km/s
Inclination	0.769 86° (6.48° to Sun's equator)
Longitude of the ascending node	74.229 88°
Argument of the perihelion	96.734 36°
Number of satellites	27
Physical characteristics
Equatorial diameter	51,118 km (4.007 Earths)
Polar diameter	49,946 km (3.929 Earths)
Oblateness	0.0229
Surface area	8.084×109 km2 (15.849 Earths)
Volume	6.834×1013 km3 (63.086 Earths)
Mass	8.6832×1025 kg (14.536 Earths)
Mean density	1.318 g/cm3
Equatorial gravity	8.69 m/s2 (0.886 gee)
Escape velocity	21.29 km/s
Rotation period	−0.718 333 333 d (17 h 14 min 24.000 00 s) 1
Rotation velocity	2.59 km/s = 9320 km/h (at the equator)
Axial tilt	97.77°
Right ascension of North pole	77.31° (5 h 9 min 15 s)
Declination	+15.175°
Albedo	0.51
Cloudtop avg. temp.	55 K
Surface temp.	min mean max 59 K 68 K N/A K
Adjective	Uranian
Atmospheric characteristics
Atmospheric pressure	120 kPa (at the cloud level)
Hydrogen	83%
Helium	15%
Methane	1.99%
Ammonia	0.01%
Ethane	0.00025%
Acetylene	0.00001%
Carbon monoxide Hydrogen sulfide	trace

Uranus (pronounced either /ˈjuɹənəs/ or /jəˈɹeɪnəs/) 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 planned. Launched in 1977, Voyager made its closest approach to Uranus on January 24, 1986 before continuing on its journey to Neptune.

In the Chinese, Korean, Japanese, and Vietnamese languages the planet's name is literally translated as the sky king star, 天王星. [1]

Physical characteristics

Composition

Uranus is composed primarily of rocks and various ices, with only about 15% hydrogen and a little helium (in contrast to Jupiter and Saturn which are mostly hydrogen). Uranus (like Neptune) is in many ways similar to the cores of Jupiter and Saturn minus 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 whilst 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 in that it is not centered on the center of the planet and is tilted almost 60° with respect to the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus. Neptune has a similarly displaced magnetic field, suggesting that this is not necessarily a result 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.

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 but 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, in 1712 and 1715. Bradley observed it in 1748, 1750 and 1753; Mayer in 1756. Le Monnier observed it four times in 1750, twice in 1768, six times in 1769, and one last time in 1771. He was a victim of his own disorderliness: one of his observations was found consigned on a paper bag used to store hair powder!

Sir William Herschel discovered the planet on March 13, 1781, but reported it on April 26, 1781, as a "comet": Account of a Comet, By Mr. Herschel, F. R. S.; Communicated by Dr. Watson, Jun. of Bath, F. R. S., Philosophical Transactions of the Royal Society of London, Volume 71, pp. 492-501.

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. In any case, 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 the Greek god; 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.

Planetary rings

Main article: Rings of Uranus

Uranus has a faint planetary ring system, composed of dark particulate matter up to 10 metres 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, but when they analysed their observations 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 the Voyager 2 space probe passed Uranus in 1986. As of 2005, 13 rings had been identified. In December 2005, NASA's 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. Most surprisingly, the orbits of Uranus's family of inner moons have changed significantly in the last decade, the new data reveal.

Natural satellites

Main article: Uranus' natural satellites

Uranus has 27 known moons. The names for these moons are chosen from characters from the works of Shakespeare and Alexander Pope. The five main satellites are Miranda, Ariel, Umbriel, Titania and Oberon (For a timeline of discovery dates, see Timeline of natural satellites).

Visibility

The brightness of Uranus is between magnitude +5.5 and +6.0, so it can be seen with the naked eye as a faint star under dark sky conditions. It can be easily found with binoculars. From Earth, it has a diameter of 4". Even in large telescopes no details can be seen on its disc. However, infrared studies of its atmosphere using adaptive optics have yielded interesting data in the years since the Voyager flyby. (1)

(1) Space.com: New Images Reveal Clouds on Uranus

Appearance

Occultation of stars by Uranus between 1800 and 2100

Uranus in fiction

• Mr. Vivenair (a pseudonym) published A Journey Lately Performed Through the Air in an Aerostatic Globe, Commonly Called an Air Balloon, From This Terraquaeous Globe to the Newly Discovered Planet, Georgium Sidus in 1784.
• In the Buck Rogers series (1928–), Uranus is portrayed as having biodomes and robots.
• R. R. Winterbotham's "Clouds over Uranus" was published by Astounding in March of 1937
• In the 1962 film Journey to the Seventh Planet, astronauts on (sic) Uranus encounter a strange intelligence.
• In the Doctor Who (1963–) serial The Daleks' Master Plan, Uranus is described as being the only location in the universe where the mineral Taranium can be acquired.
• In H. P. Lovecraft's Cthulhu Mythos (1928–), Uranus is known as L'gy'hx and is inhabited by cubical metallic many-legged creatures who worship Lrogg. They entered in religious conflict with the Shan. (Ramsey Campbell's The Insects from Shaggai, 1964)
• In the anime Bishoujo Senshi Sailor Moon (1992), Sailor Uranus is the seventh guardian soldier to appear in the series, and represents the planet. Her attacks are associated with the force of nature (e.g. World Shaking, which is basically an earthquake). Later in the series, her attacks become more of galactic-like (e.g. Space Turbulence). Her talisman is Space Sword, and her image colours are beige and sometimes, gold.
• In the animated series Futurama (1999–2003), in 2620 the name of Uranus was changed to get rid of "That Stupid Joke" once and for all - but the new name is "Urectum".
• In Wayne W. Dyer's Gifts from Eykis (2002), Eykis is a female from Uranus who imparts wisdom and spirituality to Earth.

See also

• Colonization of Uranus

External links

• NASA's Uranus fact sheet
• Keck pictures of Uranus show best view from the ground - Press release with some photographs showing rings, satellites and clouds
• News reports of 22 December 2005 rings and moons discovery
  • New Moons and Rings found at Uranus, SPACE.com
  • Two more rings discovered around Uranus, MSNBC