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Uranus

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Uranus
The planet Uranus

Click image for description

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 30,708.1600 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 6.485 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 (http://www.hnsky.org/iau-iag.htm)
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
Atmospheric characteristics
Atmospheric pressure 120 kPa
Hydrogen 83%
Helium 15%
Methane 1.99%
Ammonia 0.01%
Ethane 0.00025%
Acetylene 0.00001%
Carbon monoxide
Hydrogen sulfide
trace

Uranus (pronounced "yər-AYN-us", or "YOOR-ə-nus") is the seventh planet from the Sun. It is a gas giant, the third largest by diameter and fourth largest by mass. It was named after the Greek god Ouranos, and is the only planet in the solar system named after a Greek god: all others are named after Roman deities. Its symbol is either ♅ (Unicode U+2645, mostly astrological) or Missing image
X_-_Uranus_B.png
Astronomical symbol for Uranus

(mostly astronomical).

Contents

Physical characteristics

Composition

Uranus is composed primarily of rock 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.

Axial tilt

One of the most distinctive features of Uranus is its axial tilt of almost ninety 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' 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 of Uranus

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 dismissed as simply another 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. Lemonnier 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 England. 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 G?ngen, chimed in with Austr䡧', a goddess mentioned by Ovid (but who is traditionally associated with Virgo). The name Minerva was also proposed [1] (http://vesuvius.jsc.nasa.gov/er/seh/hersc.html). 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.

Exploration of Uranus

NASA's Voyager 2 is the only spacecraft to have visited the planet. Launched in 1977, Voyager made its closest approach to Uranus on January 24, 1986 before continuing on its journey to Neptune.

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.

Appearance

Stationary, retrorad Opposition Stationary, prograd Conjunction to sun
June 10th, 2004 August 27th, 2004 November 12th, 2004 February 22nd, 2004
June 15th, 2005 September 1st, 2005 November 16th, 2005 February 25th, 2005
June 19th, 2006 September 5th, 2006 November 20th, 2006 March 1st, 2006
June 23rd, 2007 September 9th, 2007 November 24th, 2007 March 5th, 2007
June 27th, 2008 September 13th, 2008 November 27th, 2008 March 8th, 2008
Jule 1st, 2009 September 17th, 2009 December 2nd, 2009 March 13th, 2009
Jule 6th, 2010 September 21st, 2010 December 6th, 2010 March 17th, 2010

The rings of Uranus

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.

The moons of Uranus

Main article: Uranus' natural satellites

Uranus has 27 known moons. The five main satellites are Miranda, Ariel, Umbriel, Titania and Oberon.

For a timeline of discovery dates, see Timeline of natural satellites.

Uranus has 21 named moons. List of named moons:

  • Cordelia
  • Ophelia
  • Bianca
  • Cressida
  • Desdemona
  • Juliet
  • Portia
  • Rosalind
  • Belinda
  • Puck
  • Miranda
  • Ariel
  • Umbriel
  • Titania
  • Oberon
  • Caliban
  • Stephano
  • Trinculo
  • Sycorax
  • Prospero
  • Setebos

Uranus in fiction

  • In the animated series Futurama, in 2620 the name of Uranus was changed to Urectum to get rid of "That Stupid Joke" once and for all.

Uranus in astrology

Main article: Planets in astrology#Uranus

See also

External links

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Uranus' natural satellites

edit  (http://httpa.academickids.com/encyclopedia/index.php?title=Template:Uranus_Footer&action=edit)

Cordelia | Ophelia | Bianca | Cressida | Desdemona | Juliet | Portia | Rosalind | S/2003 U 2 | Belinda
S/1986 U 10 | Puck | S/2003 U 1 | Miranda | Ariel | Umbriel | Titania | Oberon | S/2001 U 3
Caliban | Stephano | Trinculo | Sycorax | S/2003 U 3 | Prospero | Setebos | S/2001 U 2


Our Solar System
Sun | Mercury | Venus | Earth (Moon) | Mars | Asteroid belts
Jupiter | Saturn | Uranus | Neptune | Pluto | Kuiper belt | Oort cloud
See also astronomical objects and the solar system's list of objects, sorted by radius or mass
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