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Uranus (planet)

The planet Uranus

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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
Avg. 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
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
257.31° (17 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

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. Its symbol is either Unicode ♅ (mostly astrological) or (mostly astronomical).


Physical characteristics


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, and at the two sections of its orbit between these two extremes 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 exploration 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 cataloged it as 34 Tauri).

Sir William Herschel discovered the planet in 1781, and originally named it Georgium Sidus (George's Star) in honour of King George III of England. However, this name was not accepted outside of Britain. At the suggestion of Lalande, French astronomers started calling it Herschel, while the German Johann Bode proposed the name Uranus, after the Greek god. The name Minerva was also proposed, but was not widely adopted.

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.

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.


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.

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 in fiction

  • In the animated series Futurama, in 2636 the name of Uranus was changed to Urectum to get rid of "That Joke."

Uranus in astrology

See also

External Links

Last updated: 09-12-2005 02:39:13