The seventh major planet from the Sun, discovered by William Herschel in 1781; a smaller gas giant than Jupiter or Saturn, and a near-twin to Neptune. It has the following characteristics: mass 8·7 × 1025 kg; radius 25 559 km/15 882 mi; mean density 1·3 g/cm3; rotational period 17·2 h (retrograde); orbital period 84·01 years; eccentricity of orbit 0·047; inclination of equator 98°; mean distance from the Sun 2·87 × 109 km/1·78 × 109 mi. Composed mainly of hydrogen and helium, like Jupiter and Saturn, Uranus has a larger percentage of ammonia and methane. Its interior lacks sharp compositional boundaries, but has a rock-rich core, ice-rich mantle, and deep atmosphere. Its low temperature allows condensation of methane clouds (methane gas in the upper atmosphere absorbs red light and accounts for the planet's greenish hue), with ammonia and water clouds also likely at lower levels.
It is highly unusual in having a rotation axis tilted so that the poles lie almost in the ecliptic, and its equatorial plane with rings and moons lies almost perpendicular to the ecliptic. It was observed at close range for the first time by Voyager 2 in 1986. Little contrast was observed in the clouds, which do show evidence of a banded structure. There is no significant internal heat souce; the circulation is driven primarily by an unusual distribution of solar heat input and rapid rotation. The temperatures at the poles and the equator are similar, indicating some dynamical means of redistributing heat deposited at the poles. Voyager discovered two new rings to add to the nine already observed telescopically, and in 2005 two further rings were observed by the Hubble Space Telescope. The rings are very dark, narrow, widely separated, and slightly eccentric. It was discovered in 2006 that the planet has a blue ring - only the second found in the Solar System. There are five major moons (Miranda, Ariel, Umbriel, Titania, Oberon) in synchronous rotation; 10 additional small moons were discovered by Voyager, and others have since been observed, making 27 in all (as of 2006).
Uranus|
Click image for description |
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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 |
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| Orbital circumference |
18.029 Tm 120.515 AU |
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| Eccentricity | 0.047 167 71 | ||||||
| Perihelion |
2,735,555,035 km 18.286 055 96 AU |
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| Aphelion |
3,006,389,405 km 20.096 471 90 AU |
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| Orbital period |
30,707.4896 d (84.07 a) |
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| 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) |
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Longitude of the ascending node |
74.229 88° | ||||||
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Argument of the perihelion |
96.734 36° | ||||||
| Number of satellites | 27 | ||||||
| Physical characteristics | |||||||
| Equatorial diameter |
51,118 km (4.007 Earths) |
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| Polar diameter |
49,946 km (3.929 Earths) |
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| Oblateness | 0.0229 | ||||||
| Surface area |
8.084×10 (15.849 Earths) |
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| Volume |
6.834×10 (63.086 Earths) |
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| Mass |
8.6832×1025 kg (14.536 Earths) |
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| Mean density | 1.318 g/cm3 | ||||||
| Equatorial gravity |
8.69 m/s2 (0.886 g) |
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| Escape velocity | 21.29 km/s | ||||||
| Rotation period |
−0.718 33 d (17 h 14 min 24 s by convention) 1 |
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| Rotation velocity | 2.59 km/s = 9320 km/h (at the equator) | ||||||
| Axial tilt | 97.77° | ||||||
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Right ascension of North pole |
77.31° (5 h 9 min 15 s) | ||||||
| Declination | +15.175° | ||||||
| Albedo | 0.51 | ||||||
| Cloudtop avg. | 55 K | ||||||
| Surface temp. |
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| 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% | ||||||
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Carbon monoxide Hydrogen sulfide |
trace | ||||||
For other uses, see Uranus (disambiguation).
Uranus (IPA: /jəˈreɪnəs/ or /ˈjurənəs/) is the seventh planet from the Sun.
Discovery and naming
Uranus was the first planet to be discovered that was not known in ancient times; (Lemonnier is often called careless or even "sloppy" for this, but it is important to know that he realized 9 of these within a short time of Herschel's discovery and most of his observations occurred at the stationary point in Uranus' orbit.)
Sir William Herschel discovered the planet on March 13, 1781, but reported it on April 26, 1781, as a "comet."
Magnetic field
Uranus' magnetic field is peculiar since it is not originating from the geometric center of the planet and is tilted almost 60° from the axis of rotation.
Planetary rings
Uranus has a faint planetary ring system, composed of dark particulate matter up to ten meters in diameter.
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The main Uranian moons (compared to Earth's Moon) |
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|---|---|---|---|---|---|
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Name (Pronunciation key) |
Diameter (km) |
Mass (kg) |
Orbital radius (km) |
Orbital period (d) |
|
| Miranda |
mə-ran'-də /mɪˈrændə/ |
470 (14%) |
7.0×1019 (0.1%) |
129,000 (35%) |
1.4 (5%) |
| Ariel |
arr'-ee-əl /ˈɛəriəl/ |
1160 (33%) |
14×1020 (1.8%) |
191,000 (50%) |
2.5 (10%) |
| Umbriel |
um'-bree-əl /ˈʌmbriəl/ |
1170 (34%) |
12×1020 (1.6%) |
266,000 (70%) |
4.1 (15%) |
| Titania |
tə-taan'-yə /tɪˈtɑ:njə/ or /tɪˈteɪnjə/ |
1580 (45%) |
35×1020 (4.8%) |
436,000 (115%) |
8.7 (30%) |
| Oberon |
oe'-bər-on /ˈoʊbərɒn/ |
1520 (44%) |
30×1020 (4.1%) |
584,000 (150%) |
13.5 (50%) |
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.
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