Uranus

For other uses, see Uranus (disambiguation).

Uranus

Uranus, as seen by Voyager 2
Discovery
Discovered by	William Herschel
Discovery date	March 13, 1781
Designations
Adjectives	Uranian
Symbol
Orbital characteristics
Epoch J2000
Aphelion	3,006,389,405 km 20.096 471 90 AU 1,868,088,249 mi
Perihelion	2,735,555,035 km 18.286 055 96 AU 1,699,799,169 mi
Semi-major axis	2,870,972,220 km 19.191 263 93 AU 1,783,943,710 mi
Eccentricity	0.047 167 71
Orbital period (synodic)	369.65 day
Average orbital speed	6.795 km/s
Inclination	0.769 86° (6.48° to Sun's equator)
Longitude of ascending node	74.229 88°
Known satellites	27
Physical characteristics
Equatorial radius	25,559 km (4.007 Earths)
Polar radius	24,973 km (3.929 Earths)
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
Surface gravity	8.69 m/s2 (0.886 g)
Escape velocity	21.29 km/s
Sidereal rotation period	−0.718 33 day (17 h 14 min 24 s by convention)
Equatorial rotation velocity	2.59 km/s = 9320 km/h
Axial tilt	97.77°
North pole right ascension	5 h 9 min 15 s (77.31°)
North pole declination	+15.175°
Albedo	0.51
Surface temp. min mean max Surface 59 K 68 K N/A Cloudtop 55 K
Atmosphere
Surface pressure	120 kPa (at the cloud level)
Composition by volume	83% Hydrogen 15% Helium 1.99% Methane 0.01% Ammonia 0.00025% Ethane 0.00001% Acetylene trace Carbon monoxide trace Hydrogen sulfide

Uranus (/jʊˈreɪ.nəs, ˈjʊr.ə.nəs/, named after the Greek god of the sky (Uranus, [[wiktionary:οὐρανός|Template:Polytonic]]), is the seventh planet from the Sun. It is a gas giant, the third largest by diameter and fourth largest by mass. Its astronomical symbol is . The symbol is a combination of the devices for the Sun and Mars, as Uranus was the personification of heaven in Greek mythology, dominated by the light of the Sun and the power of Mars. It is also the alchemical symbol of platinum.

Uranus is the first planet discovered in modern times. Sir William Herschel formally discovered the planet on March 13, 1781; the other planets (from Mercury out to Saturn) have been known since ancient times, since they are visible to the naked eye. The discovery of Uranus expanded the boundaries of the solar system for the first time in modern human history. It was also the first planet discovered using technology (a telescope) rather than the naked eye.

Discovery and naming

Uranus is the first planet to be discovered that was not known in ancient times; although it had been observed on many previous occasions, it was often mistakenly identified as a star. The earliest recorded sighting was in 1690 when John Flamsteed cataloged Uranus as 34 Tauri. Flamsteed observed Uranus at least six more times. The record belongs to a French astronomer, Pierre Lemonnier, who observed Uranus at least twelve times between 1750 and 1769, including on four consecutive nights. (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."

On the 13th of March, 1781, between ten and eleven o'clock at night, while Herschel was examining the small stars near H Geminorum with a seven-foot telescope, bearing a magnifying power of two hundred and twenty-seven times, one of these stars seemed to have an unusual diameter; and it was, therefore, thought to be a comet. It was under this denomination that it was discussed at the Royal Society of London. But the researches of Herschel and of Laplace showed later that the orbit of the new body was nearly circular, and Uranus was consequently elevated to the rank of a planet.

Herschel originally named it Georgium Sidus (George's Star) in honour of King George III of Great Britain (c.f. American poet Elizabeth Graeme Fergusson's "Upon the Discovery of the Planet..." about the event). When it was pointed out that sidus means star and not planet, Herschel 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 astrological symbol (, "a globe surmounted by your [Herschel's] 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 another 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.

Finally, Bode, as editor of the Berliner Astronomisches Jahrbuch, opted for Uranus, a name he had proposed as early as 1781, after the Latinized version of the Greek god of the sky, Ouranos; Maximilian Hell followed suit by using it in the first ephemeris, published in Vienna and computed by the Benedictine priest Placidus Fixlmillner. The earliest publication to include Uranus in its title was in 1823. The name was in use in Germany at least as far back as 1791, however. 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" was still used infrequently (by the British alone) thereafter. The final holdout was HM Nautical Almanac Office, which did not switch to Uranus until 1850.

Jupiter (or Zeus) was the father of Mars (or Ares), and Saturn (or Cronos) was the father of Jupiter, each representing different Ages of Man; thus, it was most logic to name the planet beyond Saturn after Uranus, since he, according to mythology, was the father of Saturn.

The stressed syllable in the name is properly the first, antepenultimate syllable, since in Latin the penultimate vowel a is short (ūrănŭs) and in an open syllable, and such syllables are never stressed in Latin. The historically correct pronunciation of the name by English-speakers is therefore [ ˈjʊ.rə.nəs ]. The historically incorrect pronunciation [ jʊˈɹeɪ.nəs ], with stress on the second syllable and a "long a" (ūrānŭs) have become very common, however, perhaps through the influence of the related adjective "Uranian" (always pronounced [ jʊˈɹeɪ.ni.ən ] or the similarly-pronounced name of the element uranium, or from a mischievous delight in homophony.

In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star (天王星), while in India it is named Aruna (Devanāgarī अरुण), the charioteer of the sun god Surya in Hindu mythology.

Physical characteristics

Composition

The atmosphere of Uranus is composed primarily of gas and various ices. It is about 83 percent hydrogen, 15 percent helium, 2 percent methane and traces of acetylene. Uranus' cyan color is due to the absorption of red light by atmospheric methane. Surface temperature on Uranus' cloud cover is approximately 55 K (−218 °C or −360 °F). The interior is richer in heavier elements, most likely compounds of oxygen, carbon, and nitrogen, as well as rocky materials. This is in contrast to Jupiter and Saturn which are mostly hydrogen and helium. Uranus (like Neptune) has a core similar to those of Jupiter and Saturn, without the massive fluid metallic hydrogen envelope. Instead, it is thought that much of Uranus' interior between the core and the atmosphere may consist of superheated liquid water under very high pressure.

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's orbit the orientation of the poles towards the Sun is reversed. This gives each pole 42 years of continuous sunlight, followed by 42 years of darkness. Between these two extremes of its orbit, particularly at the equinoxes, the Sun rises and sets around the equator normally. Uranus will reach its next equinox around December 2007, and not again until 2049.

Season, Northern Hemisphere	Year	Season, Southern Hemisphere
Winter Solstice	1902, 1986	Summer Solstice
Vernal Equinox	1923, 2007	Autumnal Equinox
Summer Solstice	1944, 2028	Winter Solstice
Autumnal Equinox	1965, 2049	Vernal Equinox

At the time of Voyager 2's passage in 1986, Uranus' south pole was pointed almost directly at the Sun. The labelling of this pole as "south" uses the coordinate definitions currently endorsed by the International Astronomical Union, namely that the north pole of a planet or satellite shall be the pole which points above the invariable plane of the solar system (regardless of the direction the planet is spinning). A different system is sometimes used, defining a body's north and south poles according to the right-hand rule in relation to the direction of rotation. In terms of this latter coordinate system it was Uranus' north pole which was in sunlight in 1986. On page 47 of the September 2006 issue of the Sky at Night magazine, Patrick Moore, commenting on the issue, sums up "take your pick!"

One result of this 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 during the formation of the Solar System, an Earth sized protoplanet collided with Uranus, causing 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's overhead position relative to Uranus is approaching Uranus' equator. By 2007 the Sun will be directly over Uranus' equator.

Magnetic field

Uranus's 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. 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.

Explanation for bland atmosphere

The internal heat of Uranus is lower than that of Jupiter and Saturn. Both Jupiter and Saturn radiate more energy than they receive from the Sun. This causes many powerful convection currents to form in the atmosphere. On Uranus that heat source is much lower due to its lower mass, with the temperature of its core roughly 7000 K compared to 30 000 K at Jupiter's core and 18 000 K at Saturn. The convection currents formed in the Uranian atmosphere are not as strong and hence it lacks the atmosphere banding of the larger gas giants. However, as stated above, the weather patterns of Uranus do vary with season, being more pronounced at the equinoxes than at the solstices.

Cloud features

For a short period in Autumn 2004, a number of large clouds appeared in the Uranian atmosphere, giving it a Neptune-like appearance. On August 23, 2006, researchers at the Space Science Institute (Boulder, CO) and the University of Wisconsin observed a dark spot on Uranus' surface, giving astronomers more insight into the planet's atmospheric activity.

Planetary rings

Main article: Rings of Uranus

Uranus with its rings in false color

Uranus has a faint planetary ring system, composed of dark particulate matter up to ten meters in diameter. The first mention of Uranus's ring system comes from William Herschel's notes detailing his observations of Uranus. These include the following passage: "February 22, 1789: A ring was suspected". Herschel drew a small diagram of the ring and noted that it was "a little inclined to the red". The Keck Telescope in Hawaii has since confirmed this to be the case. Herschel's notes were published in a Royal Society journal in 1797. Between 1797 and 1977 the rings are rarely mentioned, if at all. This might be the result of the unfavourable inclination of the ring system for part of the intervening period, or due to a reduction in the brightness of the ring system. The ring system was rediscovered on March 10, 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 the star SAO 158687 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 imaged when Voyager 2 passed Uranus in 1986.

In December 2005, the Hubble Space Telescope detected a pair of previously unknown rings. The largest is located at twice the distance from the planet than the previously known rings. The new rings are so far from the planet that they are being called Uranus' "outer" ring system. Hubble also spotted two small satellites. One, Mab, shares its orbit with the outermost newly discovered ring. These two rings bring the total number of Uranus rings to 13.

In April 2006, images of the new rings with the Keck Observatory yielded the colours of the outer rings: one was blue and the other red. A marked similarity to the colourful E and G rings of Saturn may be a coincidence, or it may be related to a similar causal mechanism. The source of the rings' colours are still an area of active research. The planet's inner rings appear grey.

Exploration

Main article: Exploration of Uranus

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. Voyager 2 discovered 10 previously unknown moons; studied the planet's unique atmosphere, caused by its axial tilt of 97.77°; and examined its ring system. In its first solo planetary flyby, Voyager 2 made its closest approach to Uranus on January 24, 1986, coming within 81,500 kilometers (50,600 miles) of the planet's cloud tops.

A picture of Uranus taken by Voyager 2 as it headed to Neptune.

Voyager 2 found that one of the most striking influences of Uranus' sideways position is its effect on the tail of the magnetic field, which is itself tilted 60 degrees from the planet's axis of rotation. The magnetotail was shown to be twisted by the planet's rotation into a long corkscrew shape behind the planet. The presence of a magnetic field at Uranus was not known until Voyager's arrival.

Radiation belts at Uranus were found to be of an intensity similar to those at Saturn. The intensity of radiation within the belts is such that irradiation would quickly darken (within 100,000 years) any methane trapped in the icy surfaces of the inner moons and ring particles. This may have contributed to the darkened surfaces of the moons and ring particles, which are almost uniformly gray in color.

A high layer of haze was detected around the sunlit pole, which also was found to radiate large amounts of ultraviolet light, a phenomenon dubbed "dayglow." The average temperature is about 60 kelvins (−350 degrees Fahrenheit). Surprisingly, the illuminated and dark poles, and most of the planet, show nearly the same temperature at the cloud tops.

The moon Miranda, innermost of the five large moons, was revealed to be one of the strangest bodies yet seen in the solar system. Detailed images from Voyager's flyby of the moon showed huge fault canyons as deep as 20 kilometers (12 miles), terraced layers, and a mixture of old and young surfaces. One theory holds that Miranda may be a reaggregation of material from an earlier time when the moon was fractured by a violent impact.

All nine previously known rings were studied by the spacecraft and showed the Uranian rings to be distinctly different from those at Jupiter and Saturn. The ring system may be relatively young and did not form at the same time as Uranus. Particles that make up the rings may be remnants of a moon that was broken by a high-velocity impact or torn up by gravitational effects.

Natural satellites

Main article: Uranus' natural satellites

See also: Timeline of discovery of Solar System planets and their natural satellites

Uranus has 27 known natural satellites. The names for these satellites are chosen from characters from the works of Shakespeare and Alexander Pope. The five main satellites are Miranda, Ariel, Umbriel, Titania and Oberon.

The Uranian satellite system is the least massive among the gas giants; indeed, the combined mass of the five major satellites would be less than half that of Triton alone. The moons are low-albedo, ice-rock conglomerates.

The main Uranian moons
(compared to Earth's Moon)

Name (Pronunciation key)		Diameter (km)	Mass (kg)	Orbital radius (km)	Orbital period (d)
Miranda	IPA: [mɪˈɹæn.də]	470 (14%)	7.0×1019 (0.1%)	129,000 (35%)	1.4 (5%)
Ariel	IPA: [ˈe.ɹiˌɛl]	1160 (33%)	14×1020 (1.8%)	191,000 (50%)	2.5 (10%)
Umbriel	IPA: [ˈʌm.bɹiˌɛl]	1170 (34%)	12×1020 (1.6%)	266,000 (70%)	4.1 (15%)
Titania	IPA: [tɪˈteɪ.ni.ə ]] or [ taɪˈteɪ.ni.ə ]	1580 (45%)	35×1020 (4.8%)	436,000 (115%)	8.7 (30%)
Oberon	IPA: [ˈoʊ.bə.ɹɑn]	1520 (44%)	30×1020 (4.1%)	584,000 (150%)	13.5 (50%)

Visibility

Size comparison of Earth and Uranus

The brightness of Uranus is between magnitude +5.5 and +6.0. Thus, under dark sky conditions it can be seen with the naked eye as a faint star. From Earth, it has a diameter of four arcseconds. It is an easy target with binoculars. In larger amateur telescopes with an objective diameter greater than 12" (30 cm), the planet appears as a pale blue disk with distinct limb darkening. The larger satellites, Titania and Oberon, may be visible. Because of its faintness and slow apparent motion, it was never recognised as a planet by ancient astronomers.

As the planet approaches its 2007 equinox, dynamic cloud activity is developing. Most of the activity requires Hubble Space Telescope or large telescopes with adaptive optics to be detectable. However, some features may become bright enough to be seen with moderately large amateur telescopes. In 2006, a Dark Spot was detected in visible-wavelength Hubble images, suggesting that the atmosphere of Uranus near equinox is becoming more like that of Neptune.

See also

• Colonization of Uranus
• Uranus in fiction
• Uranus in astrology

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
• Planets—Uranus A kid's guide to Uranus.
• Uranus at Jet Propulsion Laboratory's planetary photojournal.
• Spring Has Sprung on Uranus