Uranus (astronomy)

Uranus , in astronomy, 7th planet from the sun, at a mean distance of 1.78 billion mi (2.87 billion km), with an orbit lying between those of Saturn and Neptune; its period of revolution is slightly more than 84 years. The first planet discovered in modern times with the aid of a telescope, Uranus was detected in 1781 by Sir William Herschel , who originally thought it to be a comet. Because the calculated orbit of Uranus did not compare accurately with the observed orbit, astronomers concluded that a disturbing influence was present. A study of this irregularity led to the discovery of Neptune in 1846. Uranus has a diameter of c.31,760 mi (46,700 km), roughly 4 times that of the earth, and a mass of about 15 times that of the earth. Like the giant planets Jupiter and Saturn, Uranus has a thick atmosphere of hydrogen, helium, and methane; a relatively low density; and a rapid period of rotation of about 17.9 hr, which causes a polar flattening of over 6%. However, its axis of rotation is tilted 98° to the plane of its orbit. The Voyager 2 space probe found that Uranus has the most inclined magnetic field in the solar system, and some astronomers interpret this as evidence that the magnetic field is reversing its polarity. Viewed through a telescope, Uranus appears as a greenish disk, slightly elliptical because of its rapid rotation. Its temperature is estimated to be about -330°F (-200°C), and at this temperature ammonia, the main constituent of the visible cloud cover, would exist in the form of ice crystals. Uranus has 27 known natural satellites with diameters ranging in size from 7 mi (11 km) to 986 mi (1,578 km).

Prior to 1986, only five of Uranus's natural satellites were known: Titania, the largest, and Oberon were discovered by Herschel in 1787; Ariel and Umbriel, by William Lassell in 1851; and Miranda, by Gerard Kuiper in 1948. When Voyager 2 flew by Uranus in 1986, it discovered 10 more natural satellites—Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, and Puck—and confirmed the existence of 11 rings. Two additional satellites, Caliban and Sycorax, were discovered in 1997, and three more, Prospero, Setebos, and Stephano, were found in 1999. Trinculo, a small irregular satellite, was discovered in 2002; eight other small satellites are also irregular, that is, their motion around Uranus is retrograde (motion opposite to that of the planet's rotation). The moons of Uranus are named after characters found in the works of William Shakespeare and Alexander Pope.

Titania along with Oberon and Umbriel appear geologically to be relatively quiet. Ariel has surface features that indicate past seismic activity. Miranda shows the most dramatic surface of all, with fracture patterns and sudden landscape changes that indicate that the moon fell apart and then reassembled after a collision in its early history. In 1977, during an occultation by Uranus of a star, astronomers detected a system of nine narrow rings of small, dark particles orbiting around the planet; two more rings, many tiny ringlets, and arcs of rings were later found by Voyager 2. Uranus's rings are distinctly different from those of Jupiter and Saturn. For example, Saturn's rings are very bright and easily seen but Uranus's are very dark, with only 5% of the sunlight being reflected back. Uranus's rings also are very narrow and flat. The widest part of Uranus's outermost ring, the epsilon ring, is 60 mi (97 km) across. The others are only 1 to 2 mi (1.5-3.2 km) wide and barely half a mile (0.8 km) deep.

Uranus Seventh planet from the Sun, discovered (1781) by Sir William Herschel. Uranus is visible to the naked eye under good conditions. Through a telescope it appears as a small, featureless, greenish-blue disc. Like all the giant planets, it possesses a ring system and a retinue of satellites. Like Pluto, Uranus' axis of rotation is steeply inclined, and its poles spend 42 years in sunlight, followed by 42 years in darkness. Highly exaggerated seasonal variations are experienced by the planet and its satellites. The fly-by of the Voyager 2 probe in 1986 provides most of our knowledge of the planet. The upper atmosphere is about 83% molecular hydrogen, 15% helium, and the other 2% mostly methane. The five largest satellites were known before the Voyager encounter, which led to the discovery of 11 more. Nineteen of its 22 moons are regular satellites, orbiting in or close to Uranus' equatorial plane. They are all darkish bodies composed of ice761 and rock. The main components of Uranus' ring system were discovered in 1977, and others were imaged by Voyager.

http://lpl.arizona.edu/nineplanets/nineplanets/uranus.html; http://wr.usgs.gov

Uranus (symbol ♅ or ) The seventh planet from the Sun. It is blue-green in colour due to the absorption of red light by methane in the upper atmosphere. Its mean opposition magnitude is +5.5, making it just visible to the naked eye under favourable conditions, but it was unknown until 1781 when discovered telescopically by F. W.Herschel. Uranus is distinctly ellipsoidal in shape (equatorial diameter 51 118 km, polar diameter 49 946 km). Its density, 1.3 g/cm³, is the lowest of the planets except Saturn. Its rotation axis is tilted at over 90° to its orbital plane, so that its rotation is retrograde, and it presents its poles and its equator alternately towards the Sun as it orbits. The rotation period of the visible surface ranges from about 16 hours at 70° south to about 18 hours near the equator, but radio observations indicate that the core rotates in 17 h 14 m.

Uranus has a thick atmosphere composed of 83 % hydrogen, 15 % helium, and 2 % methane (molecular percentages). Thicker clouds of methane at the 1 bar pressure level overlie deeper opaque clouds that probably consist of hydrogen sulphide. The temperature near the top of the atmosphere is around −220°C. Internally, Uranus is thought to have a small rocky core at a high temperature, probably surrounded by a layer of icy materials, topped by a layer of hydrogen and helium. Unlike the other gas giants, Uranus does not emit more heat than it receives.The interior is probably as hot as that of Neptune, but the heat may be prevented from escaping as effectively as a result of some unknown process. Its magnetic field has a strength of about 2.5 × 10⁻⁵ tesla at the equator, similar to the Earth's. Uranus's magnetic axis is not centred in the core, but one-third of the way to the surface, and is tilted at nearly 60° to its spin axis.

URANUS
Physical data
Diameter (equatorial)	Oblateness	Inclination of equator to orbit	Axial rotation period (sidereal)
51 118 km	0.023	97°.77	17.24 hours
Mean density	Mass (Earth = 1)	Volume (Earth = 1)	Mean albedo (geometric)	Escape velocity
1.27 g/cm3	14.5	63	0.51	21.3 km/s
Orbital data
Mean distance 106 km	from Sun AU	Eccentricity of orbit	Inclination of orbit to ecliptic	Orbital period (sidereal)
2870.658	19.189	0.047	0°.8	84.02 years

The atmosphere of Uranus shows few visible features. Computer-enhanced images have revealed extremely faint dark belts or bands with bright zones between them. Dark and bright spots occur, but these are also very faint. Each latitude has its own characteristic rotation period, with the shortest rotation periods at around 70° south. This may be a result of the high axial inclination of Uranus; since each pole of Uranus spends long periods facing the Sun, convection cells originating at the pole could produce winds that blow against the rotation of the planet near the equator, due to the Coriolis force.

 Uranus has thirteen known rings, all far fainter and less extensive than those of Saturn. The innermost ring, known as Zeta, is 35–40 000 km from the centre of Uranus, and the outer, brightest ring, called Epsilon, is 51 100 km from the centre. The rings differ from those of Jupiter and Saturn in that they are slightly eccentric, and do not perfectly lie in the equatorial plane of the planet. They are comparatively dust‐free and composed of much bigger particles, generally over a metre in size. The Epsilon Ring is composed mainly of large icy boulders, but is dark grey in colour with an albedo of a few per cent. Uranus has 27 known satellites.