Uranus

Uranus, in astronomy

Uranus (yrā`nəs, yr`ə–), 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.

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Uranus, in Greek religion and mythology

Uranus, in Greek religion and mythology, the heaven, first ruler of the universe, son of Gaea (the earth). He was the father of Gaea's children, the Titans, the Cyclopes, and the Hundred-handed Ones (the Hecatoncheires). Fearing that his children would rebel against him, he imprisoned them, but Kronos, a Titan, with the help of Gaea, castrated him, thereby taking away his power. From the blood of Uranus that fell on Earth sprang the three Furies (the Erinyes), the goddesses of revenge. According to Hesiod, Aphrodite was born of Uranus' discarded flesh and the foaming sea.

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Uranus

 or Ouranus

Ancient Greek personification of heaven. When Gaea emerged from Chaos, she produced Uranus, the mountains, and the sea. Her subsequent union with Uranus produced the Titans, the Cyclopes, and the Hecatoncheires. Uranus despised his offspring and hid them in Gaea's body. In response to her appeal for vengeance, Cronus castrated Uranus. From the drops of blood that fell on Earth were born the Furies, the Giants, and the ash-tree nymphs called Meliai. His severed genitals floated on the sea, producing a white foam from which sprang Aphrodite. Uranus also consorted with Clymene, Hemera, Hestia, and Nyx.

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Uranus

Seventh planet from the Sun. It was discovered in 1781 by William Herschel (see Herschel family) and named for the Greek god personifying heaven. A blue-green gas giant, it has almost 15 times the mass of Earth and over 50 times its volume. It is less dense than Earth; the gravity at the top of its atmosphere is 11% weaker. Its equatorial diameter is 31,800 mi (51,100 km). Uranus has 10 sharply defined narrow, dark rings, with broad dust bands between them; the rings consist mainly of boulder-size chunks of dark material. Uranus also has at least 27 moons (most named after Shakespearean characters) and a magnetic field about as strong as Earth's. The planet rotates once every 17 hours around an axis that, unusually, is almost parallel to the ecliptic; from Earth it appears to spin on its side. It takes 84 years to orbit the Sun, at a mean distance of 1.78 billion mi (2.87 billion km). It has no solid surface; its fluid interior is thought to consist of a mixture of rock, ices, and gas, with little or no rocky core. Its upper atmosphere is mostly hydrogen and helium; the blue-green colour comes from absorption of red light by the small amount of methane present.

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Uranus¹

Greek myth the personification of the sky, who, as a god, ruled the universe and fathered the Titans and Cyclopes on his wife and mother Gaea (earth). He was overthrown by his son Cronus

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Uranus²

one of the giant planets, the seventh planet from the sun, sometimes visible to the naked eye. It has about 15 satellites, a ring system, and an axis of rotation almost lying in the plane of the orbit. Mean distance from sun: 2870 million km; period of revolution around sun: 84 years; period of axial rotation: 17.23 hours; diameter and mass: 4 and 14.5 times that of earth respectively

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Uranus [′yu̇r·ə·nəsoryu̇′rā·nəs]

(astronomy)

A planet, seventh in the order of distance from the sun; it has five known satellites, and its equatorial diameter is about four times that of the earth.

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Uranus - Hideyuki Nakashima <nakashim@el.go.jp>, 1993. A logic-based knowledge representation language. An extension of Prolog written in Common Lisp, with Lisp-like syntax. Extends Prolog with a multiple world mechanism, plus term descriptions to provide functional programming. ftp://etlport.etl.go.jp/pub/uranus/ftp.

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Uranus 

the seventh major planet from the sun in the solar system; its astronomical symbol is or ♅. One of the giant planets, Uranus was discovered by W. Herschel in 1781. The planet had been observed accidentally even earlier but remained unrecognized among the stars.

Uranus revolves around the sun at a mean distance of 19.19 astronomical units in a nearly circular orbit. The orbital eccentricity is 0.047, and the inclination of the orbit to the ecliptic is only 0.77°. Uranus completes one revolution around the sun in 84.015 years, or 30,685 earth days. The synodic period, during which the oppositions and conjunctions are repeated, is 369.7 days. Since Uranus is a sufficiently bright object of about sixth magnitude, it is easily observed through binoculars, but a telescope with a magnification of not less than 60 × is required to resolve the disk. The planet is barely distinguishable with the naked eye. The apparent diameter of Uranus ranges from 3.4″ to 4.3″. The true equatorial diameter of the planet is 50,700 km, or 3.98 times the diameter of the earth. The volume of Uranus is 61 times that of the earth, but its mass is 14.56 times that of the earth because the mean density of the planet is low and equal to 1.32 g/cm³, which is characteristic of the giant planets. The acceleration gravity at the planet’s equator is 1,040 cm/sec² minus 60 cm/sec² due to centrifugal acceleration, and the escape velocity is 22 km/sec.

The figure of Uranus is highly flattened at the poles, the oblate-ness being about 1:33. The flattening reflects the rapid rotation of the planet about its axis; the rotation period is 10.8 hours. Because no features are observable on the disk of Uranus, the fact that the planet rotates cannot be established from direct observations. The rotation period was determined on the basis of periodic changes in the planet’s brightness with an amplitude of up to 0.15 magnitude and on the basis of the Doppler shift of the planet’s spectral lines, which makes it possible to obtain the equatorial linear rotation velocity. Most of the planets, if viewed from their north poles, rotate counterclockwise, that is, in the same direction as they revolve around the sun. In contrast, Uranus—like Venus—rotates in the retrograde direction. The axis of rotation of Uranus lies nearly in the orbital plane, forming an angle of 98° with a line perpendicular to the orbit.

Because of its great distance from the sun, Uranus receives very little light and heat from the sun, nearly 370 times less than that received by the earth. However, the reflectivity of Uranus is very great, the highest among the planets. The spherical albedo of Uranus is 0.93, and its geometric albedo is 0.57. If Uranus reflects all of the sun’s thermal radiation with such effectiveness, the planet’s surface temperature should be very low, less than 90°K (–180°C). Such a low surface temperature is confirmed by measurements in the infrared region, where the mean temperature was found to be only 55±3°K. At the same time, the temperature measured in the centimeter wavelength range is considerably greater than 100°K, which indicates the existence of a heat flux from the interior of the planet. The high albedo of Uranus indicates the presence of a thick atmosphere. Spectroscopy has revealed the presence of molecular hydrogen (H₂) in the amount of 100 kilometer-atmospheres (km-atm) above the cloud layer and of methane (CH₄) in the amount of 3 to 150 km-atm, according to various estimates. The atmospheric pressure at the level of the clouds is estimated as 3 atm. Theoretical studies of the internal structure of Uranus have yielded the following results: (1) the outer gaseous envelope consists of H₂, He, and CH₄; (2) the total mass of the three gases is about 10 percent of the mass of the planet; (3) the thickness of the envelope is 27 percent of the radius of the planet; and (4) a liquid core, consisting mostly of water, lies beneath the gaseous envelope.

Uranus has five satellites, which move in the planet’s equatorial plane and in the direction of the planet’s rotation. All five satellites are faint and can be observed only with large telescopes. Herschel discovered the two outermost and brightest satellites, Titania and Oberon, in 1787. W. Lassell discovered Ariel and Umbriel, which are not as bright, in 1851. Miranda, the innermost satellite, was discovered through photography by the American astronomer J. Kuiper in 1948; the magnitude of Miranda is 16.5. The size of the satellites can only be roughly estimated on the basis of their brightness. The largest, Titania, has a diameter of between 500 and 1,300 km; the smallest, Miranda, has a diameter of 150 to 500 km.

Rings around Uranus were discovered in 1977.

REFERENCES

Moroz, V. I. Fizika planet. Moscow, 1967.
Martynov, D. Ia. Planety: Reshennye i nereshennye problemy. Moscow, 1970.

D. IA. MARTYNOV