Uranus 天王星(英)

Uranus¹ is the seventh planet from the Sun and the third largest (by diameter). Uranus is larger in diameter but smaller in mass than Neptune².

　　orbit: 2,870,990,000 km (19.218 AU) from Sun

　　diameter: 51,118 km (equatorial)

　　mass: 8.683e25 kg

　　Hardcopy The New Solar System

　　Summarizes what we've learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets.

　　The Compact NASA Atlas³ of the Solar System

　　This 'road map' of the solar system is the definitive⁴ guide for planetary science.

　　The Tempest by William Shakespeare

　　Uranus is the ancient Greek deity⁵ of the Heavens, the earliest supreme⁶ god. Uranus was the son and mate of Gaia the father of Cronus (Saturn⁷) and of the Cyclopes and Titans (predecessors of the Olympian gods).

　　Uranus, the first planet discovered in modern times, was discovered by William Herschel while systematically⁸ searching the sky with his telescope on March 13, 1781. It had actually been seen many times before but ignored as simply another star (the earliest recorded sighting was in 1690 when John Flamsteed cataloged it as 34 Tauri). Herschel named it "the Georgium Sidus" (the Georgian Planet) in honor of his patron, the infamous⁹ (to Americans) King George III of England; others called it "Herschel". The name "Uranus" was first proposed by Bode¹⁰ in conformity¹¹ with the other planetary names from classical mythology¹² but didn't come into common use until 1850.

　　Uranus has been visited by only one spacecraft, Voyager 2 on Jan 24 1986.

　　Most of the planets spin on an axis¹³ nearly perpendicular¹⁴ to the plane of the ecliptic but Uranus' axis is almost parallel to the ecliptic. At the time of Voyager 2's passage, Uranus' south pole was pointed¹⁵ almost directly at the Sun. This results in the odd fact that Uranus' polar regions receive more energy input¹⁶ from the Sun than do its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles. The mechanism¹⁷ underlying¹⁸ this is unknown.

　　Actually, there's an ongoing¹⁹ battle over which of Uranus' poles is its north pole! Either its axial inclination²⁰ is a bit over 90 degrees and its rotation²¹ is direct, or it's a bit less than 90 degrees and the rotation is retrograde. The problem is that you need to draw a dividing line *somewhere*, because in a case like Venus there is little dispute that the rotation is indeed retrograde (not a direct rotation with an inclination of nearly 180).

　　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 (and Neptune) are in many ways similar to the cores of Jupiter and Saturn minus the massive liquid metallic²² 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' atmosphere is about 83% hydrogen, 15% helium and 2% methane²³.

　　Like the other gas planets, Uranus has bands of clouds that blow around rapidly. But they are extremely faint, visible only with radical²⁴ image enhancement of the Voyager 2 pictures. Recent observations with HST show larger and more pronounced streaks²⁵. Further HST observations show even more activity. Uranus is no longer the bland²⁶ boring planet that Voyager saw! It now seems clear that the differences are due to seasonal²⁷ effects since the Sun is now at a lower Uranian latitude²⁸ which may cause more pronounced day/night weather effects. By 2007 the Sun will be directly over Uranus's equator.

　　Uranus' blue color is the result of absorption of red light by methane in the upper atmosphere. There may be colored bands like Jupiter's but they are hidden from view by the overlaying methane layer.

　　Like the other gas planets, Uranus has rings. Like Jupiter's, they are very dark but like Saturn's they are composed of fairly large particles ranging up to 10 meters in diameter in addition to fine dust. There are 11 known rings, all very faint; the brightest is known as the Epsilon ring. The Uranian rings were the first after Saturn's to be discovered. This was of considerable importance since we now know that rings are a common feature of planets, not a peculiarity²⁹ of Saturn alone.

　　Voyager 2 discovered 10 small moons in addition to the 5 large ones already known. It is likely that there are several more tiny satellites within the rings.

　　Uranus' magnetic field is odd in that it is not centered on the center of the planet and is tilted³⁰ almost 60 degrees with respect to the axis of rotation. It is probably generated by motion at relatively³¹ shallow depths within Uranus.

　　Uranus is sometimes just barely visible with the unaided eye on a very clear night; it is fairly easy to spot with binoculars³² (if you know exactly where to look). A small astronomical³³ telescope will show a small disk. There are several Web sites that show the current position of Uranus (and the other planets) in the sky, but much more detailed³⁴ charts will be required to actually find it. Such charts can be created with a planetarium³⁵ program.

　　Uranus' Satellites

　　Uranus has 21 named moons and six unnamed ones:

　　Unlike the other bodies in the solar system which have names from classical mythology, Uranus' moons take their names from the writings of Shakespeare and Pope.

　　They form three distinct classes: the 11 small very dark inner ones discovered by Voyager 2, the 5 large ones, and the newly discovered much more distant ones.

　　Most have nearly circular orbits in the plane of Uranus' equator (and hence at a large angle to the plane of the ecliptic); the outer 4 are much more elliptical.

　　 Distance Radius³⁶ Mass

　　Satellite (000 km) (km) (kg) Discoverer Date

　　------ ------ ------ ------- ------- -----

　　Cordelia 50 13 ? Voyager 2 1986

　　Ophelia 54 16 ? Voyager 2 1986

　　Bianca 59 22 ? Voyager 2 1986

　　Cressida 62 33 ? Voyager 2 1986

　　Desdemona 63 29 ? Voyager 2 1986

　　Juliet 64 42 ? Voyager 2 1986

　　Portia 66 55 ? Voyager 2 1986

　　Rosalind 70 27 ? Voyager 2 1986

　　2003U2 75 6 ? Showalter 2003

　　Belinda 75 34 ? Voyager 2 1986

　　1986U10 76 40 ? Voyager 2 1986

　　Puck 86 77 ? Voyager 2 1985

　　2003U1 98 8 ? Showalter 2003

　　Miranda 130 236 6.30e19 Kuiper 1948

　　Ariel 191 579 1.27e21 Lassell 1851

　　Umbriel 266 585 1.27e21 Lassell 1851

　　Titania 436 789 3.49e21 Herschel 1787

　　Oberon 583 761 3.03e21 Herschel 1787

　　2001U3 4281 6 ? Sheppard 2003

　　Caliban 7169 40 ? Gladman 1997

　　Stephano 7948 15 ? Gladman 1999

　　Trinculo 8578 5

　　Sycorax 12213 80 ? Nicholson 1997

　　2003U3 14689 6 ? Sheppard 2003

　　Prospero 16568 20 ? Holman 1999

　　Setebos 17681 20 ? Kavelaars 1999

　　2002U2 21000 6 Sheppard 2003

　　Uranus' Rings

　　 Distance Width

　　Ring (km) (km)

　　------- -------- -----

　　1986U2R 38000 2,500

　　6 41840 1-3

　　5 42230 2-3

　　4 42580 2-3

　　Alpha 44720 7-12

　　Beta 45670 7-12

　　Eta 47190 0-2

　　Gamma 47630 1-4

　　Delta 48290 3-9

　　1986U1R 50020 1-2

　　Epsilon 51140 20-100

　　(distance is from Uranus' center to the ring's inner edge)