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PLANET HISTORY   Above: The Greek sign for Uranus. Uranus is the seventh planet from the Sun and the third largest (by diameter). Uranus is larger in diameter but smaller in mass than Neptune. Careful pronunciation may be necessary to avoid embarrassment; say "YOOR a nus" , not "your anus" or "urine us". Uranus is the ancient Greek deity of the Heavens, the earliest supreme god. Uranus was the son and mate of Gaia the father of Cronus (Saturn) 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 systematically 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 infamous (to Americans) King George III of England; others called it "Herschel". The name "Uranus" was first proposed by Bode in conformity with the other planetary names from classical mythology but didn't come into common use until 1850. Uranus has been visited by only one spacecraft, Voyager 2 on Jan 24 1986.     Surface and Atmosphere   Most of the planets spin on an axis nearly perpendicular 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 pointed almost directly at the Sun. This results in the odd fact that Uranus' polar regions receive more energy input from the Sun than do its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles. The mechanism underlying this is unknown. Actually, there's an ongoing battle over which of Uranus' poles is its north pole! Either its axial inclination is a bit over 90 degrees and its rotation 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 metallic 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% methane. Like the other gas planets, Uranus has bands of clouds that blow around  rapidly. But they are extremely faint, visible only with radical image enhancement of the Voyager 2 pictures. Recent observations with HST show larger and more pronounced streaks. Further HST observations show even more activity. Uranus is no longer the bland boring planet that Voyager saw! It now seems clear that the differences are due to seasonal effects since the Sun is now at a lower Uranian latitude 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 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 peculiarity 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 tilted almost 60 degrees with respect to the axis of rotation. It is probably generated by motion at relatively 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 binoculars (if you know exactly where to look). A small astronomical 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 detailed charts will be required to actually find it. Such charts can be created with a planetarium program such as Starry Night.     Uranus' Satellites and Rings   Uranus has 15 named moons plus 5 recently discovered ones which as yet have not been given official names; it thus has the most known moons of any of the planets. 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 (right), 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.   Satellite Distance (000 km) Radius (km) Mass (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 Belinda 75 34 ? Voyager 2 1986 1986U10 75 20 ? Karkoschka 1999 Puck 86 77 ? Voyager 2 1985 Miranda 130 236 6.30 x 1019 Kuiper 1948 Ariel 191 579 1.27 x 1021 Lassell 1851 Umbriel 266 585 1.27 x 1021 Lassell 1851 Titania 436 789 3.49 x 1021 Herschel 1787 Oberon 583 761 3.03 x 1021 Herschel 1787 Caliban 7100 30 ? Gladman 1997 1999U1 10000 20 ? Kavelaars 1999 Sycorax 12200 60 ? Gladman 1997 1999U2 25000 20 ? Kavelaars 1999     Ring Stats (distance is from Uranus' center to the ring's inner edge) Ring Distance (km) Width (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       Statistics and Facts   Here you will find all the stats and facts for Uranus. This includes everything from the escape velocity needed to leave its surface to surface temperatures! All these figures will be compared with Earth...   Discoverer: William Herschel Discovery Date: 13 March 1781   Uranus Earth Ratio (Uranus/Earth) Mass (1024 kg) 86.832 5.9736 14.536 GM (x 106 km3/s2) 5.794 0.3986 14.536 Volume (1012 km3) 6,833 108.321 63.08 Equatorial radius (km) 25,559 6,378.1 4.007 Polar radius (km) 24,973 6,356.8 3.929 Volumetric mean radius (km) 25,362 6,371.0 3.981 Mean density (kg/m3) 1,270 5,515 0.230 Surface gravity (eq.) (m/s2) 8.69 9.78 0.889 Escape velocity (km/s) 21.3 11.19 1.903 Ellipticity (Flattening) 0.02293 0.00335 6.84 Moment of inertia (I/MR2) 0.225 0.3308 0.680 Visual magnitude V(1,0) -7.19 -3.86 --- Bond albedo 0.300 0.306 0.980 Visual geometric albedo 0.51 0.367 1.390 Solar irradiance (W/m2) 3.71 1,367.6 0.0027 Black-body temperature (K) 58.2 254.3 0.229 J2 (x 10-6) 3,343.43 1,082.63 3.088 Semimajor axis (106 km) 2,872.46 149.60 19.201 Sidereal orbit period (days) 30,685.4 365.256 84.011 Tropical orbit period (days) 30,588.740 365.242 83.749 Perihelion (106 km) 2,741.30 147.09 18.637 Aphelion (106 km) 3,003.62 152.10 19.748 Synodic period (days) 369.66 --- --- Mean orbital velocity (km/s) 6.81 29.78 0.229 Max. orbital velocity (km/s) 7.11 30.29 0.235 Min. orbital velocity (km/s) 6.49 29.29 0.222 Orbit inclination (deg) 0.772 0.00 --- Orbit eccentricity 0.0457 0.0167 2.737 Sidereal rotation period (hrs) -17.24* 23.9345 0.720 Length of day (hrs) 17.24 24.0000 0.718 Obliquity to orbit (deg) 97.77 23.45 4.169 Min. distance from Earth (106 km) 2581.9 Max. distance from Earth (106 km) 3157.3 **Don't understand these measurements? Click here to get help with the definitions and notes**   * Magnetic coordinates (as determined by the Voyager 2 Radio Science experiment)     Atmospheric Composition (uncertainty in brackets)   82.5% [3.3%] Molecular Hydrogen (H2) 15.2% [3.3%] Helium (He) 2.3% Methane (CH4)   Trace Amounts Of (ppm): Hydrogen Deuteride (HD) - 148 Ammonia ice Water ice Ammonia hydrosulfide Methane ice       Open Issues Why doesn't Uranus radiate more heat than it receives from the Sun as the other gas planets do? Is its interior cold? Why is its axis so unusually tilted? Was it due to a massive collision? Why do Uranus and Neptune have so much less hydrogen and helium than Jupiter and Saturn? Is it simply because they are smaller? or because they're farther from the Sun? What will happen to Uranus's weather as it progresses thru its seasons?     Picture Gallery All three photos below are of great quality and very large!! So download time will be slow, but well worth it... Click on any image to view it full size!