Overview and Exploration

• Name: God of the heavens, father of Cronus/Saturn and of the Cyclopes and Titans.
• Viewing: Barely visible to the eye; It had been mistaken for a star until William Herschel in 1781 discovered that it moved (at first, he thought it was a comet); He also named it Georgium Sidus ("George").
• Orbit: Almost 20 AU from the Sun, almost circular [eccentricity 0.02, period 84 years].
• Planet: A blue-green gas ball, 4 times the Earth's size [15 times the mass, density 1.2 times that of water].
• Exploration: Visited only by Voyager 2 (in 1986), which discovered most of its moons.

Features

• Appearance: A hazy, blue-green disk from methane clouds (although the atmosphere is mostly H and He); The oddball among jovian planets, does not produce much internal heat (is the core too well insulated?), and it is not heated by the Sun as much as Saturn, so the atmosphere keeps the temperature almost uniform, and Uranus has few distinct cloud features or storms.
• Rotation: Differential (faster at the poles), 17.2 hrs, tilted about 98° (from a catastrophic impact billions of years ago?); More sunlight at the poles!
• Interior: Small core; In general, a mixture of rocky fragments and various ices (water, methane, ammonia).

  Moons and Rings

• Moons: 27 currently known ones (some found in 2005), but it may have more, with names of characters from writings of Shapespeare and Pope; Most are in the equatorial plane, and are made of rock and water ice; Miranda, which must have suffered a catastrophic impact and has 20-km deep cliffs; 2 sphepherd moons Cordelia and Ophelia (1986) keep thin epsilon ring from spreading.
• [Some recent ones: Most occupy distant, eccentric, retrograde and highly inclined orbits (Why?); Caliban and Sycorax (1997); Three discovered in 1999 appear to be 20 km across, and some discovered in 2003 about 10 km across.]
• Rings: At least 13 including a system discovered in 2005, dark [made of carbon particles from methane or broken asteroid] and narrow [10-100 km], but the outermost one is blue; Seen by stellar occultation from Earth; Sizes range from larger particles [up to several m] to fine dust; Shepherd moons.