n.
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 consisting mainly of boulder-sized chunks of dark material. It also has at least 21 moons (most named after Shakespearean characters) and a magnetic field about as strong as Earth's. The planet is estimated to rotate about every 17 hours around an unusual axis that 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). Its 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.