Mercury (planet)

Mercury (planet), in astronomy, planet closest to the sun, with a mean solar distance of 36 million mi (57.9 million km). Its eccentric elliptical orbit brings it within 28.5 million mi (46 million km) of the sun at perihelion (point nearest to the sun) and takes it 43.5 million mi (70 million km) from the sun at aphelion (point farthest from the sun). Its diameter is 3,031 mi (4,878 km), and its mass about 0.054 that of the earth. Mercury revolves around the sun in just under 88 days—faster than any other planet—and rotates on its axis in about 59 days. Albert Einstein's successful prediction that Mercury's orbit would advance by 43 in (109 cm) per century is usually regarded as a confirmation of the general theory of relativity. Night surface temperature on this dry and airless planet is believed to be about −315ÉF (−E193C), midday equatorial temperature over 648ÉF (342ÉC). No plant life is believed to exist. Mercury also has no known satellites. The U.S. Mariner space probe revealed (1974–75) that Mercury has a moonlike, heavily cratered surface and a slight magnetic field.

See also: Planet; Solar System.