The innermost planet of the Solar System. In ancient Greece it had two names – Apollo for its appearance as a morning star and Hermes as an evening star – although Greek astronomers knew that a single body was involved. Being named after the fleet-footed god Hermes, or its Roman equivalent Mercury, is especially apt as the planet races around the Sun at an average speed of 48 km/s, completing one circuit in 88 days. Until the 1960s it was thought that Mercury's day (the time it takes to spin on its axis) was the same length as its year (the time it takes to complete one orbit) so that it always kept one face to the Sun. However, Doppler radar measurements in 1965 showed that Mercury actually rotates three times in two of its years. Although Mercury's mean distance from the Sun is 57,910,000 km (0.387 AU), its orbit is very elongated. The range from perihelion to aphelion is 46 to 70 million km.

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(Source: The Internet Encyclopedia of Science »)