Mercury

Mercury is the innermost and smallest planet in the solar system, orbiting the Sun once every 88 days. It ranges in brightness from about -2.0 to 5.5 in apparent magnitude, but is not easily seen — its greatest angular separation from the Sun (greatest elongation) is only 28.3° (it can only be seen in twilight).

Comparatively little is known about the planet: the only spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only 40%–45% of the planet's surface. Mercury's surface is very similar in appearance to that of the Moon, showing extensive mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years.

The small number of unmanned missions to Mercury means that its geology is the least well understood of the terrestrial planets. It has no natural satellites and no substantial atmosphere.It is one of the four terrestrial planets, meaning that like the Earth it is a rocky body. It is the smallest of the four, with a diameter of 4879 km at its equator. Mercury consists of approximately 70% metallic and 30% silicate material

Until 1962 it was thought that Mercury's "day" was the same length as its "year" so as to keep that same face to the Sun much as the Moon does to the Earth. But this was shown to be false in 1965 by doppler radar observations. It is now known that Mercury rotates three times in two of its years.

Mercury has been known since at least the time of the Sumerians (3rd millennium BC). It was given two names by the Greeks: Apollo for its apparition as a morning star and Hermes as an evening star.

Greek astronomers knew, however, that the two names referred to the same body. Heraclitus even believed that Mercury and Venus orbit the Sun, not the Earth.