Mercury is the closest planet to the Sun and the eighth largest. Mercury is slightly smaller in diameter than the moons Ganymede and Titan but more than twice as massive.
orbit: 57,910,000 km (0.38 AU) from Sun
diameter: 4,880 km
mass: 3.30e23 kg
Mercury was the god of commerce, travel and thievery, in Roman mythology;the Roman counterpart of the Greek god Hermes, the messenger of the Gods. The planet probably received this name because it moves so quickly across the sky.
Since at least the time of the Sumerians (3rd millennium BC). it was seen and recognized. As a morning star and as an evening star it was sometimes given different names. Greek astronomers knew, however, that the different names referred to the same body. Heraclitus even believed that Mercury and Venus orbit the Sun, not the Earth.