... Mercury is the closest planet to the Sun and the eighth largest. ... An account of the non-discovery of a planet inside Mercury's orbit. ... Heraclitus even believed that Mercury and Venus orbit the Sun, not the Earth. ... 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. ... Actually Earth's density is due in part to gravitational compression; if not for this, Mercury would be denser than...