... M ercury is the closest planet to the Sun and the eighth largest. ... Heraclitus even believed that Mercury and Venus orbit the Sun, not the Earth. ... The perihelion of its orbit precesses around the Sun at a very slow rate. 19th century astronomers made very careful observations of Mercury's orbital parameters but could not adequately explain them using Newtonian mechanics. ... Actually Earth's density is due in part to gravitational compression; if not for this, Mercury would be denser than Earth...