Mercury Planet Facts. For comparison, mercury’s size is about a third of earth, and earth has a density of 5.51 g/cm³. In classical greece it was called apollo when it appeared as a morning star just before sunrise and hermes, the greek equivalent of the roman god mercury, when it appeared as an evening star just after sunset.hermes was.

The planet mercury was known by ancient people thousands of years ago. Mercury is often identified with the greek god hermes, the messenger of the gods in roman mythology.