The ancient civilizations looked up to the sky and saw that some of the shiny things moved against the fixed backdrop of stars. They assumed in their simplicity that these moving things in the sky must be gods of great power, and so they named them and made up stories about them. Such is the nature of simple folk with no science to resort to. However, while looking up and seeing all of this, the were standing on the GROUND (Latin TERRA, Old English EORTHE). They didn't realize that the TERRA/EORTHE that the were standing on was just another of those shiny things that move around in the sky, so they did NOT call it a god and invent a mythology about it. (Actually, that's not entirely true -- there are whole mythologies just dealing with the spirits that inhabit the ground and water and the flora and fawna, but that's a different story.) By the time anyone got around to realizing we lived on a planet, everyone was already calling it by whatever local name they had for "ground / dirt / this thing we live on", which, in English, became Earth.