Hm... but when talking about the moon/sun, you usually say "they arrived at the moon" or "they could see the sun" and not "they arrived at moon", etc.

So would that mean if you say "they returned to the earth", it's not capitalized, but saying "they returned to Earth", it is?

It would be easier if the language made it easy to use the Latin terms for everything... eg. "They could see Sol in the day and Luna at night when they arrived on Terra". But no one talks like that. >_<