I always thought they were doing it just for the coolness. It makes Earth sound different, like it's not the same Earth we live on but changed in some way, which in Sci-Fi is often the case, even if that change is one of politics or ethics or...
Another thought that just occurred whilst writing the above, with Terra, you get to call people who come from there Terrans, but Earthlings which I assume is the equivalent for Earth, sounds rather cheesy imo. Granted you could just say Humans, but then that's not identifying the place of origin if you assume we've spread out across the galaxy.
Harry: I've often heard Astronomers use the term Lunar to talk about the Moon, never heard the term Luna although I suppose it would be hard to distinguish from a pronunciation only. Frequently hear Lunar mentioned anytime the topic is the surface of the moon though.
As for the Sun, outside of games and Sci-Fi books which do use Sol, I've only ever heard our sun referred to as the Sun. Not saying that's always the case, just the frequent one for me.
Caleb: Add another Amateur Astronomy to your list then. Well, Amateur may be too strong a word, but certainly hobbyist of many years
