[PvK]

Personally, I think astronomers are a step behind sci-fi writers on this one. Sol, Earth or Terra, and Luna are acceptable proper names. Having more than one name for the same thing is passable, too, especially with multiple Languages and cultures.

However, "the solar system" and "the moon" are just generic terms, and naturally refer to a particular one when the speaker happens to be in a solar system, or when on a planet with one moon, and not talking about space travel at the time. They don't cut it for proper names, though. Basic logic failure there. Yes, when defining a language based on how it's used, naturally those will pass that test. Kind of like a medieval peasant talking about "the river" because he never leaves his home village and never sees another village. Or someone talking about "the cat" meaning their cat.

PvK