What does 'Astra' here mean? Surely they must be referring to the Latin word.
Because in another dead language of antiquity, Sanskrit, it means "weapon". Which would be a bit too on-the-nose.
Luna, Terra, Sol, Astra. Though Sun is also a star, should have called it Galaxy or something.
Quite clearly in the same vein as Sol, Terra, Luna.
It's clearly an extension of the previous naming: Luna, terra, sol