With games, you can often cheat your way through those laws of physics... Is there a enough reason, gameplay-wise that you should keep track of earth movements and universal expansion to impliment it, or can you cut some corners for the sake of being able to spend resources where they matter more?
The prime rule of games is "it must be fun" and complete realism isn't a requirement of fun.
If you let a player travel back to the 14th century in your game, then if you want to take celestrial movements into account, the player would end up in the vacuum of space at the other side of the time-rift because the earth he left from is nolonger in the same place. But since there is no perpose for including such rules of physics, you simply don't.