Basically, you could mark a place and time, then have a way for the game to teleport you to that spot and time, so if there's a 6 way eclipse or something, you can see it again.

It would be cool if you were able to place "surface labels" onto areas of a planets surface. So for instance, you find a noteable feature
like a particularly large crater or ocean or something, you can then place a label there (separate from planet/moon labels etc) with a name or description of the location.