Hi all,
Had to go away to Sydney for a couple of days, so only just saw your replies. I'm starting to think I'm completely on the wrong track with the astronomy thing. The puzzle writer has given a little clue, which refers to earlier puzzles, so I'll have a look at that. Thanks very much everybody.
big_dav_2001, Here's the link to the puzzle site. They're not easy, but they're fun.
http://www.aenigma-puzzles.net/welcome.html