Lionel Windsor has a post over at the Sola Panel talking about God, the universe and all that. Kinda fun.
Well, you could point out that weather and tide and climate predictions need detailed solar, lunar and planetary modelling. You could also point out that car engines need modern mechanics, which is all based on the laws of motion formulated by Isaac Newton, who used the orbits of planets to calculate and build his theories. Or you could point to the humble GPS satellite navigator, which relies on Einstein's theory of relativity and orbiting satellites. Of course, astronomy is useful; after all, it helps us to work out whether it's raining, and how to drive quickly to the cricket and back without taking a wrong turn!