The answer to the riddle, "What has been around for a million years but is only a month old?" is "the moon." This is just one of many astronomy riddles.
Some riddles play with the phases of the moon, such as "When is the moon the heaviest?" Naturally, the answer is, "When it's full." Space travel is also covered in "Have you heard about the cow astronaut?" If you have, you know, "He landed on the mooooooon." Lots of riddles focus on the sun, including, "Why didn't the sun go to college?" It did not attend because, "It already has a million degrees." Planets come into play, as well. "What did Mars say to Saturn?" The correct response is, "Give me a ring sometime."