I think the comic speaks for itself. Just great. This and other rather dry comics can be found at The Perry Bible Fellowship. Some I like, but most of it is just to dark for me - black is not the color of my humor, maybe a darkish grey sometimes.
Because some of you are not as geek as some of the astronomers wannabes (and even if you are friends with an astronomer wannabe you may have more interesting things to care about), here's a good article to clarify the joke.