Astronomers sometimes call it a naked-eye event, and while it has little significance for the experts, this week’s sky show involving four heavenly bodies is one anybody can enjoy. On Thursday, April 6, three planets and the moon will all huddle together in a small patch of the late evening sky. In astronomical terms, Mars, Jupiter, Saturn and our only natural satellite will be within 9 degrees of each other (a degree is about the width of two full moons as seen from Earth.)