Since the 20th century there have been two main types of astronomy, "observational" and "theoretical" astronomy. Observational astronomy uses telescopes and cameras to "observe" or look at stars, galaxies and other astronomical objects. Theoretical astronomy uses maths and computer models to explain the observations and predict what might happen. Working together, theories predict what should happen and observations show whether the predictions work. The main work of astronomy is to explain puzzling features of the universe. For thousands of years the most important issue was the motions of planets; now many other topics are studied.