Thousands of years ago in ancient Mesopotamia, somebody noticed that there were two different sets of lights in the night sky (other than the moon). One set consisted of a large number of tiny lights that rose and set every night, but always seemed to stay in the same position relative to each other. The ancients organized some of these well-behaved lights into groups that today we call constellations. The spot on the horizon where a particular constellation rose moved westward a little bit every night until, a year later, it circled all the way back where it had started (some constellations actually disappeared for a while during parts of the year.)<\/p>\n\n\n\n\n