From Chris Madden, http://www.goma.demon.co.uk/moon/moonselect.html

The orbital period and phases of the moon provide the basic time keeping of month and week. Also, since the moon is relatively close, ancient astronomers were able to deduce that it orbited the earth, was round, was shaded by the earth in lunar eclipses, and so forth -- a good beginning to understanding the way the solar system works!