saros

The period of time over which the sequence of lunar and solar eclipses repeats. Its length, 6,585.32 days (about 18 years), has been known since antiquity. After this period, the Earth, Sun and Moon will have returned to the same relative positions. Succeeding eclipses in a particular saros series occur about 8 hours later and fall nearly 120 degrees of longitude further west. This was known to the ancient Babylonian and Mayan astronomers and to the builders of Stonehenge.