22. The Saros period

Avid sky-watchers like the Babylonians had no difficulty working out the rudimentary pattern of lunar eclipses. They occur every 5th or 6th month, discounting about 1/3 of eclipses that occur entirely in daylight hours.

A more elusive pattern operates on a lengthy cycle of 223 months. This period of about 18 years is known as a Saros. (The term is loosely applied to a cycle of eclipses separated by 18 years, perhaps better labeled a Saros cycle.) The eclipses of a Saros cycle share a striking feature - they look the same. Hence the timing and prognostic characteristics of an impending eclipse can be determined by checking earlier eclipses of its Saros cycle.