saros

(astronomy) A cycle of time after which the centers of the sun and moon, and the nodes of the moon's orbit return to the same relative position; this period is 18 years 11⅓ days, or 18 years 10⅓ days if 5 rather than 4 leap years are included.

time The inherent period for recurrence of eclipses, essentially the least common multiple of eclipse year and synodic month = 6 585.32 days, = 18.998~ eclipse years = 223.000 synodic months = 18 y 11.32 d. A period for cruder recurrence of eclipse sequences has 1 386.38 days (3 y 291.38 d), close to 4 eclipse years and to 47 synodic months.

The fractional part of the day count causes each solar eclipse of any one sequence episode in a given series to be centred about a third Earth's rotation westward from the corresponding one of the previous episode (115° for the saros, 137° for the lesser period). See also Metonic cycle.

A sequence of eclipses separated by 1 saros is called an eclipse series. A solar eclipse series contains about 70 eclipses spread over about 1 300 years; a lunar eclipse series extends over about 900 years. Many eclipse series of each type are running at any one time.