Saros

The Saros is an eclipse cycle in astronomy that describes a period after which solar and lunar eclipses return with nearly identical geometry. The cycle lasts about 6,585 days, which is 18 years, 11 days, and 8 hours, and it causes eclipses to recur at roughly the same geometry of the Sun, Earth, and Moon.

The length of a Saros arises from near-integer relationships among three Moon-Earth orbital periods: the synodic month (new Moon to new Moon), the draconic month (node passage), and the anomalistic month (perigee to perigee). A Saros equals about 223 synodic months, 242 draconic months, and 239 anomalistic months. The extra eight hours shifts the location of the eclipse westward by about one third of the globe, so the next eclipse in the series is visible from a different region.

There are solar and lunar Saros cycles. A single Saros series comprises a sequence of eclipses that share similar geometry and type, evolving over centuries from partial eclipses near the poles to total or annular eclipses near the middle, and then back to partial eclipses before the series ends.

Historically, the Saros was used by ancient civilizations, including the Babylonians and Greeks, to predict eclipses. The term is now widely used in modern eclipse prediction and analysis, remaining a fundamental concept in understanding long-term eclipse patterns.