Saroscyclus

Saroscyclus, also known as the Saros cycle, is an approximate 18-year, 11-day, and 8-hour period after which the relative geometry of the Sun, Earth, and Moon repeats closely enough to produce eclipses with similar characteristics. The cycle is the result of the near alignment of three lunar orbital periods: the synodic month (about 29.53 days, the Moon’s phase cycle), the draconic month (about 27.212 days, the node passage), and the anomalistic month (about 27.554 days, the perigee).

Because these three periods drift at different rates, their alignment recurs only after many lunations. The canonical Saros length is about 6,585 days, or 223 synodic months, and corresponds to nearly 239 anomalistic months and 242 draconic months as well. When the alignment occurs, the Sun-Earth-Moon geometry is similar enough that a near-identical eclipse type tends to recur.

In practice, an eclipse in a given Saros cycle is followed, after one Saros, by another eclipse of similar kind. The 8-hour difference in the cycle means the subsequent eclipse appears about 120 degrees of longitude west of the previous one, so visibility shifts to a different region of the globe. Each Saros cycle comprises a series of eclipses, progressing across the sky over centuries as the Moon’s nodes and orbital elements slowly evolve.

Historically, the Saros cycle was known to ancient civilizations, including the Babylonians, and later became a foundational tool for predicting eclipses. Today it remains a useful framework in eclipse studies, though modern predictions rely on precise orbital computations.