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A Saros cycle is an eclipse cycle of 223 synodic months (period from one new moon to the next), or approximately 242 draconic months (the period of the Moon to return to the ascending node of its orbit, i.e. cross the plane of Earth's orbit twice). Thus an eclipse (which happens when a conjunction or opposition of the Sun and Moon occurs in one of the nodes, that is, crossing the plane of the orbit) occurs again one Saros later.

A Saros is also close to 239 anomalistic months (period of the Moon to return to its perigee, i.e. the period of its elliptic orbit), and to 18 anomalistic years. Therefore the circumstances of an eclipse are also very similar to an eclipse one Saros earlier.

A Saros is about 6585 + 1/3 days (18 years and 11 days). So an eclipse one Saros after another occurs about 8 hours later in the day. A solar eclipse will then be visible about 120 deg. of longitude West of the previous location. Therefore another cycle of 3 Saroses, called exeligmos (Greek: "turn of the wheel"), has been used: after an exeligmos, a solar eclipse will again be visible not far from the original location.

There can be 223 possible Saros cycles.

The Saros cycle was probably known to the Chaldeans (ancient Babylonian astronomers), and later to Hipparchos, Pliny (Naturalis Historia II.10[56]) and Ptolemy (Almagest IV.2), but not under this name. The Babylonian "Saros" appears to have been a name for a period of 3600 years. The name "Saros" was first given to the eclipse cycle by Edmund Halley in 1691, who took it from a lexicon by the Byzantine scholar Suidas from the 11th century.