Saros (astronomy)

The Moon's orbit around Earth is inclined to the plane of Earth's orbit around the Sun, so eclipses can occur only when a new or full Moon is near one of the two orbital nodes. The Saros results from a close alignment among three lunar periods: the synodic month, from new Moon to new Moon; the anomalistic month, from perigee to perigee; and the draconic month, from node to node. About one Saros equals 223 synodic months, 239 anomalistic months and 242 draconic months, with the latter two matching the interval to within a few hours. This near recurrence brings the Sun, Earth and Moon into similar eclipse-producing geometry again.

A Saros lasts about 6,585.3 days, or approximately 18 years, 11 days and 8 hours. The extra roughly one-third of a day means Earth rotates about 120 degrees farther before the next event in the series. As a result, a solar eclipse's path shifts westward by about 120 degrees with each Saros, rather than repeating over the same locations. After three Saros periods—about 54 years and 34 days—the series returns to roughly the same geographic region; this triple-Saros cycle is called the Exeligmos.

A Saros series is the ordered family of eclipses separated from one another by one Saros. The similarity is not exact, and the series has a beginning and an end because the lunar orbital periods are not perfectly commensurate. A typical solar Saros series starts with partial eclipses near one polar region, progresses through central eclipses (total, annular or hybrid), and ends with partial eclipses near the opposite pole. NASA's eclipse reference describes solar series lasting about 1,226 to 1,550 years and comprising 69 to 87 eclipses, with the precise count and duration varying between series.

The Saros is useful for recognizing and organizing recurring eclipse patterns, not for predicting that an eclipse will be visible from a particular location: detailed circumstances and visibility maps are still needed. NASA notes that the cycle was known to the Chaldeans as a period when lunar eclipses seemed to repeat, and that it applies to solar eclipses as well. The term's use as the name of an eclipse period has a separate history: Encyclopaedia Britannica traces its adoption for that purpose to the English astronomer Edmund Halley in 1691, after an earlier mistaken association of the word with a 223-month cycle.