Saros Cycle

SAR-ohs SY-kuhl

Definition

The Saros is the cycle over which eclipses repeat — about 18 years, 11 days, and 8 hours, equal to 223 synodic months (New Moon to New Moon) and very close to 242 draconitic months (returns of the Moon to the same node). Each eclipse in a Saros series looks much like the one before it in type and in node position. The leftover 8 hours shifts the place of greatest eclipse about 120° westward each time, so a Saros series usually makes a full turn around the globe every three cycles — the "exeligmos" of roughly 54 years.

In Tradition

In the history of Babylonian astronomy, the Saros is the central observed pattern behind System A's eclipse prediction. Hunger and Pingree state that the "Saros possibly goes back to -746, though more likely the late sixth century B.C." Rochberg writes that "for the moon, the data are collected for 18 years before the goal year (the Saros cycle)," tying it to the Goal-Year Texts — Babylonian forecasts for a target year. Modern Western practice uses the Saros to group eclipses into families.

In Practice

To forecast within an eclipse family, an astronomer identifies the Saros series of an eclipse from its node, type, and date; the next eclipse in that series comes about 18 years, 11 days, and 8 hours later, shifted around 120° west in longitude, with much the same geometry. Modern Western astrology tracks the Saros series of the eclipse nearest before a person's birth — the pre-natal eclipse Saros — as a long thematic thread across the life, with each later eclipse in that series re-stirring the family theme. The Saros also underpins the Babylonian Goal-Year method: what the Moon will do in a target year is anticipated from the same calendar position 18 years earlier.

Historical Origin

The Saros is attested in Late Babylonian mathematical astronomy as Column Φ of System A (defined as Φ = 223 synodic months − 6585 days). Rochberg works the parameter out on p. 144 of The Heavenly Writing as the basis for predicting eclipses from the Moon's node and latitude; Hunger and Pingree treat it on pp. 239 and 245-246 as the period at the foundation of System A's lunar theory. The 18-year forerunner period for the lunar columns of Goal-Year Texts is documented per Sachs 1948.

Further Reading

  • Francesca Rochberg, The Heavenly Writing: Divination, Horoscopy, and Astronomy in Mesopotamian Culture
  • Hermann Hunger, Astral Sciences in Mesopotamia
  • Hermann Hunger, Astrological Reports to Assyrian Kings