The Saros Cycle: Periodicity and Repetition of Eclipses

The Saros Cycle: The Clock of Eclipses

Since antiquity, astronomers have noticed that eclipses are not random but follow a remarkable periodicity called the Saros cycle. This cycle, lasting about 18 years, results from a subtle combination of three fundamental periods of the Moon's orbit: the synodic month, the draconic month, and the anomalistic month.

The Saros cycle represents one of the most fascinating regularities in celestial mechanics. Understanding this cycle not only allows for the precise prediction of eclipses but also helps appreciate the wonderful harmony governing the movements of our Earth-Moon-Sun system. The ancient astronomers who discovered this cycle without modern tools testify to human ingenuity in the face of the mysteries of the cosmos.

N.B.: The word "Saros" appears in European astronomy in the 17th century. It was Edmond Halley (1656–1742), the famous British astronomer, who introduced this term in 1691. Halley had read a work by the Byzantine Georgios Syncellos (8th century), where the Greek word (saros), meaning "repetition" or "cycle," appeared—but in reality, it designated among the ancient Chaldeans a period of 3600 years, not the cycle of eclipses.

Fundamental Orbital Periods of the Moon and the Sun

The Saros cycle emerges from the approximate numerical relationship between these periods:

• Synodic month: average duration between two consecutive new moons, about 29.53 days (Moon's evolution around the Earth).
• Draconic month: interval between two passages of the Moon through the same orbital node (ascending or descending), about 27.21 days.
• Anomalistic month: interval between two consecutive passages of the Moon at perigee, about 27.55 days.
• Ecliptic year: The approximate return of the Sun to the same nodes (346.62 days)

Eclipse Families of the Saros

Each series of eclipses belonging to the same Saros cycle is called a "Saros series." A series usually begins with a partial eclipse at one of the Earth's poles, then evolves to become central (total, annular, or hybrid) before returning to partial eclipses at the other pole. A complete Saros series lasts between 1226 and 1550 years and contains 69 to 87 eclipses.

Predicting Eclipses Using the Saros

Due to the additional 8 hours in the Saros period (⅓ of a day), each successive eclipse in a series occurs about 120 degrees of longitude further west. This particularity means that three Saros cycles (called Exeligmos) are needed for an eclipse to recur approximately at the same geographic location.

Table of Main Characteristics of the Saros Cycle

Sources: NASA Eclipse Web Site (Fred Espenak), NASA ADS.