Periodicity and Repetition of Eclipses

Saros Cycle: the clock of eclipses

A cosmic periodicity revealed by observation

Since antiquity, astronomers have noticed that eclipses (from ancient Greek: abandonment, defection) 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 lunar orbit: the synodic month, the draconic month and the anomalistic month.

A celestial harmony within reach of prediction

The Saros cycle represents one of the most fascinating regularities of celestial mechanics. Understanding this cycle makes it possible not only to predict eclipses with precision, but also to appreciate the wonderful harmony that governs the movements of our Earth-Moon-Sun system. The ancient astronomers who discovered this cycle without our modern tools bear witness 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), in which the Greek word (saros) appeared, meaning "repetition" or "cycle", but which, in reality, designated among the ancient Chaldeans a period of 3600 years, and not the cycle of eclipses.

The fundamental orbital periods of the Moon and the Sun

The three determining lunar cycles

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 (evolution of the Moon 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).

The Saros Eclipse Families

Birth and evolution of a Saros series

Each series of eclipses belonging to the same Saros cycle is called a Saros series. A series generally 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 central eclipses.

Predicting Eclipses Using the Saros

Geographic offset and the Exeligmos

Because of the 8 extra hours in the Saros period (⅓ of a day), each successive eclipse in a series occurs about 120 degrees of longitude farther west. This peculiarity means that three Saros cycles (called the Exeligmos (period of 54 years and 33 days)) are necessary for an eclipse to recur at approximately the same geographic location.

Summary of active lunar Saros series

Active lunar Saros series during the last two centuries

The table below summarizes the main lunar Saros series whose activity extends over the period 1800-2100. Each series is characterized by a total duration of 12 to 15 centuries and includes between 71 and 83 successive eclipses.

N.B.:
The eclipse sequence uses the following notation: N = penumbral eclipse, P = partial eclipse, T = total eclipse. The numbers indicate the number of eclipses of each type in the chronological order of the series. Each series begins with penumbral eclipses, evolves toward partial then total eclipses, before returning to partial eclipses and finally penumbral ones.

Summary of active solar Saros series

Active solar Saros series during the last two centuries

The table below summarizes the main solar Saros series whose activity extends over the period 1800-2200. Each series is characterized by a total duration of 12 to 15 centuries and includes between 69 and 82 successive eclipses.

N.B.:
The eclipse sequence uses the following notation: P = partial eclipse, A = annular eclipse, H = hybrid eclipse, T = total eclipse. The numbers indicate the number of eclipses of each type in the chronological order of the series. Each series begins with partial eclipses near one pole, produces several dozen central eclipses, then ends with partial eclipses near the opposite pole.