The Saros cycle: how the Babylonians predicted eclipses
223 lunations, 18 years and a handful of days: how the Saros cycle works, what the Babylonians knew and why its name was born of a misunderstanding.
An eclipse is not an isolated event. Eighteen years and ten or eleven days later a very similar eclipse recurs, with the Moon in the same position relative to the nodes and at almost the same distance from the Earth. This interval, exploited for more than two thousand years, is called the Saros and is one of the oldest tools of astronomical prediction on record.
Three cycles that meet
The Saros lasts 223 synodic months, equal to about 6,585.3 days. Its effectiveness comes from a triple coincidence: 223 synodic months are almost exactly equal to 242 draconic months and to 239 anomalistic months. After one Saros, therefore, Sun, Moon and nodes return to nearly the same configuration and the Moon lies at a similar distance, so the next eclipse is of similar type and duration.
The fraction of a day, about eight hours, has a curious consequence. In that time the Earth turns a third of the way round, and the next eclipse in the series occurs about 120 degrees of longitude further west. After three Saros, that is, about 54 years and 33 days, the eclipse returns roughly to the same region of the globe. The Greeks called this triple period exeligmos, meaning «turn of the wheel».
From the tablets of Babylon
Babylonian astronomers kept systematic records of observations for centuries, the so-called astronomical diaries, and in the second half of the first millennium BC they used a cycle of 223 months to determine when an eclipse was possible. Some cuneiform tablets, largely kept in the British Museum, arrange eclipses in eighteen-year schemes. The method did not always say whether the eclipse would be visible at Babylon, but it indicated the months in which to expect one.
The name Saros, on the other hand, is modern. It was introduced by the English astronomer Edmond Halley at the end of the seventeenth century, who took it from the Byzantine lexicon known as the Suda, where the Babylonian term sar, which denoted the number 3,600, was mistakenly linked to a period of lunar months. The name, born of a philological misunderstanding, has remained in use to this day throughout astronomical literature.
Series that are born and die
Eclipses linked by a Saros form a series. Each series begins with a small partial eclipse near one of the poles, continues with central eclipses ever closer to the equator and fades out at the opposite pole, after a period ranging from about 1,200 to 1,500 years and comprising from 69 to 87 eclipses. The series are numbered: the total solar eclipse of 12 August 2026, for example, belongs to series 126, the same as the eclipses of 1990 and 2008.
A sky that repeats itself
In the ancient world the periodic recurrence of eclipses strengthened the idea that the sky followed a regular order and that celestial events came round again as on a wheel, an image dear to astrological tradition as well. Pliny the Elder, in his Natural History, records that eclipses return in cycles of 223 months, showing how widely that knowledge, born in Mesopotamia, had spread through the Greek and Roman world.
The Saros shows how patient observation, carried on for generations, can turn a feared phenomenon into an expected event. What once appeared to be a sudden sign from the gods became, thanks to the scribes of Babylon, the return of a measurable rhythm, written in cuneiform on small clay tablets and later handed down to the Greek astronomers.