The Saros

There is also a long-term cycle over which conjunctions and oppositions repeat, making eclipses possible. This period is known as the saros, a Greek word meaning "repetition" that is itself derived from the Babylonian sharu. In eclipse calculations the saros is of huge importance.

On our calendar we will see eclipses repeating with gaps of18 years plus 10 or 11 days (a length of time very close to 19 eclipse years, or 18.03 solar years). Take, for example, the eclipse ofAugust 11, 1999. It was preceded by a similar event on July 31, 1981, and will be followed by another on August 21, 2017. The first saros gap had four leap years (1984, 1988, 1992, 1996) so each date within the year was 11 days earlier, while the second saros gap contains five leap years (2000, 2004, 2008, 2012, 2016) leading to the next date being but 10 days later. Knowledge of this cycle (the saronic cycle) therefore allows a sky watcher or astronomer to make long-term eclipse predictions.

At any time there are several distinct saronic cycles in action, interwoven but distinguishable. By now you will have got the picture that due to a host of coincidences of celestial mechanics, there are various underlying cycles that make eclipses repeat in a rather predictable way.

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