Saros cycle

The Saros (ⁱ/ˈsɛərɒs/) is a period of approximately 223 synodic months (approximately 6585.3211 days, or 18 years, 11 days, 8 hours), that can be used to predict eclipses of the Sun and Moon. One saros period after an eclipse, the Sun, Earth, and Moon return to approximately the same relative geometry, a near straight line, and a nearly identical eclipse will occur, in what is referred to as an eclipse cycle. A sar is one half of a saros.

A series of eclipses that are separated by one saros is called a saros series.

The earliest discovered historical record of what we call the saros is by the Chaldeans (ancient Babylonian astronomers) in the last several centuries BC. It was later known to Hipparchus, Pliny and Ptolemy.

The name "saros" (Greek: σάρος) was applied to the eclipse cycle by Edmond Halley in 1691, who took it from the Suda, a Byzantine lexicon of the 11th century. The Suda says, "[The saros is] a measure and a number among Chaldeans. For 120 saroi make 2222 years according to the Chaldeans' reckoning, if indeed the saros makes 222 lunar months, which are 18 years and 6 months." The information in the Suda in turn was derived directly or otherwise from the Chronicle of Eusebius of Caesarea, which quoted Berossus. (Guillaume Le Gentil claimed that Halley's usage was incorrect in 1756, but the name continues to be used.) The Greek word apparently comes from the Babylonian word "sāru" meaning the number 3600.

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