A Saros cycle is a period of 6585 + 1/3 days (approximately 18 years 10 days and 8 hours) which can be used to predict eclipses of the sun and the moon. Every saros the Sun, the Earth and the Moon return to approximately the same places, therefore, almost identical solar and lunar eclipses repeat one saros apart. For example, there was a total eclipse of the Sun on 11 August 1999 centred over Europe and there will be a similar eclipse of the Sun on 21 August 2017. Due to the one third of a day fraction the Earth will be one third of a turn (120 degrees) through its daily rotation so the eclipse will happen 120 degrees west ie. over North America.
The saros was discovered by ancient astronomers and was very useful to them since the calculations involved are simple. The only problem is that the next eclipse of the same Saros cycle occurs about 8 hours later in the day. In the case of an eclipse of the Sun this means the region of visibility shifts west one third of the way around the world and most places which saw the first eclipse do not see any of the second one. In the case of an eclipse of the moon the next eclipse might still be visible as long as the moon is above the horizon. Therefore a longer cycle of 3 Saroses (54 years and exactly 31 days), called exeligmos (Greek: "turn of the wheel"), has been used: after an exeligmos, an eclipse will again be visible at or near to the original location.
In astronomical terms the saros is due to several lunar and solar cycles repeating at about the same time:
Therefore the circumstances of an eclipse are also very similar to an eclipse one Saros earlier and an eclipse (which happens when a conjunction or opposition of the Sun and Moon occurs in one of the nodes, that is, crossing the plane of the orbit) occurs again one Saros later.
At any one time there can be 223 possible Saros cycles running simultaneously.
- For solar eclipses, they have been numbered by van den Bergh (1955). Currently (2003) the 39 series numbered 117 to 155 are active, i.e. a solar eclipse occurs at a New Moon that belongs to one of these series. Solar Saros series last for 69 to 86 eclipses (1226 to 1532 years), but on average 77 eclipses (1370 yr). They start and end with partial eclipses, but have about 48 central (total or annular) eclipses around the middle of the series.
- For lunar eclipses, there are now 41 series active. They last from 71 to 87 eclipses (1262 to 1551 years), but on average are not as long lived as for solar eclipses: 72 eclipses (1280 years), of which 40 to 58 are total.
The Saros cycle was probably known to the Chaldeans (ancient Babylonian astronomers), and later to Hipparchos, Pliny (Naturalis Historia II.10) and Ptolemy (Almagest IV.2), but not under this name. The Babylonian "Saros" appears to have been a name for a period of 3600 years. The name "Saros" was first given to the eclipse cycle by Edmund Halley in 1691, who took it from the Suda, a Byzantine lexicon of the 11th century. Halley's naming error was pointed out by Guillaume Le Gentil in 1756, but the name stuck.
- G. van den Bergh, Periodicity and Variation of Solar (and Lunar) Eclipses, 2 vols. H.D. Tjeenk Willink & Zoon N.V., Haarlem, 1955