Saros cycle

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The Saros cycle is an eclipse cycle. It is a period of about 6585 + 1/3 days (approximately 18 years 11 days) which can be used to predict eclipses of the Sun and the Moon. One Saros period after an eclipse, the Sun, Earth, and the Moon return to approximately the same places and 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 centered 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, Earth will be one third of a turn (120 degrees) further through its daily rotation so the eclipse will happen 120 degrees west, i.e. over North America.

The Saros was discovered by Babylonian astronomers. It is very useful 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 from which the first eclipse was visible do not see any of the second one. In the case of an eclipse of the Moon the next eclipse might still be visible from the same location as long as the Moon is above the horizon. Therefore a longer cycle of three Saroses (54 years and a month, almost 19756 full days), known as a Triple Saros or exeligmos (Greek: "turn of the wheel"), has been used. After an exeligmos, an eclipse will again be visible at or near the original location.

In astronomical terms the Saros is due to several lunar and solar cycles repeating after about the same period of time:

223 synodic months (period from one New Moon to the next),
approximately 242 draconic months (the period of the Moon to return to the ascending node of its orbit, i.e. cross the plane of Earth's orbit twice)
239 anomalistic months (period of the Moon to return to its perigee, i.e. the period of its elliptic orbit) and
18 anomalistic years.

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.

So in principle one Saros after an eclipse, there will be another eclipse: however this does not repeat indefinitely because the match of the underlying periods (223 synodic = 242 draconic = 239 anomalistic months) is not perfect. In practice there is a long series of eclipses separated by one saros, that lasts many centuries but has a definite first and last eclipse.

At any one time theoretically there could be at most 223 possible Saros series of solar eclipses running simultaneously, because there are only 223 New Moons in the time span of a Saros. Similar for lunar eclipses at Full Moons.

For solar eclipses, these Saros series 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 Saros 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 Hipparchus, Pliny (Naturalis Historia II.10[56]) 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.