A Saros cycle
is an eclipse cycle
of 223 synodic months (period from one new moon
to the next), or approximately 242 draconic months (the period of the Moon
to return to the ascending node
of its orbit
cross the plane of Earth
's orbit twice). Thus an eclipse
(which happens when a conjunction
of the Sun
and Moon occurs in one of the nodes, that is, crossing the plane of the orbit) occurs again one Saros later.
A Saros is also close to 239 anomalistic months (period of the Moon to return to its perigee, i.e. the period of its elliptic orbit), and to 18 anomalistic years. Therefore the circumstances of an eclipse are also very similar to an eclipse one Saros earlier.
A Saros is about 6585 + 1/3 days (18 years and 11 days). So an eclipse one Saros after another occurs about 8 hours later in the day. A solar eclipse will then be visible about 120 deg. of longitude West of the previous location. Therefore another cycle of 3 Saroses, called exeligmos (Greek: "turn of the wheel"), has been used: after an exeligmos, a solar eclipse will again be visible not far from the original location.
There can be 223 possible Saros cycles.
- 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
II.10) and Ptolemy
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 a lexicon by the Byzantine scholar Suidas
from the 11th century.
- Bergh, G. van den (1955): Periodicity and Variation of Solar (and Lunar) Eclipses, 2 vols. H.D. Tjeenk Willink & Zoon N.V., Haarlem, 1955.