Saros (astronomy)

The earliest discovered historical record of what is known as the saros is by Chaldean (neo-Babylonian) astronomers in the last several centuries BCE.

[6] The name "saros" (Greek: σάρος) was applied to the eclipse cycle by Edmond Halley in 1686,[7] who took it from the Suda, a Byzantine lexicon of the 11th century.

"[8] The information in the Suda in turn was derived directly or otherwise from the Chronicle of Eusebius of Caesarea,[9] which quoted Berossus.

(Guillaume Le Gentil claimed that Halley's usage was incorrect in 1756,[10] but the name continues to be used.)

[12] The Saros period of 223 lunar months (in Greek numerals, ΣΚΓ′) is in the Antikythera Mechanism user manual on this instrument, made around 150 to 100 BCE in Greece, as seen in the picture.

Two eclipses separated by one saros have very similar appearance and duration due to the distance between the Earth and Moon being nearly the same for each event: this is because the saros is also an integer multiple of the anomalistic month of 27.5545 days, the period of the moon with respect to the lines of apsides in its orbit.

In addition, because the saros is close to 18 years in length (about 11 days longer), the Earth will be nearly the same distance from the Sun, and tilted to it in nearly the same orientation (same season).

At some point, eclipses are no longer possible and the series terminates (Sun leaves the beginning of the node).

For solar eclipses the statistics for the complete saros series within the era between 2000 BC and AD 3000 are given in this article's references.

[16][17] It takes between 1226 and 1550 years for the members of a saros series to traverse the Earth's surface from north to south (or vice versa).

In each successive saros, the Moon's orbital path is shifted northward with respect to the Earth's shadow, with the first total eclipse occurring in 1950.

Because of the +1⁄3 fraction of days in a saros, the visibility of each eclipse will differ for an observer at a given locale.

For the lunar saros series 131, the first total eclipse of 1950 had its best visibility for viewers in Eastern Europe and the Middle East because mid-eclipse was at 20:44 UT.

This cycle of visibility repeats from the start to the end of the series, with minor variations.

[22] For example, if the Moon's penumbra partially covers the southern limb of the Earth during a solar eclipse, 9 years and 5+1⁄2 days later a lunar eclipse will occur in which the Moon is partially covered by the southern limb of the Earth's penumbra.

Antikythera Mechanism Saros cycle for the prediction of eclipses ΣΚΓ′, in the red rectangle, and means 223 months. Written between 150 and 100 BCE
Lunar eclipses occurring near the Moon's descending node are given odd saros series numbers. The first eclipse in such series passes through the southern edge of the Earth's shadow, and the Moon's path is shifted northward each successive saros, while lunar eclipses occurring near the Moon's ascending node are given even saros series numbers. The first eclipse in such series passes through the northern edge of the Earth's shadow, and the Moon's path is shifted southward each successive saros.
Visualization of a period of one saros cycle in 3D.
Solar eclipses occurring near the Moon's descending node are given even saros series numbers. The first eclipse of each series starts at the southern limb of the Earth and the eclipse's path is shifted northward with each successive saros, while solar eclipses occurring near the Moon's ascending node are given odd saros series numbers. The first eclipse of each series starts at the northern limb of the Earth and the eclipse's path is shifted southward with each successive saros.