Saros cycle

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The Saros cycle is an eclipse cycle with a period of 223 synodic months (approximately 6585.3213 days, or nearly 18 years 11 1/3 days), that can be used to predict eclipses of the Sun and Moon. One cycle after an eclipse, the Sun, Earth, and Moon return to approximately the same relative geometry, and a nearly identical eclipse will occur.

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

Contents

History

The earliest discovered historical record of the Saros cycle is by the Chaldeans (ancient Babylonian astronomers) in the last several centuries BC,[1][2][3] and was later known to Hipparchus, Pliny[4] and Ptolemy,[5] but under different names. The Sumerian/Babylonian word "šár" was one of the ancient Mesopotamian units of measurement and as a number appears to have had a value of 3600.[6] The name "saros" (Greek: σάρος) was first given to the eclipse cycle by Edmond Halley in 1691, who took it from the Suda, a Byzantine lexicon of the 11th century. Although Halley's naming error was pointed out by Guillaume Le Gentil in 1756, the name continues to be used.

Description

The Saros cycle of 6585.322 days (14 normal years + 4 leap years + 11.322 days, or 13 normal years + 5 leap years + 10.322 days) is useful for predicting the times at which nearly identical eclipses will occur, and derives from three periodicities of the lunar orbit: the synodic month, the draconic month, and the anomalistic month. For an eclipse to occur, either the Moon must be located between the Earth and Sun (for a solar eclipse) or the Earth must be located between the Sun and Moon (for a lunar eclipse). This can happen only when the Moon is new or full, respectively, and repeat occurrences of these lunar phases are controlled by the Moon's synodic period, which is about 29.53 days. Most of the times during a full and new moon, however, the shadow of the Earth or Moon falls to the north or south of the other body. Thus, if an eclipse is to occur, the three bodies must also be nearly in a straight line. This condition occurs only when the Moon passes close to the ecliptic plane which is the case around the time when it passes through one of the two nodes of its orbit (the ascending or descending node). The period of time for two successive passes through the ecliptic plane at the same node is given by the draconic month, which is 27.21 days. So the conditions of an eclipse are met at a new or full moon around one of the nodes, which occurs every 5 or 6 months (the Sun, being in conjunction or opposition to the Moon, is also at a node of the Moon's orbit at that time - this happens twice in an eclipse year). However, if two eclipses are to have the same appearance and duration, then also the distance between the Earth and Moon must be the same for both events. The time it takes the Moon to orbit the Earth once and return to the same distance is given by the anomalistic month, which has a period of 27.55 days.

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