Saros cycle of Moon; lunar Saros cycle

This Saros cycle of the Moon: Lunar Saros cycle page and the entire website are excerpted from You and the Universe, a handmade, individualized fine art book on astrology, mythology and astronomy through which the recipient's complete astrological reading is woven.

There are two types of eclipses: one in which the Moon moves between the Sun and the Earth and blocks the light of the Sun (a solar eclipse) falling on the Earth, and the other in which the Moon moves behind the Earth relative to the Sun, and the Earth blocks the Sun's light falling on the Moon (a lunar eclipse). Both of these were of great interest to early man;, the Sun and the Moon were considered gods, and the sudden extinction of their light was thought to be a powerful and evil omen. To be able to predict when this would happen would therefore be to gain some control over evil power, and perhaps to propitiate it in some way as well.

Three things must all be true in order for a solar or lunar eclipse to occur:

1) The Moon must be at new (solar eclipse) or full (lunar eclipse) phase. At new Moon the Moon is between the Earth and the Sun, and at full moon the Earth is between the Sun and the Moon; one of these geometries must obtain for the sun's light to be blocked at all.

2) The Moon must be at or near one of its nodes: The Moon's orbital plane around the Earth and the Earth's orbital plane around the Sun (the ecliptic) are inclined at a little over 5° to each other. The Moon must be on or near the intersection (node) of these two planes in order for its shadow to fall on the Earth (it's descending node), or for the Earth's shadow to fall on the Moon (it's ascending node). That is, the Moon must be the line joining the Earth to the Sun (vertically and horizontally aligned) in order for eclipses to occur (see upper or lower pictures).

3) The Moon must be at or near perigee: The Moon's orbit is eccentric—an ellipse—as Kepler demonstrated to be true for the orbits of all orbiting bodies. At its closest to the Earth (perigee) it is 226,420 miles away; at its most distant (apogee) it is 252,724 miles from the Earth. Because of the actual size of the Earth and the Moon relative to their distance apart, the Moon's shadow is only long enough to reach the Earth (solar eclipse) when it is closest to the Earth. And similarly, the Earth's shadow is only long enough to reach the Moon (lunar eclipse) when it is closest to the Moon.

Actually, the third criterion can be relaxed for some types of solar eclipses to still occur. Depending on the Earth-Sun distance at eclipse, a solar eclipse can be

complete: in which the shadow at the observer's location on Earth completely covers the Sun; or

partial: in which the Moon's shadow partially covers the observer's location on Earth; this latter can occur in two ways, as shown by the right two images below.

Fortunately for the early Babylonian, Assyrian and Greek astrologers and astronomers, the Moon has a "Saros" cycle of 18 years, 11 days and 8 hours, after which period lunar and solar eclipses repeat themselves almost exactly. This was all early man needed to predict eclipses to the hour. If, for example, there was an eclipse of the Sun at dawn on the 10th of June, 980 B.C., then you would know with great certainty that there would be an eclipse of the Sun on the 21st of June in 962 BC in the late afternoon. By keeping track of all eclipses in an 18-year period, a Babylonian astronomer could predict future eclipses with great certainty! (Since there are two to five solar eclipses every year, there are about 40 different Saros series in progress at any one time.) And since lunar eclipses were regarded as evil omens by the Babylonians and the Assyrians, being able to predict them was of great importance. Substitute kings were appointed to rule and to bear the wrath of the gods during a known, upcoming eclipse. The substitute was killed after the eclipse, fulfilling the prophecy and simultaneously ensuring the safe continuity of the true monarch's reign. And during the reign of king Nabu-Nasir, the Babylonian astronomers discovered that 235 lunar months are almost equal to 19 solar years, the difference being only about two hours—very helpful in creating a repeating lunar calendar.


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