Greek mythology pictures: Sol, your Sun

Greek mythology pictures: Luna, your Moon





 Saros cycle of the Moon: Lunar Saros cycle

For thousands of years astronomy and astrology were one. The heavens were studied to know the gods, to predict eclipses and to know when the Nile would flood. Since the Sun and the Moon were considered gods, the sudden extinction of their light was thought to be a powerful and evil omen. Thus to be able to predict when this would happen would be to gain some control over evil and perhaps to propitiate it in some way as well. 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 real—but hidden—monarch's reign. But how were eclipses foretold over 2500 years ago?

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).

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

1) The Moon must be at its new (solar eclipse) or full (lunar eclipse) phase. The new Moon is between the Earth and the Sun, and at full Moon the Earth is between the Sun and Moon. One of these geometries must obtain for the Sun’s light to be blocked at all.

2) Since the Moon’s orbital plane around the Earth and the Earth’s orbital plane around the Sun are inclined to each other at about 5°, the Moon must be at or near the intersection of these two planes (a node) for either its shadow to fall on the Earth (at its descending/south node) or for the Earth’s shadow to fall on the Moon (at its ascending/north node); i.e., the Moon must be on the line joining the Earth to the Sun for eclipses to occur (see the two diagrams below).

3) Since the Moon’s orbit is an ellipse (as Kepler demonstrated for all orbiting bodies, page 35), and because the Moon’s shadow is only long enough to reach the Earth (solar eclipse) when it is closest to the Earth (perigee), the Moon must be at or near perigee for a solar eclipse to occur. Also for lunar eclipses, the Earth’s shadow is only long enough to reach the Moon when Earth is closest to the Moon. Depending on the Earth-Sun distance (226,420-252,724 miles) at eclipse, a solar eclipse can be complete or partial.

Actually, the third criterion above 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 diagram below.

Fortunately for early astronomers lunar and solar eclipses repeat every 18 years 11 days and 8 hours in a "Saros" cycle. The earliest eclipse record is over 4000 years old: for failing to predict the solar eclipse of Oct. 22, 2134 BC, the Chinese emperor, caught unprepared, ordered his royal astronomers beheaded. Early eclipse reports from Babylon in 1375 BC, China in 1063 BC and Assyria in 763 BC (in the Bible: Amos 8:9), along with Herodotus’ report that Thales of Miletus predicted the solar eclipse of 585 BC ending a five-year war between the Medes and Lydians, suggest the Babylonians knew of the Saros cycle by the 5th or 4th century BC (see tablet below).

A Babylonian tablet of eclipses from 490 BC to 374 BC. The eclipse events are listed in 38-row/18-year Saros columns from front (shown here) to back, with 223 months between columns. The 223 months between consecutive columns of 38 rows apiece is because a Saros cycle has 38 possible eclipses in 223 synodic months. So the Babylonians knew of the Saros cycle many hundreds of years BC (continue to the Triple Saros/exeligmos cycle).





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© Carl Woebcke, Saros cycle of the Moon: the Lunar Saros Cycle, 1991-2011. All rights reserved.