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Astro*Dictionary by Michael Erlewine





3 articles for "Saros"

Saros [DeVore]

(1) A Chaldean and Babylonian interpretation of a cycle of 60 days as 60 years. (2) 60 sixties, or 3,600. (3) A lunar cycle of 6,585.32 days — 223 lunations; or 18 years, 11 1/3 days. In this period the centers of Sun and Moon return so nearly to the same relative places that the eclipses of the next period recur in approximately the same sequence — but with their zone of visibility shifted 120° to the Westward. (v. Eclipses.)

Because the Node recedes 19.5 a year, the Sun meets the same Node in 346.62 days – the eclipse year. As this does not coincide with the Lunar periods, the Sun moves past the node a degree a day for as many days as it takes for the Moon to reach a conjunction or opposition. Thus either a Solar or Lunar Eclipse may occur before or after the Sun reaches the Node, or both before and after. If the Lunation occurs within 2 or 3 days before or after the Sun reaches the Node there may be no accompanying Lunar Eclipse, as on Dec. 3, 1918 and May 29, 1919 (Saros Series II).

If the Lunation or Full Moon occurs from 4 to 9 days before the Sun reaches the Node, there will be a Lunar Eclipse followed by a Solar Eclipse, or the reverse. If the Lunation occurs from 10 to 12 days before the Sun reaches the Node there may be a series of three Eclipses: a Solar before the Node, a Lunar at the Node, and another Solar when the Sun has passed beyond the Node. Associated with this are certain values:

242  returns of the Moon to a particular Node6585.36
019  returns of the Sun to the same Node6585.78
233  Synodic months6585.32


See also:
♦ Eclipse ♦ Eclipse Cycles ♦ Eclipse Year ♦ Lunar ♦ Saros Cycle
Saros Cycle [Astro*Index]

[pl. Saroces] Discovered by Babylonian and Chaldean astronomers. A cycle of coincident eclipses at 18 year 11.33 day intervals. This phenomenon is based on the eclipse year, the time it takes the sun to complete one revolution of the zodiac with respect to the lunar nodes, or 346.62 days. 19 such eclipse years (6585.78 days) equal 223 lunar months (6585.32 days). Every 223 months, eclipses occur with the sun, moon, and nodes in the same positions relative to each other. 223 lunar months is the same as 18 tropical years plus 11.33 days. The nodal axis retrogrades through the zodiac through a synodic cycle of 18.6 years. The occurrence of an eclipse requires two conditions: The Moon must be in the correct phase (near New Moon for a solar, near Full Moon for a lunar eclipse), and the Sun must be close to one of the Moon's Nodes. Thus, a solar eclipse should repeat itself after a interval which is an exact multiple of both the synodic month (29.53059 days) and the eclipse year (346.62005 days). The smallest interval which approximates this condition is 223 synodic months, or 6585.32 days, or 18 years, 11.32 days (or 10.32 days if the interval contains 5 leap years). This interval was known to the ancient Chaldean astronomers, who named the interval the Saros, which means repetition. During one Saros interval, about 29 lunar eclipses and 41 solar eclipses occur; 10 of these solar eclipses are total, and will be repeated in the next Saros, but will be visible at a geographic longitude farther to the west by approximately 120°. After 3 Saroses (54 years, 1 month), the eclipses recur at nearly the same longitude. At that time, the Sun is farther west with respect to the Node; thus, eclipses in the 3 Saros period occur farther south if at the ascending node, or farther north if at the descending node. Even greater accuracy is afforded by using 48 Saroses (865.4 years).

See also:
♦ Eclipse ♦ Eclipse Year ♦ Lunar Eclipse ♦ Lunar Node ♦ Lunar Phases ♦ Solar Eclipse ♦ Synodic Month ♦ Tropical Year
Saros Cycle [Munkasey M.]

The time interval between successive Solar or Lunareclipses which occur at the same zodiacal longitude. This is about 6585 1/3 days (18 years, 11 to 13 days - depending on leap years in between), which is 223 Lunations. There is also an important third harmonic of this cycle which occurs at 54 years and 33 days. This harmonic has to do with the Eclipses occuring in the the different decanates of the signs, which they will do, in order of their occuring. When completing this harmonic the Eclipses return to the same Terrestrial Longitude they started from. Another variation of the Saros Cycle goes 1260 years and has 70 or 71 Solar Eclipse repetitions at 18+ years each. See also: "Metonic Cycle".

See also:
♦ Eclipse ♦ Eclipse Year ♦ Lunar Eclipse ♦ Lunar Node ♦ Lunar Phases ♦ Solar Eclipse ♦ Synodic Month ♦ Tropical Year


Astro*Index Copyright © 1997 Michael Erlewine


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