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October 2004 Feature: The Saros Eclipse Cycle

On the night of October 27-28, residents of the Americas, Greenland, Iceland, and western Europe and Africa will be in a grand position to watch a total eclipse of the Moon. Mid eclipse will take place at 11:04 p.m. EDT. Click here or here to read more about this eclipse.

It's quite possible to predict future lunar eclipses from this Oct. 27-28 eclipse -- or any eclipse for that matter. Every 223 lunar months (the lunar month being the period of time between successive full Moons) following an eclipse, another eclipse of similar duration will faithfully take place. This 223-lunar month eclipse cycle equals 6,585.321 days, or 18 years and 11.321 days (the figure sometimes varying by one day, depending on the number of intervening leap years).

Notice that the Saros period is NOT an even number of days. It's about one-third day (8 hours) longer than 18 years and 11 days. Therefore, each successive Saros takes place about 8 hours later and 120 degrees west of the previous eclipse. At the third return of the Saros (669 lunar months), the eclipse swings full circle (120 degrees x 3 = 360 degrees), happening at roughly the same time and the same geographical location. This triple saros is called an Exeligmos, a period of about 54 years and 34 days.

Starting with the October 27, 2004 total lunar eclipse, we project this triple Saros (Exelgimos) to the year 2058. Click on the date to see an eclipse map, all maps courtesy of Eclipse Predictions by Fred Espenak, NASA/GSFC:

DateTime of Mid Eclipse
October 27, 200410:04 p.m. Eastern Standard Time
November 8, 2022 5:59 a.m. Eastern Standard Time
November 18, 2040 2:03 p.m. Eastern Standard Time
November 30, 205810:14 p.m. Eastern Standard Time

This Saros series -- like any family of Saros eclipses -- won't continue on forever. The changing dynamics between the full Moon and its node constantly beget and devour Saros eclipse cycles -- just like Chronos, or Father Time, begets and devours all his children. A Saros cycle springs into being when the Moon's node comes to within 18 degrees or so west of the full Moon. (Before that time, no eclipse was possible, because the Moon's node was too far west of the full Moon.)

As the eclipses recur every 223 lunar months, the Moon's node keeps shifting roughly one half degree east in relationship to the full Moon. After many, many centuries, the Moon's node finally drifts about 18 degrees or so eastward of the full Moon. At this critical juncture, the Saros cycle begins to dissolve. Once the Moon's node travels so far east of the full Moon, that particular Saros cycle becomes history.

We can figure the approximate lifespan of a Saros series. If after one Saros eclipse cycle of 18.031 years, the node moves about one half degree east in relation to the full Moon, then a Saros series must have roughly 72 eclipses in a series (36 degrees divided by 1/2 degree = 72). Given that each Saros eclipse cycle spans 18.031 years, then a whole Saros series of 72 eclipses must have an approximate lifespan of 1300 years (72 x 18.031 years = >1298 years).

According to Fred Espenak's lunar eclipse page, the Saros series that includes the Oct. 27-28, 2004 eclipse -- Saros Cycle 136 -- will endure for some 1280 years. Saros Series 136's first eclipse marked its birth on April 13, 1680. Its last eclipse will fall on May 31, 2960, the date that Saros Series 136 finally gives up the ghost. The October 27-28 total lunar eclipse is the 19th of this succession of 72 lunar eclipses.

For further reading on the Saros eclipse cycle, click here .

copyright 2004 by Bruce McClure

October 27 Total Lunar Eclipse

Note on the Half Saros: Rereading A Catalogue of Eclipse Cycles, I recently learned about the half saros, a period of 111.5 lunar months (3292.66 days). One half saros period (9.015 years) following the October 27, 2004 total lunar eclipse, a total SOLAR eclipse will take place on November 3, 2013. Click here for a map of this total solar eclipse, this eclipse belonging to solar saros series 143, the counterpart of lunar saros series 136. Cool!! More solar eclipse information can be attained at Fred Espenak's Solar Eclipse Page.