Saros Cycle

A interesting fact about the eclipse is that the eclipse repeats itself in a cycle of 18 years 11 days and 8 hours. Due to the 8 hour factor, and on account of the earth's rotation, the eclipse gets shifted by a hundred and twenty degrees to another place on the earth. But again, with every 3 repetitions of the Saros cycle accounting for 54 years and 34 days, the eclipse could happen over the same region all over again (with a shift in the latitudes, of course).

For example, the eclipse that is anticipated for 11th August '99 seems to be a repeat of the one that had happened on July 9th 1945. In fact, this knowledge that the eclipses repeat in cycles, could well be used to predict eclipses across large stretches of time periods - both into the future as well as into the past.

Cycle of the Century

Eclipses can be used as a calendar in units of 54 years to form an important chronological marker, and has been useful for dating some important documents in history. This technique has been used by historians for fixing exact dates of past events.

Knowledge of Eclipses

Quite early on, the Babylonians discovered the Saros Cycle. The earliest record of a solar eclipse, however, is known to have come from ancient China. The date of this eclipse has been dated as October 22, 2134 BC Prediction of eclipses gave the predictors extraordinary power over society. They kept this knowledge as a secret to themselves and allowed for mystification of the sciences.

LUNAR ECLIPSE

From time to time, the Moon (or a portion of it) enters the Earth's shadow in what is known as a lunar eclipse .The Moon is dimmed partially or almost completely, depending on whether it passes through the less dark part of the Earth's shadow, the penumbra , or the darker part, the umbra .

The Lunar Prospector relied on sunlight to recharge its batteries. Scientists were concerned that whenever the Lunar Prospector was in the darkness of the Earth's shadow, its batteries could have drained to the point where they could not be recharged.

Prospector survived penumbral lunar eclipses on September 6, 1998, and January 31, 1999, without damage. It also survived a much darker umbral eclipse the following July.

Patterns of Eclipses

Two different cycles of the Moon determine the pattern of eclipses over time. One cycle -- the familiar monthly lunar phases -- is easy to understand: a solar eclipse may occur only at a new Moon, as the Moon passes between the Earth and the Sun, casting its shadow toward the Earth. The other cycle involves the gradual shift in orientation of the Moon's orbit. Only when these two cycles are favorably combined (about every six months) can a solar eclipse occur.

We have a new Moon every month, but we don’t have an eclipse every month. Usually the Moon's shadow passes completely above or completely below the Earth. This is because the Moon's orbit is tilted at about a five-degree angle to the Earth's orbit, so that the Moon usually passes above or below the direct line of sight between the Earth and the Sun.

Only at those times when the new Moon is near one of its nodes can a solar eclipse occur. (The nodes are the two points where the Moon’s orbit intersects the plane of the Earth’s oribt, the ecliptic.) For a solar eclipse to occur, the new Moon must be close enough to the ecliptic plane so that it’s shadow will touch some part of the Earth. As it turns out, when the new Moon appears within 18-3/4 days before or after the alignment of a node, a solar eclipse will take place. This creates a 37-1/2-day time window for eclispes, called an eclipse season, when the conditions are favorable for an eclipse to occur.

If the lunar nodes were stationary with respect to the stars, each node would be lined up between the Earth and the Sun at the same time each year, and eclipses would occur at the same two periods of time every year, six months apart. In fact, this is what almost happens, except that the nodes of the lunar orbit are gradually shifting their orientation in space. By the time one node is in line with the Sun again, it has regressed slightly. The alignment happens 18.6 days sooner than if the nodes were not moving, creating the shorter eclipse year (about 346.6 days). This regular regression of the Moon’s nodes is the other cycle that determines the patterns of eclipses over time.

The result is that the eclipse seasons gradually shift earlier and earlier each year, with a solar eclipse at a new Moon that falls within the window. The solar eclipse on March 9, 1997, is followed by successive eclipses in the same season on Feb. 26, 1998 (total), Feb. 16, 1999 (annular), and Feb. 5, 2000 (partial).

These two cycles – the lunar month (or synodic month) and the eclipse year – plod along year after year without much apparent coincidence. An eclipse year (346.62 days) does not come close to being an exact multiple of these periods (324.83 days in eleven synodic months, 354.36 in twelve). A longer cycle, close to an exact multiple of these two periods, would be useful for making eclipse predictions.

Just such a longer cycle, called the saros cycle, was discovered by Babylonian astronomers in ancient times. The saros (meaning "repetition") lasts exactly 223 synodic months. That's a period of 18 years 11-1/3 days (or 18 years 10-1/3 days if five February 29ths fall within the period). And the saros coincides closely with 19 eclipse years: 223 synodic months (29.5306 days) = 6,585.32 days
19 eclipse years (346.6200 days) = 6,585.78 days

This resonance between the periods of these two cycles produces a repetition of eclipses in a remarkably short time. (In terms of astronomical cycles, 18 years is a short time!) The eclipses on the following map are all in the same saros series, each separated by 18 years and 10 or 11 days.

The paths of totality for successive eclipses in this saros series change in a regular pattern every 18 years. The paths, which are similar in shape, gradually widen and shift to more northerly latitudes. The longitude for each successive eclipse in the series shifts to the west a little more than one third of the way around the globe.

A series of eclipses, each separated by this 18-year 11-1/3-day cycle, is called a saros series. Because the resonance between 19 eclipse years and the saros is not exact (0.46-day difference), a saros series cannot go on indefinitely. Eventually a series reaches a point when the eclipses are no longer visible; the umbra passes too far above or below the Earth to be seen. A single saros series spans over 1,200 years and includes between 68 and 75 solar eclipses.

The repetition of eclipses follows very regular patterns in time. Eclipse seasons and saros cycles come and go like clockwork. The repetition of eclipses at a given place on the Earth, however, appears to follow no discernible cycle. Partial phases of solar eclipses can be seen about every 2-1/2 years from any particular spot on the Earth. The best estimate for total eclipses is to say they recur at the same location about every 360 years on the average. This figure is based on the average area of the paths of totality, the total surface area of the Earth, and the overall frequency of total eclipses. But because we are dealing with averages over the time span of many millennia, the actual circumstances for particular locales vary, sometimes widely, from this estimate. For example, the Moon’s umbra never passed over the city of London during a period of 837 years between consecutive total solar eclipses in the years 878 and 1715. On the other hand, if you visit the coast of Angola in southern Africa to witness almost 5 minutes of a total eclipse on June 21, 2001, you can return to the same spot three eclipse seasons later for two minutes of totality on Dec. 4, 2002.