The Role Of The Metonic Cycle In Eclipses

Eclipses recur in sequences separated by one saros, which lasts for 18.03 solar years (very close to 19 eclipse years). At any time there are many interleaved saronic cycles in action: 39 at present. Astronomers label these cycles with numbers. For example, the total solar eclipse ofAugust 11, 1999 is part of saros 145, a sequence that began with an eclipse on January 4, 1639, and will end with the

77th on April 17, 3009. The next in this sequence is that cutting across the United States on August 21, 2017: book your viewing location now. Similarly the total solar eclipse of June 21, 2001 is part of saros 127, which consists of 82 eclipses between the years 991 and 2452.

(It may be noted that for the sake of clarity I have been a little lax in my usage of the term "saros." Correctly the word applies to the period of about 18.03 years after which eclipses repeat, whereas a phrase like "saros 145" refers to a whole sequence of eclipses spaced by such gaps. The intended meaning in each case should be clear enough.)

The saros is not the only cycle important in eclipse prediction. Earlier we met the Metonic cycle of 19 solar years and saw that it is of fundamental significance in calendar matters. After 19 years, 235 synodic months have elapsed, bringing the conjunctions and oppositions back to the same phase, to within a few hours. The Metonic cycle lasts for 6,939.6 days.

Break out the pocket calculator again. Multiplying the eclipse year (346.62 days to five figures) by 20 you will derive 6,932.4 days, which is just 7.2 days short of the Metonic cycle.

The implication of this is that after 19 years the Moon comes back to be not much more than seven degrees from its node, and another 19 years later it returns to a position again advanced by seven degrees. The maximal eclipse season we described above lasts while the Sun moves through 37 degrees, and the Moon's position may skip through that taking steps separated by seven degrees but 19 years apart. That is, there may be a short sequence of four or five (and just possibly six) eclipses separated by 19-year gaps, occurring at the same time of year.

The ecliptic limit chosen there is the largest possible, which is appropriate for partial eclipses. Regarding total and annular eclipses, three or four will occur in these brief sequences related to the Metonic cycle. For example, consider the total solar eclipse due on December 4, 2002. This will be followed by another such event on the same date in 2021. Looking back in time, there was an annular eclipse in 1983, and a partial eclipse in 1964, all on December 4. After 2021 the lunar node slips out the ecliptic limit, but re-enters on the other side a month earlier in the calendar with a partial solar eclipse on November 4, 2040, followed by three annular eclipses on similar dates in 2059, 2078, and 2097. These are all instances of the 19-year Metonic cycle gap, then.

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