The Metonic Cycle

You may recall that, at the beginning of the book, we mentioned the period of 235 synodic months, which lasts for almost exactly 19 years. The difference between these is just 125 minutes. Astronomers call this 19-year period the Metonic cycle, after the mathematician Meton who lived in Athens in the fifth century B.C., although there is evidence that the Babylonians knew of the synchrony earlier.

Meton invented a calendar cycle containing 6,940 days, but the Greeks never adopted it for widespread use. The 19-year cycle is employed, though, in various other calendrical spheres. Many past and present calendar schemes using leap months rather than the familiar leap days have seven extra lunar-based months spread over 19 years. Thus a dozen of the years each contain 12 months, while seven of them have 13, making 235 in all.

A form of the Metonic cycle is used today in the Hebrew calendar, and many other luni-solar calendars. It is also employed in the calculation of Easter. Its inaccuracy—that discrepancy of 125 minutes—has affected history in various ways, most especially in the evolution of ecclesiastical calendars. Again the intricate details are discussed in the Appendix.

The discovery of the Metonic cycle by the ancients would have been possible simply by watching for a repeated full or new moon at the same time of year. The fact that there are almost exactly 235 lunations in 19 years is a phenomenon that could have been identified by quite early societies, such as those who were building megalithic monuments in Britain and elsewhere in Western Europe from at least the middle of the fourth millennium B.C. This simple coincidence between the lunar and solar cycles would have been fairly impressive, and most important would have allowed the subsequent prophecy of various celestial events. Starting from that basis further coincidences would soon be unveiled. Such considerations may underlie the gradual evolution of impressive sites such as Stonehenge.

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