Sky Spy

The Metonic Cycle

2013-12-26T00:00:00Z 2014-07-03T11:39:27Z The Metonic CycleBy Tim Hunter Special to the Arizona Daily Star Arizona Daily Star

The moon is a waning (getting smaller) crescent. New moon, when the moon is positioned between the Earth and sun, will be New Year’s Day next Wednesday.

The next time new moon will be on New Year’s Day is 2033. The last time new moon was on New Year’s Day was 1995. It turns out that approximately every 19 years new moon falls on Jan. 1.

It also turns out that every 19 years full moon occurs on Jan. 1 and a second full moon (sometimes known as a “blue moon”) occurs on Jan. 31. This last happened in 1999 and will happen next in 2018.

In fact, you can choose any date of the year, and the phase of the moon on that date repeats every 19 years. The Hebrew calendar, which is based on the moon, observes that the sun, Earth, and moon come back into the same relative positions every 19 years.

I happened to discover this cycle while running a software program that shows the phases of the moon. This represents the complex relationship of the Earth’s rotation around the sun every 365.25 days and the length of the lunar month of 29. 5 days.

In this context a lunar month is defined as the time for the moon to go from one full moon to the next full moon or from one new moon to the next new moon.

These relationships are superimposed on our modern Gregorian calendar, which defines a year as 365 days plus a leap year day every four years on even years.

I can’t claim discovery of this relationship, which is actually known as the Metonic cycle.

Meton of Athens introduced the concept in 432 B.C., though it probably was known by Neolithic peoples more than two thousand years earlier.

We may have modern technology, but the ancients were just as clever and much better observers of the sky.

Contact Tim Hunter at skyspy@azstarnet.com

Copyright 2014 Arizona Daily Star. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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