| Metonic Cycle |
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19 tropical years = 6939.602 days 235 synodic months = 6939.688 days This approximation is used by the Hebrew Calendar . It was known to the Greek Astronomer Meton , who introduced it about 432 BC , and the Chaldea n astronomer Kidinnu (4th century BC). It is also used in the Computation Of The Date Of Easter . In a typical Lunisolar Calendar , most years are lunar years of 12 months, but some years have an extra month, known as an Intercalary or ''embolismic'' month. There are 7 of these intercalary months in the 19 years of a Metonic cycle. Traditionally (in the ancient Babylonian , Hebrew , and Attic calendars), the years: 3, 6, 8, 11, 14, 17, and 19, are the long (13-month) years of the Metonic cycle. The Cycle incorporates two less accurate subcycles, for which 8 years = 99 lunations to within 1.5 days, with an error of one day every 5 years (an Octaeteris ), and 11 years = 136 lunations within 1.5 days, with an error of one day every 7.3 years. The Metonic cycle itself is a subcycle of the next more correct 334 years = 4131 lunations to within 41 minutes, with an error of one day every 11598 years. Meton approximated the cycle to a whole number (6940) of days, obtained by 125 long months of 30 days and 110 short months of 29 days. The Callippic Cycle was a more accurate approximation, obtained by taking one day away from every fourth of Meton's cycles, so creating a 76-year cycle with a mean year of exactly 365.25 days. The 19-year cycle is also close (to somewhat more than half a day) to 255 Draconic Month s, so it is also an Eclipse Cycle , which lasts only for about 4 or 5 recurrences of eclipses. SEE ALSO |
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