What is Metonic cycle?
The '''Metonic cycle''' is a period of about 19 years used in [[astronomy]] and [[calendar]] studies. In particular, it is often used in the structure of [[lunisolar calendar]]s, because 19 [[Sun|solar]] years is almost exactly the same length of time as 235 [[Moon|lunar]] [[month]]s. ==History== The Metonic cycle is named for the [[astronomer]] [[Meton of Athens]], who used the cycle in the calendar of [[Classical Athens|Attica]] in the 5th century [[BC]]. But it was used in the calendar of many [[ancient history|ancient]] cultures, including [[Babylonia]] and [[China#Ancient (2100 B.C. – 1500 A.D.)|China]], before then. ==Using the Metonic cycle in a calendar== Nineteen years of 12 lunar months each would last a total of <math>19	imes12=228</math> lunar months. But from astronomy we know that 19 solar years are the same length as <math>235</math> lunar months. So over 19 years, <math>235-228=7</math> extra lunar months need to be added to make the years line up correctly. Adding a thirteenth month in a year is called ''intercalation.'' There are different ways to do it: * The ancient Babylonian calendar and the modern [[Hebrew calendar|Hebrew (Jewish) calendar]] add an extra month just before the [[spring equinox]] on a regular schedule of years: years 3, 6, 8, 11, 14, 17 and 19 of each 19-year cycle * The modern [[Chinese calendar]], and other Asian calendars that follow it, use a calculation of solar periods and moon [[Phases of the Moon|phases]] to decide when to add the extra month * Other cultures use [[observation]]—for example, seeing if a [[crop]] is [[ripe]]—to decide when to add an extra month. The [[computus|calculation of the date of Easter]] uses the [[astronomy|astronomical]] observation of the spring equinox to determine whether the following [[Easter]] will come 12 or 13 lunar months after the last one. ==Metonic cycle and eclipses== A Metonic cycle is close to the length of 20 [[year|eclipse years]]. For that reason, some [[eclipse]]s repeat for a time at 19-year [[interval]]s. ==External