Application in Traditional Calendars
Traditionally (in the Babylonian and Hebrew lunisolar calendars), the years 3, 6, 8, 11, 14, 17, and 19 are the long (13-month) years of the Metonic cycle. This cycle can be used to predict eclipses, forms the basis of the Greek and Hebrew calendars, and is used in the computation of the date of Easter each year.
The Chaldean astronomer Kidinnu (4th century BC) knew of the 19-year cycle, but the Babylonians applied it since the late sixth century. They measured the moon's motion against the stars, so the 235:19 relation may originally have referred to sidereal years, instead of tropical years as it has been used in various calendars.
The Runic calendar is a perpetual calendar based on the 19-year-long Metonic cycle. Also known as a Rune staff or Runic Almanac, it appears to have been a medieval Swedish invention. The calendar does not rely on knowledge of the length of the tropical year or of the occurrence of leap years. It is set at the beginning of each year by observing the first full moon after the winter solstice. The oldest one known, and the only one from the Middle Ages, is the Nyköping staff, believed to date from the 13th century.
The Bahá'í calendar, established in the middle of the 19th century, is also based on cycles of 19 years.
Read more about this topic: Metonic Cycle
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