The Metonic Cycle
The ways people categorize time is mostly based on the placement of the celestial bodies around us. But, unlike the Gregorian calendar, which only relies on the sun, the Jewish Calendar utilizes both the sun and moon. The Jewish calendar has 354 days each year with a leap year every 2-3 years. Because of this, each month lasts an average of 29.5 days. This period of time in between each full moon is called a מולד (pronounced molad). The time between these is assumed to be about 29 days, 12 hours, and 793 חלקים (pronounced chalakim). Since there are 1080 חלקים in an hour, 1 חלק is about 3.33 seconds. Therefore, the entire interval lasts approximately 29 days, 12 hours, 44 minutes, and 3.33 seconds.
In the Jewish calendar, the years are counted from the last day of creation. The year 2017 on the Gregorian calendar is actually 5778 A.M (Anno Mundi, Year of the World), since the Judaic calendar starts at 3761 B.C.E. The leap years for the Judaic calendar are determined using the numerical value of the year in A.M. If the year has a remainder of 0, 3, 6, 8, 11, 14, or 17 when divided by 19, it’s a leap year and we add a month to the typical 12 months. This rule is based on a calendar pattern that was discovered by Meton in 432 B.C.E, the Metonic cycle. This cycle assumes that 19 years on the Gregorian calendar is equal to 235 lunar cycles. This cycle is incredibly accurate, with both time intervals being equals to about 6939 days, with a difference of about 2 hours. Due to this pattern, a person’s Hebrew birthday coincides with their Gregorian birthday every 19 years, though it gets less accurate as you get older. Using this overlap, you can calculate your Hebrew birthday using the traditional Gregorian date. There are a number of calculators such as the one offered on Chabad.org that perform this function.
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