"The Jewish Calendar" by Robert Gorelik
The Jewish calendar is a type of lunisolar calendar, which is a lunar calendar with a solar adjustment. A lunar cycle—based on the synodic month, or complete cycle of phases of the moon as seen from the earth—is about 29.5 days, and that is calculated so the full moon always lands on the 15th of each month. To compensate for that extra half day, the different Hebrew months alternate between 29 and 30 days.
If you do the math, 12 lunar months only add up to 354 days, which is about 11 days less than a solar year (365.25 days). Without some type of adjustment, the holidays would be 11 days earlier every year. They would travel around the calendar and come back to the same spot only once every 33 years. Yet that doesn’t happen because the Torah (Deu 16:1) mandates that Passover has to be celebrated in the spring (similar to the original Passover as chronicled in the book of Exodus). In order for that to work out, a system was eventually adopted that adds seven leap months over the course of a 19-year cycle, which works out to about once every three years.
It is frequently assumed that the sun and the moon were created in order to shine their light on Earth. While this may be a secondary purpose of these celestial bodies and obviously essential for life on earth, Genesis describes their purpose as “signs” that serve to “to mark seasons and days and years” 1:14). In other words, to facilitate the celebration of the Sabbath and the “Feasts of the LORD.” In this study, we will talk about how these things work.
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