Project Genesis

The Calendar and Holidays (incl. Sabbath)

The Jewish Calendar

Calculating a New Jewish Month

I had a question in regard to Rosh Chodesh, determining a new lunar month on the Jewish calendar. Is Rosh Chodesh determined by the first sliver of the new moon, or is it determined by the dark moon? Could you send me a few Rabbinic sources that speak of this matter (calculating the new moon)?

In Temple times, and afterwards as long as there was a court in Israel, Rosh Chodesh was determined by the sighting of the new moon. Since then it has been determined by calculations, based on the Molad. The Molad if the halfway point between the last time the moon is visible one month and the first time it will be visible the next month.

The best source I can give you would be Maimonides in his laws of the New Month. You might want to try “Understanding the Jewish Calendar” by Nathan Bushwick which
is excellent.

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