A Modern Commentary to Rambam’s Laws of Sanctification of the Month הלכות קידוש החודש Chapter 1

תקציר

In the first chapter, Rambam states the calendrical commandment to calculate, understand, and to set the day of the new lunar month. As 12 lunar months fall almost 11 days short of a solar year, an intercalated (leap) month is required every 2-3 years. The moon is not visible for one day before, and one day after, its conjunction with the sun. The moon may become visible on the 30th or 31st night of the previous month. The decision to consecrate the new month lies with Bet Din, who are in fact experts in astronomy. In fact, the astronomical knowledge of moon visibility is a positive commandment. Only in the land of Israel may Bet Din calculate and set months and intercalate years.