Rosh Chodesh means "head of the month." The month begins with the appearance of the new moon (Rosh Chodesh). The ancient Rabbis understood Exodus 12:1-2 as requiring that the new month be determined by eyewitness testimony: just as God "pointed out" the appearance of the new moon to Moses and Aaron, so must witnesses declare before the judges of the Sanhedrin, the Great Court in Jerusalem, who stood in place of Moses and Aaron, that they had seen that form in the heavens.
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