Page "Hebrew calendar" Paragraph 38

According to the Mishnah and Tosefta, in the Maccabean, Herodian, and Mishnaic periods, new months were determined by the sighting of a new crescent, with two eye witnesses required to testify to the Sanhedrin to having seen the new lunar crescent at sunset.

The practice in the time of Gamaliel II ( c. 100 CE ) was for witnesses to select the appearance of the moon from a collection of drawings that depicted the crescent in a variety of orientations, only a few of which could be valid in any given month.

These observations were compared against calculations.

When thirty days elapsed since the last new moon, the witnesses were readily believed.