1,239. Calculating the Moon's Visibility

Hilchos Kiddush HaChodesh 1:5

Observing the moon in order to determine the month is not something that each individual does as he does for Shabbos. When it comes to Shabbos, each person counts six days and rests on the seventh. Declaring the new month, however, is a responsibility designated for the court. The day that the court sanctifies as Rosh Chodesh is Rosh Chodesh. Exodus 12:2 says, “This month will be for you…” meaning that the testimony regarding the moon has been entrusted to us (to be administered by the court).

Hilchos Kiddush HaChodesh 1:6

The court would make calculations like those of astronomers who were familiar with the location and paths of the stars. They would carefully research whether or not the moon would be visible at its proper time, meaning the 30th night. If it was determined that it would be possible to see the moon, they would wait the entire 30th day for witnesses to arrive. If witnesses came, the court would examine their testimonies according to the law in order to verify them and sanctify the new month. If the moon was not seen and witnesses did not come, they would complete the 30th day, making the previous month full. If calculations determined that the moon would not be seen, the court would not convene on the 30th day or wait for witnesses. If witnesses came, they would know either that they were false witnesses or that they had seen clouds that resembled the moon to them, but they had not really seen the moon.