In Jerusalem there was a great courtyard called Beit Yaazek, and its only business was to receive witnesses. Every month, two Jews who had seen the thin sliver of the new moon hanging over the horizon would make their way to this courtyard. There the rabbis of the Sanhedrin questioned them: where did you see it, at what hour, in which direction, how curved, how bright?
If their testimony matched, the court declared the new month. The Jewish calendar turned on their eyes.
Because their work was so important, the rabbis made their journey comfortable. Grand feasts were prepared for them in Beit Yaazek as a standing invitation. Come, eat well, tell us what you saw. If the witnesses knew they would be treated kindly, more of them would travel to Jerusalem, and the calendar would stay accurate.
There was one problem. In the old days, if a witness was caught on the road when Sabbath began, he was stranded wherever he stood. He could not walk the remaining miles to Jerusalem, and the testimony was lost. Sometimes the new month itself was delayed because two observers were stuck in a field waiting out the Sabbath.
Rabban Gamliel the Elder ruled that witnesses on their way to testify could walk two thousand cubits in any direction even after Sabbath had begun, enough to reach the city. The calendar could not be held hostage to geography.
This teaching from Rosh Hashanah 21b, preserved in Hebraic Literature (1901), is a quiet reminder that Judaism runs on the eyes of witnesses. A Jewish month begins when two reliable people tell the court what they saw in the sky.