It happened once that [a group of Jews traveling by boat on Friday afternoon] did not reach port before dark. They asked Rabban Gamliel, “May we disembark?” He told them, “You may, for I have already calculated and seen that we were within the boundary before it grew dark.”
Rabbinic law permitted a Jew to walk up to two thousand cubits from the city on Shabbat. This distance could be extended by placing a meal at the limit before Shabbat, thereby extending one’s domain another two thousand cubits. However, if one wound up outside the Shabbat limits, he or she was supposed to stay put, moving no more than four cubits (about seven feet) until the end of Shabbat. Our Mishnah tells a story in which the travelers arrive in port after Shabbat begins. Are they considered within the city, and therefore permitted to disembark and move around, or must they stay put on the boat until Saturday night? Rabban Gamliel, who employed some sort of mechanical device for calculating distances (as recounted in the Talmud), gave a lenient answer—they had entered the two-thousand-cubit limit of the port before Shabbat began and could therefore disembark.