Eiruvin 4:2-3

Eiruvin 4:2

One on occasion, the rabbis’ ship did not reach port until after Shabbos started. They asked Rabban Gamliel if they could disembark and he replied that they could because he was looking when Shabbos started and he ascertained that they were already within the Shabbos limit. (The Talmud – Eiruvin 43b – says that Rabban Gamliel had a tube through which he could see great distances. Rashi describes the tube as being able to expand and contract in order to focus on different distances. This is all the more noteworthy when one considers that not only did Rabban Gamliel and the gemara precede the known invention of the telescope, but even Rashi lived five centuries too early to be familiar with the device he describes!)

Eiruvin 4:3

If a person left the Shabbos boundary for a permitted reason, such as to save a life or to testify about the appearance of the moon, and he was then informed that the task has already been taken care of, he may travel 2,000 cubits in each direction. If doing so brings him back within the Shabbos boundary, it’s as if he never left. This is because one who leaves the Shabbos boundary permissibly is allowed to return to his place. (If not for this leniency, people would hesitate to leave the boundary for permitted reasons.)