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The Gemora relates an incident: Rabban Gamliel and Rabbi Yehoshua were once traveling on a ship. Rabban Gamliel had with him some bread while Rabbi Yehoshua had with him bread and flour. When Rabban Gamliel’s bread was consumed, he relied on Rabbi Yehoshua’s flour.
Rabban Gamliel asked him, “How did you know that we would be so much delayed that you brought flour with you?” Rabbi Yehoshua answered him, “There is a certain star that rises once every seventy years and leads the sailors off course, and I suspected that it might rise and lead us astray.” Rabban Gamliel said to him, “You possess so much knowledge and yet you must travel on a ship (in order to earn a livelihood)!”
Rabbi Yehoshua knew that a comet would likely be visible during his sea voyage, and that its light would confuse the sailors who navigated by the stars. That comet returned about once every 70 years.
We explore Jeremy Brown’s on the Jewish reception of the Copernican Revolution and Halley’s comet.