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The Martian day is 24 hours, 37 minutes long. After approximately 39 Martian days, 40 days have passed on Earth. As a Martian colonist, should I rest every seventh Martian day, and have my Shabbat differ from that of my brothers in Jerusalem?

Additionally, how would one synchronize the remaining Jewish holidays with my kin on Earth? Does the Halacha prescribe any method to determine the times to observe holidays and fasts when the local year differs from the Earth year?

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Rabbi Azriel Rosenfeld says that when on a different planet, pro-rate the length of the day or the year. On Mars, a "day" is 24 hours and 39 minutes in earth time and a "year" is 687 days in earth time, so you must modify observance of the calendar accordingly.

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  • Interesting, thank you. From when does the epoch begin? Will I begin prorating the year from my landing date on Mars, as per the current date in Jerusalem on Earth?
    – dotancohen
    Oct 16, 2020 at 17:27
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    That would be one way. I doubt you will find ironclad halacha on the matter. Rabbis like to wait until the matter is of wide and practical interest.
    – Maurice Mizrahi
    Oct 16, 2020 at 17:55
  • The situation is already here, we have had Jewish astronauts serving significant periods of time on both MIR (the Soviet Space Station) and the ISS (International Space Station). Granted, you can say the ISS is in Earth orbit, and therefore tied to Earth time keeping (day, year, etc.) for the purposes of Shabbos and Yomtif, but if you go by the basic definition we use for day, sunset to sunset, a day on the ISS is only little over an hour and a half (92.68 minutes to be precise), the time to complete an orbit. So is a week of 7 days on ISS only 10.8 hours?
    – Dennis
    Oct 16, 2020 at 20:06
  • @Dennis: As you suspected, timekeeping on the ISS is tied to timekeeping on Earth. But yours is a salient point. Consider, e.g. a Muslim on the ISS observing Ramadan. Would he fast only while the ISS is on the night side of the Earth?
    – dotancohen
    Oct 17, 2020 at 20:29
  • @dofancohen: Yes, my point. Or on the moon, where a day is 29.5 earth days long, does Shabbos on the moon only come once every seven months? It also raises the question, if you are on the moon, when do you commemorate Rosh Chodesh?
    – Dennis
    Oct 18, 2020 at 19:09

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