Would it be possible theoretically to celebrate Rosh Chodesh and Yomim Tovim on planets that have more than one moon? As the Jewish Calender is based on the moon.

  • 1
    Possible duplicate judaism.stackexchange.com/q/30683/759
    – Double AA
    Feb 21, 2019 at 18:02
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    Heavily related judaism.stackexchange.com/questions/65874/…
    – DonielF
    Feb 21, 2019 at 18:52
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    Can this question be made broader to discuss any and all halachot based on time? The length of a "day", a week, a month and a year all relate, somehow, to the cycle of the sun but the answers presented here star-k.org/articles/kashrus-kurrents/515/… aren't all easily transferable.
    – rosends
    Feb 21, 2019 at 20:08
  • Follow up to @rosends comment - The entire concepts of Rosh Hodesh and the molad are based on the moon orbiting specifically around Earth. All the Biblical holidays (well, the rabbinical ones, too, but, I'm trying to simplify it, a bit.) are based on specific calendar dates within the month. We would then have a problem if say, a planet's moon orbit is 7 "days". How could you celebrate Pesach on the 15th of the month when it doesn't exist? There's a separate problem coordinating "days" which is defined as a planet's rotation. Venus's rotation is slower than it's orbit. (cont.)
    – DanF
    Feb 21, 2019 at 21:57
  • (..cont.) So, that means that on Venus, it's "day" is longer than its "year". You can see how difficult things can get.
    – DanF
    Feb 21, 2019 at 21:58

1 Answer 1


Some background - why your question cannot be answered "truly" Halachicly:

There are two types of Halacha - derived from other sources and made-up from nowhere. Yes, many Halochos have no existing sources whatsoever, but because something has to be Halachic they are made-up. The probably best example is the use of electricity on Shabbos - we had no precedent or even true understanding of electricity when the Rabbis were "forced" to rule it out.

Why go so far to other planets - What about the time at the poles? How do you keep time at the poles that have no hours? As this Halacha cannot be derived from existing sources that were not aware of the existence of the poles, you just make it up, for example, you align with Jerusalem.

How about going to the orbit and having 6 sunrises in 24 hours? Same here, you make the Halachah up.

So your question is from the same area - it can not be derived from the existing sources - once it will be relevant, the Rabbis will make something up.

To your question - no, we only have one specific Moon, not just any moon that orbits the planet you're currently on. Our Moon is one that orbits the Earth and has the same relative size as the Sun and metaphorically represents the Jewish people as we say in Birkas Levonah. Outside the Earth, it makes no sense.

Moreover, the time as we know it in Judaism has no sense outside the Earth, because the only way of telling time in Judaism is by knowing the relative position of the Sun (hours) or the Moon (days). There's no idea of a minute or a second or fixed hour or else. THerefore once you're up in the skies you're on your own - the Halacha does not cover it. Come back soon!

  • 1. What kind of halachos are completely made-up? 2. When Rav Moshe paskened about poles and orbit, he didn't just make it up - he had actual sources that, in his view, indicated that one should follow Yerushalayim when one doesn't have local zemanim.
    – DonielF
    Feb 21, 2019 at 18:57
  • 3. Most importantly for the purposes of this question: this doesn't answer the question. You throw out there claims like "not just any moon that orbits the planet you're currently on" and that somehow the fact that the Jews are compared to the moon implies that all halachos of the moon don't apply to other moons (where the metaphor still applies, usually). Either you're making stuff up, or you're throwing out unsourced claims, or this is completely off.
    – DonielF
    Feb 21, 2019 at 18:58
  • @Doniel Where does R Moshe (Feinstein?) write about the poles? What sources did he bring (obviously excluding any recent sources that just made things up)?
    – Double AA
    Feb 21, 2019 at 19:19
  • @DonielF If one asks for Halochos he needs to know how they are made (to rely on them). Some (young me included) hope to think that Judaism covers it all. Turns out it doesn't. All the rest the Rabbies "make up" (based on Ruach Hakodesh of course!). Claims like "ליבי אומר לי " or "לא מצאתי בכתוב ואני אומר מעצמי" are very common. THis question is a good example of something not possibly covered in our sources because they had no idea of the real structure of the solar system (not talking about heliocentrism).
    – Al Berko
    Feb 21, 2019 at 20:40
  • @DonielF So if you find a Rabbi who dares to think that far, most conclusions would be plain sourceless speculations.
    – Al Berko
    Feb 21, 2019 at 20:42

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