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If I fly westward on a fast day, and travel through one/several time zone/s, thereby going back in (clock) time, must I then fast until it ends in my current time zone? What if I fly eastward, thereby "skipping" several hours?

Essentially the question is, do I end my fast after a certain period of time, or at a certain time of day?

(Bonus: If I end the fast according to where I currently am (in the west), may I eat in public, even though others around me are still fasting? (I strongly suspect that the answer to this is no.))

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I second your strong suspicions. –  Double AA Jan 4 '12 at 3:34
    
The two oldest answers to this question were merged in from judaism.stackexchange.com/questions/1187/fasting-on-an-airplane –  Isaac Moses Jan 31 '12 at 15:48
    
You have not considered the more difficult situation of determaning when a fast begins: The problem with the fast start time is that there is no astrological event viewable out your window to tell you when dawn is. By the time you realize that dawn has come the fast has already started. Therefore you either need a great app (which you probably can't run on an airplane) to calculate your exact position vs dawn or you need to know what time the fast begins over the place you are flying. Of course you may not be flying over land. The complicating factor is flying near the north pole and then cont –  user2303 Jan 21 '13 at 9:04
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Hello user2303 and welcome to Mi Yodeya! This type of response is best left as a comment on the question, which you'll be able to create once you gain the necessary reputation. Also, you may wish to see this question: judaism.stackexchange.com/q/4916/883 –  HodofHod Jan 21 '13 at 9:15

4 Answers 4

up vote 11 down vote accepted

You end your fast when it becomes dark, independent of how long you have been already fasting.

Source: Igrot Moshe OC 3:96

See also Shevet HaLevi 8:261:2 who argues and says to stop based on you original location's times. It's not clear if he would hold this lechumra as well.

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@HodofHod Thanks! Better late than never. –  Double AA Feb 13 '12 at 5:30
    
I had to take your word for it anyway. I'm tethering on an EDGE connection right now, so I couldn't load the pages. Thanks for your answer! –  HodofHod Feb 13 '12 at 5:34
    
It seems that if you fly from America to EY, The Shevet HaLevi might say that you do fast the whole thing, since his only reason for following the place you left, (when you go the other way), is that it would be a serious problem for people to fast an extra 10 or more hours. –  HodofHod Jul 8 '12 at 3:15
    
@HodofHod Yes IIRC that's what I was trying to say by "it's not clear..." ie would he make you fast more time past Tzeit Hakochavim as in your case. I agree your assessment is good, but he doesn't say it explicitly. –  Double AA Jul 8 '12 at 5:08

I know the Journal of Halacha & Contemporary Society had an article about this many years ago. If I recall correctly (and that's a big "if", and that may not be the only opinion), if it was a Torah-prohibited fast you'd have to wait until you saw actual sunset; for the minor fasts you could stop at what your sunset should be. I don't remember which way Tisha B'Av goes, I think it's stricter.

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I would think that one could be lenient and eat in public, with the only exception being Yom Kippur (or possibly the Tenth of Tevet). –  Adam Mosheh May 2 '12 at 22:26
    
@AdamMosheh Why 10 Tevet? –  Double AA Jul 8 '12 at 5:10

The English book The Date Line in Halacha (a super-abridged translation of the massive ספר תאריך ישראל) states

When flying westward on the fast day, and 10 Teves becomes 11 Teves, the aforementioned machlokes would apply. Rav Schienberg and Yisroel Vehazemanim would permit one to break the fast midday, while Rav Elyashiv and Rav Chaim Kanievsky would hold that one must wait until sunset.

Earlier in the chapter it references שולחן ערוך אורח חיים תקסב:א as the basis for these הלכות.

The 10th of Teves is just an example. This applies to fast days in general.

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would R' Elyashiv and R' Kanievsky hold the same going eastward? i.e. can you shorten the fast by e.g. 7 hours? or is it a safek l'chumra kind of thing? Also, would R' Scheinberg hold that flying from japan eastward, leaving on 10 teves, and crossing the dateline at sunset, would have to fast 48 hours? –  Jeremy Apr 26 '10 at 15:15
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Quoting from the same source: "If one flies eastward, from the U.S. to Israel on a fast day, one need not wait until the fast is over in New York, for the nightfall in Israel marks the end of the fast. On the other hand, what would the halacha be in the case where one flies eastward and crosses the date line in the middle of the day, (e.g. and 10 Teves becomes 9 Teves)?...Rav Chaim Pinchas Scheinberg and Yisroel Vehazemanim opine, that one may eat as soon as he crossed the date line from 10 Teves to 9 Teves...Rav Elyashiv and Rav Chaim Kanievsky disagree..." They say no premature termination. –  WAF Apr 27 '10 at 3:04

Fasting, shabbos, praying, and any other time-dependent mitzvah always goes by where you are right now.

I'm still searching for more sources on this - but I have never heard any halachic authority say anything different on this issue (with the exception of sfiras haomer, but for a reason that only applies to sfiras haomer)

This idea is mentioned on Young Israel's website, without citation.

http://www.youngisrael.org/content/PDFs/Halacha_Central/Halochoscope/hs13-41a.pdf

There are even websites that help you to calculate what time the day (and therefore the fast) starts and ends, while you are in mid-air!

http://travelzmanim.com/

This is helpful not only on a fast day, but for any day, to figure out exactly when to pray on the airplane.

If one were to board tonight's El Al flight (Jan 4, 2012) from JFK airport in New York City at 23:50, he/she would land in Israel after a 9 1/2 hour flight, where the time would be 17:15. on Jan 5.

The fast does not begin until sunrise on Jan 5. Assuming one sleeps for the first eight hours of the flight, one would only be awake for the last hour or so of the fast (10th of Teves) - since the fast ends, according to the latest opinion, at 17:30.

Is it worth the $1,100 roundtrip ticket to effectively skip a fast day? Maybe for some very wealthy and frail people, it is.

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