Sources and reasoning:
1). Responsa Radbaz, vol. I (siman 76), writes: Know that Shabbat was entrusted to every Jew, as it is stated: “For it is a sign between Me and you.” Just as the sign of the covenant applies to each and every individual, so Shabbat was entrusted to each and every individual. Since Shabbat was entrusted to every individual wherever he may be, he counts six days and at the end of the six performs Shabbat, which is a remembrance of the Creation, as it is stated, “For in six days Hashem made,” etc. For if you do not say so, even in Eretz Yisrael there would be some discrepancy. End quote.
2). See Responsa Rav Pe’alim, vol. II, Sod Yesharim (siman 4), which discusses a person who traveled by kefitzat haderech on Shabbat through a Divine Name and arrived in a place where the sun sets sooner, such that it is now Motzaei Shabbat for him, and he has not observed Shabbat for twenty-four hours. May he now perform melachot? He replies as follows: However, it seems that there is no basis whatsoever for uncertainty in this matter. Since we see that this country observes Shabbat on Shabbat, while in another country they treat as weekday this very same time during which Shabbat is observed here, the reason is that regarding Shabbat it is written: “in all your dwellings” — one follows the place. Therefore, likewise in the case under discussion, we follow the place. Since Shabbat has ended in this place and weekday has begun, a person located in that place must conduct himself accordingly. For the verse did not make Shabbat observance and its sanctity dependent on a number of hours; it did not say that one must rest for twenty-four hours. Rather, it said “day” and “in all your dwellings,” thus making the day dependent upon location. If he is standing in a place where the day has passed and night has entered, certainly none of Shabbat’s sanctity remains for him. If we were to tell him to observe Shabbat during unequivocal weekday hours in order to complete twenty-four hours, this would be absurd. It resembles one who did not observe Shabbat on the seventh day and then comes to rest on Sunday, during the weekday, as compensation for Shabbat. Certainly this is foolish, for there is no compensation for Shabbat during the weekdays. End quote.
Likewise, the later authorities conclude that we follow the place where one is located. Responsa Chavatzelet HaSharon, vol. I (siman 47), writes that in all matters we follow one’s location and its time, and the same applies regarding stoning and Shabbat. Kaf HaChayim (siman 261, se’if katan 19) likewise writes in the name of Responsa Sho’el UMeishiv (fourth edition, vol. II, 154). See also Responsa Minchat Yitzchak, vol. VI (siman 84), regarding seven clean days, where the time is shortened from the place one departed because we follow one’s present location. This is also written in Responsa Minchat Elazar, vol. IV (siman 42), and Responsa Divrei Yosef (siman 8). In Chazon Ovadia—Four Fasts (p. 409), it is likewise written that regarding fasts we follow the place where one is presently located; see there. See also the work LeLaket Shoshanim—BeChavut, vol. IV (shoresh 2, siman 2), which discusses the date line with regard to the counting of the Omer.
3). However, this case may be distinguished in two ways: a. In the present case, one has already observed Shabbat and, by crossing the International Date Line, arrives in a place where it is again Shabbat. It may be argued that one is not obligated to observe Shabbat with them again, since he has already commemorated the Creation. Once he has already treated it as weekday, how can he treat it again as holy? There is no obligation to commemorate the Creation twice in one week. This differs from the case of Rav Pe’alim, where Shabbat’s hours are shortened or where one arrives in a place where it is presently Shabbat. b. There is further room to distinguish and say that the case before us differs because he was not in that location on Erev Shabbat. It may be said that the essential sanctity of Shabbat takes effect upon its onset, and that the primary factor depends on where a person is located at the beginning of Shabbat. At that moment, the obligation to observe Shabbat for the entire Shabbat takes effect upon him. This is unlike Rav Pe’alim’s case of a person who travels on Shabbat to a place where Shabbat has now ended. Responsa Eretz Tzvi, vol. I (siman 44), likewise makes this distinction. See Responsa Mishpatim Yesharim (vol. I, siman 77), which raises the question of one for whom it is Motzaei Shabbat in his location and who comes to a place where it is still Shabbat, whether he is obligated by Torah law to observe Shabbat there, but reaches no conclusion. See also Michtavei Torah (Gur, siman 35), which questions whether one who makes Havdalah at the conclusion of Yom Kippur and then comes to a place where it is still daytime is liable to karet if he eats there; likewise, regarding Pesach, whether he is liable to karet for eating chametz. It writes: It appears that since he has already fulfilled that which is stated, “from evening until evening you shall observe your Shabbat,” and likewise “until the twenty-first day in the evening,” he is no longer obligated by Torah law. End quote. See also Responsa Minchat Elazar, vol. IV (siman 42), which discusses a similar uncertainty.
4). On the other hand, Responsa Or LeTzion, vol. I (siman 14), appears to maintain that even in such a case one follows the place where he is located, and that he must observe Shabbat again even if he arrives in the middle of Shabbat, including with regard to prayer. This is also implied by Responsa Sho’el UMeishiv (ibid.). Responsa BeTzel HaChochmah, vol. IV (siman 83), writes similarly. According to their view, seemingly the same applies in the present case: one must observe Shabbat with them while on the plane.
5). Since the matter remains uncertain, the practical ruling appears to be that as soon as the flight enters a place where it is currently Shabbat below, one should not perform melachot due to the doubt, especially melachot prohibited by the Torah, since in cases of doubt concerning Torah law we rule stringently. As for the flight itself, there is no prohibition, because most of its passengers are non-Jews and they act for their own purposes. Nor is it prohibited because of uvda dechol, since one departed permissibly, especially as this is a matter of doubt. However, there is room to discuss traveling more than twelve mil beyond the techum on Shabbat. The Rema (siman 404, סעיף 1) explains that one who travels by a miraculous leap through a Divine Name more than twelve mil beyond the techum is subject to stringency according to the opinion that techumin are prohibited by Torah law. In the present case, if the plane passes over seas and rivers, this is only a rabbinic matter, since it is unlike the encampments in the wilderness, as the Shulchan Aruch writes there. Even if we say that it is presently passing over deserts, Rabbi Akiva Eiger writes in the name of the Maggid Mishneh that even in a karmelit, according to our practice where we have no reshut harabim, there are presently no Torah-level laws of techumin at all. The Alter Rebbe likewise writes this (se’if 2). Although Biur Halachah (siman 404, s.v. ho’il) questions this view, in the present case, which is doubtful as explained, it may be permitted.
6). Regarding Havdalah, in my humble opinion one should hear it from local residents. If there are no Torah-observant Jews nearby, one should make Havdalah due to the doubt without Hashem’s Name and Kingship (and without the blessings over the candle and spices, since one need not seek them out). This is especially so because Maran the Chazon Ish (Kuntres Yud Chet Sha’ot) maintains that one follows the point ninety degrees east of ירושלים, where the land ends and the ocean begins, and that the new day begins at this point. Many leading poskim agree with him; see Piskei Teshuvot, vol. III (p. 956, note 16). According to his view, Shabbat must be observed in every respect according to the local Sunday. Accordingly, certainly according to his view, one should not make Havdalah at the local conclusion of Shabbat with Hashem’s Name and Kingship. Although I saw that Responsa BeTzel HaChochmah, vol. IV (siman 83), writes regarding one who crossed the date line and arrived in a place where it is Shabbat that he is legally required to make Havdalah again, since the Havdalah he made the previous night does not suffice for him, as Shabbat’s sanctity had not yet taken effect upon him. Nevertheless, it concludes that it is proper for him to hear Havdalah from another person. See also Responsa Or LeTzion (ibid.).
7). Regarding the Shabbat prayers, if one has already prayed the three prayers, he is not obligated to pray more, because Chazal instituted three prayers per day and no more, regardless of one’s location. This is what I have written according to my limited understanding.
This response was translated automatically from Hebrew and has not yet been reviewed by a person. Therefore, it may contain inaccuracies and should not be considered a definitive reference.
