Shabbos 19a: One may not depart on a ship less than three days before Shabbos. This is only on a pleasure trip, but for a mitzvah it is permitted.

שבת יט ע”א: תנו רבנן: אין מפליגין בספינה פחות משלשה ימים קודם לשבת. במה דברים אמורים ־ לדבר הרשות, אבל לדבר מצוה ־ שפיר דמי.

A girl named Amalia took a flight from New York to South Korea. The plane took off an hour after Shabbos, and flew northwest over Canada and the Arctic Ocean. When she landed, it was Monday morning in Korea. However, during the Arctic part of the flight, before crossing the dateline (according to any opinion as to where the halachic dateline is located), it became light for a while. She asked: Did she have to keep Shabbos during that period? And was she allowed to take the flight in the first place?

Let’s assume she was traveling for pleasure. The Rishonim disagree on the reason why it’s forbidden to embark on a ship in the last three days before Shabbos. The Mishnah Berurah lists five opinions in his introduction to OC 248. Let’s go through them and see if any of them apply to our case.

1. Rabbeinu Chananel, as well as Rabbi Shmuel ben Eli, Gaon of Baghdad in the Rambam’s time (quoted in Igros Harambam p. 278), held that the reason is techumin. If the bottom of the boat is less than 10 tefachim from the bottom of the river, then there is definitely a problem of techumin. And even if the water is deep, there is a sofek (Eiruvin 43a) whether techumin apply above 10 tefachim. The rule is sofek d’oraisa l’chumra, so one would not be allowed to travel more than 12 mil on such water, because 12 mil is the techum d’oraisa. In Amalia’s case, however, there is no problem of techumin because she departed from New York AFTER Shabbos, and one’s techum can only be established at the onset of Shabbos.

2. The Rif and Rambam argued that if the problem were techumin, it would not be permitted even on Sunday, Monday or Tuesday, or for a mitzvah. Rather, it must be that on a body of water there is never a techum d’oraisa. Therefore, if the boat’s bottom is more than 10 tefachim off the bottom of the water (or in an airplane flying over water), techum is always m’drabbanan and one may travel as far as one wants. What then is the prohibition of embarking on a ship within three days of Shabbos? It is because sea-sickness will prevent him from enjoying Shabbos. In our case, there is no sea-sickness on an airplane.

The Shulchan Aruch OC 248:2 and 404 paskens like the Rambam’s leniency about techumin on water. Here, since the flight traversed land in the Arctic too (Canada), the Rambam’s leniency in techumin does not apply. However, as explained above, one’s techum cannot be established in the middle of Shabbos, only at the onset.

3. The Baal Hamaor holds the reason is because the Jew might be forced to do melacha in the middle of the voyage. On an airplane, passengers never have to do any melacha.

4. The Ramban holds that the non-Jews on the ship might have to do melacha for the Jew. The Mishnah Berurah says this would not apply when the ship belongs completely to a non-Jew. Presumably, it must also be true that most of the passengers are non-Jews – see Shabbos 122a. This is usually the case on flights to South Korea.

5. Tosafos hold that the reason is because water travel is similar to swimming. This reason would not apply on a plane.     

This was all written regarding the ships of the Gemara’s time, which were sailboats. What about ships with a steam or internal combustion engine, or airplanes? Seemingly, two new problems would arise.

1. By sitting in the ship or plane, the Jew is adding weight and thus causing more fuel to be burned. This would apply if he boarded on Shabbos; however, when boarding before Shabbos it would seem to be a passive transgression, which would be mutar. It would be similar to sitting down on the gas pedal of a vehicle before Shabbos and remaining there for the duration of Shabbos.

2. The Ramban’s reason – that the non-Jews do melacha for the Jew, might become a problem now even when the majority of the passengers are non-Jews. The reason is that we consider the work to be done primarily for the majority only in a situation like lighting a candle, where the Gemara says, “a candle for one is a candle for a hundred.” But here, every time the pilot accelerates, he is adding a bit more fuel for the Jew. This is called ribuy shiurim – adding on to the same melacha. This would be analogous to the case of a non-Jew who warmed up water for the use of two non-Jews and one Jew, or for a sick person and a healthy person – would it be allowed to use that water? Tosafos in Gittin 8b is uncertain about this question.

The Shemiras Shabbos K’hilchasa (30:55) says that it’s allowed to travel on a motorized ship driven by non-Jews, provided that the ship departs on a fixed schedule, whether there are enough passengers or not. Apparently, he paskened that ribuy shiurim by a non-Jew is permitted. Thus in our case – the plane traveling on Shabbos – it would be permitted as well.

Finally, even if someone were to disagree with any of the logic above, and they were to argue that one or more of the Rishonim’s reasons do apply to planes (e.g. they would argue that people do feel motion sickness on a plane, or that there is some chance that the Jew will have to do melacha, or that one could indeed have a techum when popping into the middle of Shabbos) I would still contend that it is mutar because in our case, she departed on Motzaei Shabbos, not on Wednesday, Thursday or Friday. Just as Chazal permitted departure at the beginning of the week even though they knew that the voyage would include a Shabbos, so too it is allowed to depart on Motzaei Shabbos even though one knows that part of the flight will be on Shabbos.

In conclusion, Amalia may take the flight, but she should be careful not to do any melacha during the part of the flight when it is Shabbos.

Shabbos 65a: Shmuel’s father made mikvaos for his daughters in the springtime, but in the fall they immersed in the Euphrates River with mats under them to prevent the mud from sticking to their feet. Why did he not let them use the Euphrates in the springtime as well? Because he held that the springtime swelling of the Euphrates is due to rainfall in the west, and he was afraid that the majority of the river’s water might be rainwater, which cannot be used as a mikveh while it is flowing.

שבת סה ע”א: אבוה דשמואל… עביד להו מקואות ביומי ניסן, ומפצי ביומי תשרי… מסייע ליה לרב, דאמר רב: מטרא במערבא ־ סהדא רבה פרת. ־ סבר: שלא ירבו הנוטפין על הזוחלין.

Over the hot springs in a certain city in England (possibly Bath), a famous spa was built in which the boiling hot groundwater flowed into a pool, where tap water was mixed in to cool it down to bathing temperature. There was always a large amount of hot spring water present – more than 80 se’ah. The spring water continued to flow out of the pool, replenished by the spring, and the tap water likewise flowed out, replaced by new tap water. The question, presented to Dayan Aryeh Leib Grossnass of the London Beis Din, was whether the pool could be used as a women’s mikveh.

Seemingly, this would be the same problem Shmuel’s father had with the Euphrates in the spring. Just as in that case, despite the fact that the river contained a tremendous amount of groundwater, it was still invalid as a mikveh while flowing due to the presence of a majority of rainwater, here too a majority of the spa’s water was tap water.

However, Dayan Grossnass found that this depends on a dispute between the Taz and the Shach on Yoreh Deah 201:2. The Mechaber brings the halacha of Shmuel’s father, and the Taz (3) asks: Why is the problem of flowing water (zochalin) worse than the problem of drawn water (mayim sh’uvim – water that has been contained in a vessel, such as tap water)? In the case of drawn water, we know that once you have a kosher mikveh of 40 s’ah, you can add even 1000 s’ah of drawn water and it will still remain kosher. But here, though a spring can be used as a mikveh even when flowing, rainwater cannot, and once the rainwater becomes the majority of the mikveh, it cannot be flowing!

The Taz answers that actually, a spring can still act as a mikveh while flowing even if a majority of rainwater is added, but only in a place where the spring water itself would have reached. Shmuel’s father feared that the amount of spring water in the Euphrates was so little that, if not for the rainwater, mixed into it, it would have dried up before reaching Babylonia, so the rainwater becomes dominant. According to the Taz, in the English spa there was plenty of hot spring water, so the mikveh would be kosher.

However, the Shach (32) disagrees and says that the rule of Shmuel’s father applies in all parts of the river, even where the spring would have reached alone. Dayan Grossnass followed the Shach and therefore did not permit women to use this spa as a mikveh.

Then he sent the question and his teshuva to the Chazon Ish, who replied that it was permitted for two reasons: 1) When the spring water is there first, you are allowed to add drawn water and it can still be flowing. 2) Flowing is only a problem if the water is visibly moving. But here it was not visibly moving, although it was possible to detect the outflow by measurement. Therefore, said the Chazon Ish, the spa could be used if there was no other mikveh available in that city.

Source: Shailos Uteshuvos Lev Aryeh 37