Dear Rabbi Bitton, I often travel to the Far East on business. When I'm done with my business dealings, I'm anxious to get back to New York as soon as possible and spend Shabbat with my family. One convenient flight that exists is a flight that leaves Hong Kong Friday at noon and arrives in New York Friday afternoon at 2:00 PM. My only concern is that during this long flight there is a time when I cross the International Dateline, and I believe, for the local Jewish community of the country I am flying over, it is Shabbat. Is this segment of the flight considered Shabbat for me as well? If so, should I avoid such a flight in order to avoid a desecration of Shabbat?
A: Dear friend, in the case you are describing, traveling from Hong Kong to New York, the plane flies against the sun, into the sunset and the plane is in the air during Friday evening (Shabbat). The question then becomes the following: do we consider Shabbat according to the personal situation of the passenger (in which case such travel will be forbidden), or according to the location where the passenger boarded and is going to land (in which case, since in both cities it is before Shabbat, such travel would be permitted).
Obviously, the Talmud could not possibly contemplate this technological scenario, and the contemporary authorities who dealt with the issue, differ in their opinions.
There is another consideration that you are mentioning, which is a bit more technical, and adds another element to the equation, and that is the issue of the International Dateline. This is the imaginary line that divides the planet in two hemispheres and determines the beginning and end of the day. So, are we allowed to cross this line into Shabbat if we are not in the land? If you want to know all the details involved in this fascinating discussion, I recommend you to read a very good summary in the book "Practical laws of Shabbat, Volume 1" (which we have in our Kanissah library!) pages 68 to 80.
Now, to answer your practical question, there are 3 opinions on the matter:
1. The first one strictly forbids traveling on such a flight because you are crossing the International dateline on Shabbat.
2. The second opinion holds that only for very special circumstances (i.e. Mitzvah, emergency or great financial loss) such travel will be permitted.
3. The third opinion allows such travel (according to such authorities as the Chazon Ish and Rav Pesach Frank) based on the fact that in their opinion the Torah does not establish a Halakhic dateline, and therefore the defining criteria is whether you board the plane before Shabbat and the plane landed before Shabbat.
In our Community, based on the ruling of Rabbi Eliyahu Ben Hayim Shelit'a we follow this third opinion.
Having said so, it is important to remember that in principle one should avoid taking flights on Friday because of the possibility of unexpected delays.