Shalom Rabbi, I have a question about the 17 Tammuz fast. I will be flying from Israel to Brazil on 17 Tammuz (leave israel on 28 June at 11:35 pm, arrive in Brazil on 29 June at 8:45 am). Should I fast in accordance with Israeli time or Brazilian time? I reside in Brazil.
Thank you very much.
Because you are not crossing the date-line, your flight does not involve major complications. You are leaving Israel at night, before the fast commences, and therefore Israel-time is not relevant for the fast. However, at the time of alos ha-shachar, from when the fast commences, you will be on the airplane, and it is from this time that you must begin fasting. It is hard to determine the precise time, because you cannot be sure of where the airplane is, but you will be able to approximate based on the flight path.
Sources: A number of sources indicate that while on a journey, one bases halachic times on the place where one is — irrespective of the point of embarkation and destination. This applies both to the question above, and to the (more involved question) of crossing the international dateline. See Shoel U’meishiv 4:2:154; Or Le-Zion 1:14; Minchas Shlomo 23; and see at length Vedarashta Vechakarta, p. 612ff.