Question:
When should Sabbath Services be held by troops of Jewish faith in the northern latitudes — Iceland, etc.? The problem involves the extent of the period of twilight.
Answer:
Twilight in rabbinic law marks the end of the day and the beginning of night. Hence, the beginning of the Sabbath depends upon the extent of twilight in the various latitudes. However, in the far northern latitudes the normal twilight is extended in the summer months by what is known as the astronomical twilight, which often lasts all night; hence, there is continuous twilight or half-twilight for months in succession. Therefore, it is impossible to know when the Sabbath should begin or end. This problem is referred to by Chaim Mordecai Margolis, Shaarey Teshuva, to Shulchan Aruch Orach Chayyim, No. 344. The Shulchan Aruch there discusses the question of the man who is travelling in the desert and finds he has lost count of the days. Joseph Karo, basing his opinion on the Talmud, says that he must begin counting days from the time that he became aware of his forgetfulness and must celebrate the seventh day as sabbath and mark it with Kiddish and Havdalah. This traveller, not knowing the exact day of the Sabbath, selects a day and observes it as the Sabbath.
Shaarey Teshuvah (ad loc.) applies the same principal of the selection of a time for the Sabbath to those who live in the northern latitudes and do not know the day and the hour of Sabbath, just as the man in the desert does not know the day of the Sabbath. The statement of Margolis is as follows: "Those who journey near the North or South Pole where the day lengthens into a month or two months and sometimes even six months, should count six days of twenty-four hours". In other words, the Jew in the Artic latitudes selects a time and an hour-length equivalent to those in the lower latititudes, This is the best that he can do.
Dr. Pool reports that such is actually the practice among communities of northern Norway and Sweden. They follow the hours for the Sabbath independently of their sunset and follow that of Hamburg.
Jewish soldiers in the northern latitudes should pick the hours of the Sabbath as observed in New York or Seattle and fix their Sabbath Service accordingly, regardless of whether twilight (in summer) or darkness (in winter) has yet come.