Many of the opinions about the halakhic date line are structured as a response to the question of what days someone should observe as Shabbat and Jewish holidays.
The concept of a halakhic date line is mentioned in the Baal HaMeor, a 12th-century Talmudic commentary,[2][3][6] which seems to indicate that the day changes in an area where the time is six hours ahead of Jerusalem (90 degrees east of Jerusalem, about 125.2°E, a line now known to run through Australia, the Philippines, China and Russia).
[7] If this line of longitude were used strictly, people could simply avoid Shabbat altogether by crossing the street.
As a result, he rules that the date line runs along 125.2°E when over water, but curves around the eastern coast of mainland Asia and Australia.
By this view, Russia, China and mainland Australia are west of the date line and observe Shabbat on local Saturday.
Rabbi Yechiel Michel Tucazinsky ruled that the International Date Line is 180 degrees east of Jerusalem.
According to this point of view, all of the major populated areas of the Pacific (such as New Zealand, Japan, Alaska and Hawaii) observe Shabbat on local Saturday (that is, consistent with the conventional International Date Line).
According to Rabbi Menachem Mendel Kasher, there is no clear tradition or Talmudic source dictating any of the preceding opinions as binding.
But the de facto result of this position is consistent with the conventional International Date Line, at least anywhere there is an established Jewish community.
The travelers' guide of the Star-K kosher supervision service, compiled according to the rulings of its rabbinic administrator, Rabbi Moshe Heinemann, uses the following approach,[2] which is also cited by others.
However, the act of crossing the date line (wherever it may be drawn) introduces a number of additional issues under Jewish law.
Questions potentially affected include:[2] In some cases, crossing the date line (wherever it may be drawn) has a specific impact on practice or prohibitions under Jewish law.
In others, an individual's count of days (by the experience of sunset and sunrise) is the determining factor, regardless of the crossing of the date line.
[23][24][25] Judith Resnik, the first American Jewish astronaut in space, lit (electronic) Shabbat candles according to the time in Houston, TX, her home and the location of Mission Control.