The observance of Jewish law (halakhah) in the polar regions of Earth presents unique problems.
Many mitzvot, such as Jewish prayer and Shabbat, rely on the consistent cycle of day and night in 24-hour periods that are commonplace in most of the world.
The problem was first identified in the 18th century when Jewish émigrés began to move in greater numbers to the northern parts of Scandinavia.
(A "variable hour" is one-twelfth of the time between sunrise and sunset, or according to another opinion between dawn and the appearance of stars at twilight.
[citation needed] The Bible, Talmud, and individual pre-modern Jewish writers do not address this issue, because Jews of this period did not visit the polar regions and were unaware of its distinctive nature.
[13] In the polar regions, such as in northern Sweden, where the midnight sun can be as long as two or three months out of a year (in the summer months), or where the sky is dark even at 2 o'clock PM (in the day) for several weeks during the winter, or what is called the polar night, the author of Sefer Ha-brit (Article 4) asks the question, what shall a Jew do when he goes to either the North Pole or the South Pole where daylight is prolonged for as much as two to three months, and, particularly, when wanting to know at what hour he must begin observing the Sabbath day and religious holidays (days that are usually ushered-in at nightfall), or when he must begin his fast on Yom Kippur?
[14] Pinchas Elijah Horovitz (18th century) stated that polar regions should observe Shabbat based on calculating 24-hour days, although without establishing a date line.