If you just want the briefest answer, here is an excerpt:

“Whoever lives in a land in which the sun does not set during the summer and does not rise during the winter, or who lives in a land in which the day lasts for six months and the night lasts for six months, for example, has to perform the five daily prayers in each twenty-four hour period. They should estimate their times based on the nearest country in which the times of the five daily prayers can be distinguished from one another.”

“Similarly they also have to fast Ramadaan. They can set the time for their fast and determine the beginning and end of Ramadaan and the times of starting and breaking the fast each day by the dawn and sunset each day in the closest country in which night can be distinguished from day. The total period must add up to twenty-four hours…”