Islamic Law

Each of the four imams, in his approach to Islamic Law, had at least one unique methodological principle.
It is established in the Sunnah that the Prophet (peace be upon him) offered the Witr prayer in various ways.
At the poles, the Sun rises and sets only once each year, with a half-year day & a half-year night.
What about someone who begins the fast at home then embarks on a journey during the day?