Paying attention to a compass while praying on a moving train

Question

Before starting the prayer on a moving train, [a person] determined the qibla direction with a compass, and then [the train] turned to the north or to the south — is the prayer valid or not? Or may one keep the compass open and keep turning towards wherever the qibla is — but in this case one's attention will be on the compass: will this cause a deficiency in the prayer?

Answer

If, having looked at the compass at the outset, he prayed in the correct direction, then the prayer is valid, so long as during [the prayer] he does not come to a predominant belief (zann ghalib) that the direction has changed.

Source: Fatawa Mahmudiyya

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