Answered by Sidi Mostafa Azzam
For six years, someone was praying in a certain direction based on his best estimate of the qibla. Then he bought a compass and found that he was off by about thirty degrees. I know that if one finds out with certainty that one prayed in the wrong direction, then those prayers are invalid in our school. Can certainty be established with a compass? Does this person have to repeat six years of prayers? Can he follow another school in this matter?
1. If one’s most recent estimate (Ar. ijtihad) indicates that one prayed towards the wrong direction, one’s previous prayers remain valid.
2. If one discovers with certainty that one was off the precise direction (such that if far from the Kaaba, one would not be considered aimed at it), then one’s previous prayers are invalid. What is meant here by certainty is by
witnessing the Kaaba,
having someone upright inform of his witnessing the Kaaba, or
witnessing a direction to which it is established the Prophet (Allah favor and salute him) prayed (Bughyat al-Mustarshidin, 39).
Given this criterion, relying on a compass does not seem to entail certainty, so the questioner’s previous prayers would be valid.
3. Worth noting is that point 2 is according to the relied-upon position of the Shafi’i school, whereas according to a weaker position that is also strong, if one discovers with certainty that one was off the precise direction but remained within the general direction, then one’s previous prayers remain valid (Bughyat al-Mustarshidin, 39). This is the case with the questioner, who was only off by 30 degrees, so even had he discovered with certainty that he was off the precise direction, his prayer would remain valid. There, therefore, seems no need to research another school on the issue.
And Allah knows best.