If brothers have gone to see the time of isha, is it allowed to doubt this sighting with the excuse of light pollution? Also is it allowed to take a ruling based on mathematical calculation that the time of isha starts after islamic midnight even though the redness disappears way before it?

Ustadh AbdulAziz al-Haqqan

The sighting has to be correct. The red twilight (Al-Shafaq Al-Ahmar) has to disappear, and calculations are not a way for that. Therefore, making calculations is an incorrect method.