Question: How is the shar‘ī midnight calculated for maghrib and ‘ishā’ prayers?

Answer: Midnight for maghrib and ‘ishā’ prayers is the middle time between sunset and fajr (time for morning adhān).
There are two fajrs: true fajr and false fajr. False fajr is when a ray appears in the sky which is reflected vertically upwards instead of spreading on the horizon. Fajr sādiq (true fajr) is when the white light attached to the surface of the horizon appears with low intensity and spreads on the horizon, and over time, its light intensity increases. Due to the weakness of fajr sādiq, observing it requires a completely clear and dark eastern horizon and it is very difficult to see it in cities like Tehran. Due to the fact that it is difficult to accurately detect the true fajr, the morning prayer should be performed as a precaution, later than abstinence from fast invalidators by a person who wants to fast.