Using Calculations to Begin Ramadan

Q: Can we use astronomical calculations to begin the month of Ramadan?

A: In the Hanbali school, there is no valid opinion that permits astronomical calculation as a method for establishing Ramadan, even for the sake of unifying the Ummah or making scheduling easier.

Physical sighting, or its legally recognized substitutes, such as completing thirty days or a blocked horizon, is non-negotiable.

Fasting based solely on calculations or astronomical predictions does not count, even if the calculation later proves correct, because it is not a recognized legal trigger.

The reason given by Hanbali jurists is that intention follows knowledge, and knowledge in this matter is only established through what the law has authorized, namely sighting or its legal equivalents, not predictive certainty.

From a scientific standpoint as well, astronomical calculations are predictive, probabilistic models, whereas sighting is an empirical observation that establishes certainty.

Sourse: Sharh al-Muntaha & Kashshaf al-Qina