Why don't we rely on astronomical calculation to determine the beginning and end of Ramadan, in accordance with His, the Exalted, saying, "The sun and the moon [run] by precise calculation," instead of visual sighting, especially since modern science has proven the possibility of doing so?

It is from Allah's mercy upon the Ummah that the Sharia is easy, and its rulings are tied to observable matters, such as linking the start of the month to the sighting of the crescent moon. This is based on the Prophet's (peace be upon him) saying: "We are an unlettered nation; we neither write nor calculate. The month is like this, and this, and this."

Scholars have not relied on astronomy to confirm the beginning or end of the month. They have agreed that what is considered is the visual sighting, based on the Prophet's (peace be upon him) saying: "Do not fast until you see it, and do not break your fast until you see it."

Muslims have unanimously agreed on the impermissibility of acting upon the news of a calculator (astronomer) concerning the sighting of the crescent moon. There is no known ancient divergence on this matter. Whoever claims its permissibility is an outlier, preceded by the consensus against it.