Questioner:

What is the your opinion regarding astronomers who use calculations to detect the crescent of the new month?

Shaykh ʿAbd al-ʿAzīz ibn Bāz:

We should not act upon the astronomers’ report. The Prophet ﷺ  said: “We are an illiterate nation. We do not use astronomical writing or calculations [in our fasting]. A month is so and so and so (and he pointed with his hands three times, folding the thumb on the third time, meaning twenty nine days) or so and so and so (and he pointed with his hands three times, meaning thirty days).” He ﷺ also said: “Fast when you see it [the new moon] and stop fasting when you see it, and if it is cloudy then complete the month with thirty days.”

Astrologers must be prevented from publishing their reports, whether it is about sighting the crescent or solar eclipse, as they would confuse people. It is not allowed to act upon their reports. Ibn Taymiyyah conveyed the consensus of scholars that the opinion based on calculations is not to be relied on in the beginning or at the end of Ramaḍān.