Shaykh ‘Abdul-‘Aziz ibn Bāz (rahimahullah):

The entrance of the month of Ramaḍān is determined by one person sighting the new moon. If the new moon is not seen, Sha‘bān is completed as thirty days, and fasting in Ramaḍān begins after the thirtieth day. It is impermissible to rely on astronomical calculations to determine the start of Ramaḍān, Shawwāl, or Dhul-Ḥijjah.

Acts of worship are not based on personal choices or opinions but are ‘tawqifiyyahʼ, derived from the Quran and the authentic Sunnah of the Prophet ﷺ. It is not allowed for anyone to make something up out of thin air. Allāh commanded the Prophet ﷺ to guide us on when to fast and when to stop fasting. The Prophet ﷺ said:  “Fast when you see the moon, and cease fasting when you see it but if it is cloudy for you then complete the month as thirty days.” (Bukhāri and Muslim)

And he ﷺ said: “Do not fast until you see the new moon [of Ramaḍān] and do not stop fasting until you see the new moon [of Shawwāl] but if it is cloudy for you then count the month [as thirty days].” (Bukhāri and Muslim)

And he ﷺ said: “Fast when you see the moon, and cease fasting when you see it but if it is cloudy for you then complete the month of Sha’bān as thirty days.”(Bukhāri and Muslim)

Believers rely on the Prophet’s guidance to determine the start and end of the month. If Sha‘bān is completed as thirty days, fasting begins the next day. At the end of Ramaḍān, if the new moon of Shawwāl is sighted, fasting ends, making Ramaḍān twenty-nine days. Otherwise, Ramaḍān is completed as thirty days. This is what the Prophet commanded with.

Astronomical calculations are not relied upon by all the scholars. The scholars, from the time of the companions (Ṣaḥābah) and those who followed them, have consensus (ijmā‘) that astronomical calculations are not relied upon in determining the entrance of the month of Ramaḍān. This consensus has been relayed by Abū al-‘Abbās Ibn Taymiyyah (rahimahullah) and many scholars, whilst some of them mentioned that there is an extremely weak alternative opinion on it.

In summary, astronomical calculations are not relied upon as is explicitly mentioned in the statement of the Messenger ﷺ. The claim that astronomical calculations should determine the start of Ramaḍān contradicts the statement of the Prophet ﷺ: “We are an unlettered Ummah, we do not use astronomical calculating nor writing”, and he also said: “So do not fast until you see the new moon.” (Bukhāri and Muslim)

The Prophet ﷺ, being the most sincere, eloquent, knowledgeable, and trustworthy among the people, would have explicitly permitted reliance on astronomical calculations if it were permissible. However, he did not say such a thing despite his ability and obligation to convey the message clearly. Therefore, he did not instruct people to rely on astronomical calculations even if there were individuals capable of doing so.

Even today, much of the ummah, from East to West, lack familiarity with astronomy. However, even if a significant portion or the majority were knowledgeable in this field, it remains impermissible for them to rely solely on it. It is obligatory upon them to follow and not innovate — they must follow the guidance of the Messenger ﷺ.

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Source: Majmū’ al-Fatāwa of Ibn Bāz, by way of ‘al-Iktiyārāt al-Fiqhiyyahʼ compiled by Khālid ibn Su‘ūd al-‘Ajmi (pp. 313-314)