Is there any evidence for setting the Islamic calendar through calculations only?
The question of whether the Islamic calendar can be set through calculations rather than solely by sighting the moon is addressed in various sources. In the Shia tradition, a narration from Al-Kafi states, It is correct, but count five days in every four years, and six days in the fifth year between the former and the latter; otherwise, only five days. This is due to the intercalation.
Al-Kafi 2:57 This indicates that calculated methods can be utilized for determining the timing of months based on lunar cycles.
However, the Sunni tradition emphasizes the importance of sighting the moon for beginning and ending months as well. For example, a hadith states: The month is twenty-nine; do not begin fasting until you see it, nor break your fast until you see it.
Sunan Abu Dawud 14:8 This highlights a reliance on physical observation rather than purely calculated methods.
Thus, while there is some evidence in the Shia tradition for using calculations to set the Islamic calendar, the dominant practice across both traditions remains the sighting of the moon as the primary method for determining the months.
Sources
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Al-Kafi 2:57
Al-Kafi · Book 2, Hadith 57
I read in the book of Ali (peace be upon him) that Allah did not take a covenant from the ignorant to seek knowledge until He took a covenant from the scholars to give knowledge to the ignorant, since knowledge existed before ignorance. — OR — If your skin shivers and your eyes weep, then proceed, proceed: your intention has been accepted. This was also reported similarly by Muhammad ibn Isma'il from Abu Isma'il al-Sarraj from Muhammad ibn Abu Hamza from Sa'id. — OR — The postpartum woman sits for the number of days she usually does in her menstruation and then waits two days. — OR — Muhammad ibn al-Faraj wrote to al-Askari (peace be upon him) asking about the calculation of fasting five days between the beginning of one year and the beginning of the following year as reported from your ancestors. He replied, 'It is correct, but count five days in every four years, and six days in the fifth year between the former and the latter; otherwise, only five days. This is due to the intercalation.' Al-Sayyari said, 'This method was approved by our companions and they found it to be accurate.' Muhammad ibn al-Faraj wrote to him in the year 832, 'This calculation is not something everyone can follow; it is intended for those who know the years and when the leap year occurs. Then, if the crescent of Ramadan is confirmed on the first night, it is correct for him. And if the crescent is confirmed for its night and he knows the years, it will be sound, God willing.' — OR — The Commander of the Faithful (peace be upon him) often said, "Know with certain knowledge that Allah, Glorious and Exalted, has not allowed any servant, no matter how much effort he exerts, how great his strength, or how long he endures, to surpass what has been decreed for him in the Wise Reminder (the Qur'an). Nor does it become permissible for a servant in his weakness and little power to attain more than what has been decreed for him in the Wise Reminder. O people, a stingy person will not increase by his cleverness, nor will a stingy person decrease because of his foolishness. The scholar who acts upon this is the most at ease regarding his benefit, and the scholar who abandons it is the busiest in his harm. Many who are favored are gradually led astray by the kindness shown to them, and many are deluded among the people while being manipulated. So awaken, O one who strives, moderate your haste, be alert from the sleep of heedlessness, and contemplate what has come from Allah, Glorious and Exalted, through the tongue of His Prophet (peace be upon him and his family). Guard these seven letters, for they are from the words of the eloquent people and part of Allah's determinations in the Wise Reminder, that no one meets Allah, Glorious and Exalted, with any of these faults: associating partners with Him in what Allah has enjoined; or quenching anger by destroying his own soul; or admitting to something another does; or seeking victory through showing innovation in his religion; or taking pleasure in people praising him for what he did not do; or being arrogant, boastful, and ostentatious. O people, the ambition of wild beasts is to transgress, the ambition of animals is their bellies, the ambition of women is for men, while the believers are fearful, anxious, and cautious. May Allah make us and you among them. — OR — He sends to her and says: the Messenger says, "Be mindful, for so and so has separated from you." — OR — In the case of a man who died leaving his parents, to the father belongs two shares and to the mother one share.
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Sunan Abu Dawud 14:8
Sunan Abu Dawud · Book 14, Hadith 8 · Narrated by Ibn Umar
The Messenger of Allah (ﷺ) said, 'The month is twenty-nine; do not begin fasting until you see it, nor break your fast until you see it. But if it is obscured for you, then complete thirty days.' Ibn Umar said that when Sha’ban reached twenty-nine days, he would look for the moon: if he saw it, that was it; if not, and nothing blocked his view, he would break his fast. But if clouds or darkness obstructed the moon, he would continue fasting. Ibn Umar also fasted with the people and did not rely solely on calculations. Umar bin Abdul Aziz wrote to the people of Basra indicating the hadith of Ibn Umar from the Prophet (ﷺ), saying, 'We add what is best we can estimate. When we see the crescent of Sha’ban on a certain evening, fasting (Ramadan) will, God willing, begin at a particular time, unless you see the crescent earlier than that.'
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