Praise be to Allah.
Islam is easy and its rules apply to everyone, mankind and jinn alike, regardless of whether they are scholars or illiterate, settled or nomadic. So Allaah made it easy for them to know the times for the acts of worship, and made their beginning and end signs which everyone can know. He made the setting of the sun the sign for the beginning of Maghrib and the end of ‘Asr, and He made the disappearance of the red glow in the sky the sign of the beginning of the time of ‘Isha’. He made the sighting of the moon after its disappearance at the end of the month the sign of the beginning of the new lunar month and the end of the previous month. He did not give us the task of knowing the beginning of the lunar month from something which is known to only a small number of people, namely astronomy and its calculations. Thus the texts of the Qur’aan and Sunnah make the actual sighting of the new moon the signal for the Muslims to start fasting Ramadaan; they stop fasting when they see the new moon of Shawwaal; the dates of Eid al-Adhaa and Yawm ‘Arafaat are similarly determined. Allaah says (interpretation of the meanings):
“… So whoever among you sights (the crescent on the first night of) the month (of Ramadaan), he must observe sawm (fast) that month…” [al-Baqarah 2:185]
“They ask you (O Muhammad) about the new moons. Say: these are signs to mark fixed periods of time for mankind and for the pilgrimage…” [al-Baqarah 2:189]
The Prophet (peace and blessings of Allaah be upon him) 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.”
The Prophet (peace and blessings of Allaah be upon him) made the beginning of the fast conditional upon the confirmed sighting of the new moon of Ramadaan, and the ending of the fast conditional upon the confirmed sighting of the new moon of Shawwaal; he did not connect this to calculations of the movements of stars or other heavenly bodies. This is how it was done at the time of the Prophet (peace and blessings of Allaah be upon him), during the rule of the Khulafa’ al-Raashidoon, at the time of the four imaams, and during the three centuries which the Prophet (peace and blessings of Allaah be upon him) said were the best. Referring to calculations of the lunar months to start and end periods of worship, instead of actually sighting the new moon, is a bid’ah (reprehensible innovation) that has no good in it and has no basis in sharee’ah. The best of all is to follow the salaf in religious matters, and the worst of all is to follow newly-invented innovations in religion. May Allaah protect us and you and all the Muslims from tribulation, both obvious and hidden.