How the onset of Ramadaan is determined

(9) The onset of Ramadaan is confirmed by the sighting of the new moon, or by the completion of thirty days of Sha’baan. Whoever sees the crescent of the new moon or hears about it from a trustworthy source is obliged to fast.

Using calculations to determine the onset of Ramadaan is bid’ah, because the hadeeth of the Prophet (peace and blessings of Allaah be upon him) clearly states: “Fast when you see it (the new moon) and break your fast when you see it.” If an adult, sane, trustworthy, reliable Muslim who has good eyesight says that he has seen the crescent with his own eyes, then we should take his word for it and act accordingly (i.e., start fasting).