Dr. Zakir explains that in Islam, the beginning of the month is determined solely by sighting the new crescent moon with the naked eye, not through astronomical calculations. He discusses hadith that indicate Prophet Muhammad (peace be upon him) did not rely on calculations and said Muslims should fast when they see the moon with their eyes. The testimony of two just Muslim men who sight the moon is sufficient to begin Ramadan, though one witness is enough for the start and two are needed for the end. Astronomical evidence should not be used to confirm sightings, as this contradicts the teachings of the Prophet.