What celestial observation traditionally marks the beginning of Ramadan?

The sighting of the crescent moon traditionally marks the beginning of Ramadan. The new month begins after the astronomical new moon when the young crescent, called the hilal, is observed shortly after sunset.

Because the Islamic calendar is lunar, Ramadan moves through the seasons over a cycle of roughly 33 years. A lunar month lasts 29 or 30 days, so the start of Ramadan can differ by a day between countries and communities.

Many Muslim authorities use local or national moon sightings, while others accept verified sightings from elsewhere or use astronomical calculations. These approaches can produce different announced dates without changing the religious importance of Ramadan. The astronomical new moon itself is not normally visible, so it is distinct from the thin crescent used to confirm the new month.

The same lunar-calendar principle is relevant to Eid al-Fitr: it begins with the start of Shawwal, the month following Ramadan, and its date may also vary according to moon-sighting practice.