Ramadan this year started on Monday in some countries but on Tuesday in others. How is that possible? 

The exact start time of Ramadan depends on when local Islamic authorities around the world declare the sighting of the new moon, the astronomical event that marks the start of the observance. 

the exact start time of Ramadan varies from place to place because it depends on a range of factors, including who observes the moon and how, and whether the sky is clear or cloudy at the time.

New technologies make moon readings more accurate, but they can also make the process more complex, the study said. For instance, does it count as an official sighting if a new crescent moon can be seen by an optical aid but not by the naked eye?

It isn’t just the start date of Ramadan that varies from place to place. The exact amount of time that people hold their dawn-to-dusk fasts depends on which year it is, and where they live in relation to the Equator.