It is simply due to the absence of a legitimate unifying authority, namely the caliphate. A caliph can adopt and impose one view and all Muslims are obliged to adopt and follow it.
Contemporary Muslim countries will begin Ramadan on different days in its absence.
Fundamentally there is a difference in method employed to establish Ramadhan which is based on an unpredictable lunar calendar. There are one of three possible methods:
1. Calculations without any moon sighting
Turkey has used this method for several years and was employed by Muslims in Australia this year. This will always mean a fixed date for Ramadan and Eid and an early start date for both.
2. Sighting via a powerful telescope
Saudi Arabia has employed this method using what they called "innovative technology" and have admitted that it is near impossible to see the new crescent with naked-eye, which is probably why they are the first to announce a positive sighting every year. These sightings should not be dismissed based on visibility curve maps using outdated criteria.
3. Naked-eye sighting
This is employed in Morocco, Oman, Jordan and other countries. It mostly creates a negative sighting as it will be extremely difficult or impossible to see the new crescent. You'll find "Ramadan is tomorrow based on the completion of Sha'ban as 30 days" is a common message.
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