When is a moon not a moon? That is the question vexing Islamic astronomers after accusations that Saudi Arabia had jumped the gun in declaring the festival of Eid al-Fitr because they mistook Saturn for the Moon.
The festival is the most joyous moment in the Islamic calendar and celebrates the end of the fasting month of Ramadan. But every now and then celebrations are marred by bickering between competing Islamic authorities over when Eid starts. This year's row is over whether the festival should have begun last Monday or Tuesday.
According to the instructions laid down by the Prophet Mohamed, Muslims can only break their fast when the crescent of a new moon is spotted above the horizon.
Where once Muslim communities relied on local committees of star-gazers to locate the new moon - known in Arabic as the Hilal - a growing number of mosques around the world now look to the Saudi authorities to decide when Eid celebrations can begin.
Last week Saudi scholars declared that the end of Ramadan began on Monday evening and gave the signal that Eid celebrations could commence. But others were not so sure.