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 ul-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.

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