The Astronomical Rules Governing the Umm al-Qura Calendar

The dates for Umm al-Qura calendar, used by the government of Saudi Arabia for civil purposes, are now determined at the Institute of Astronomical & Geophysical Research of the King Abdulaziz City for Science and Technology (KACST) in Riyadh from modern astronomical theories of the sun and the moon. In the past, the calculation of the Umm al-Qura calendar has been based on the following astronomical rules:

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