Al-Azhar Mosque
The accuracy of applying these methods to actual locations depend on the accuracy of its input parameters—the local latitude and the latitude of Mecca, and the longitude difference. At the time of the development of these methods, the latitude of a location could be determined to several arc minutes' accuracy, but there was no accurate method to determine a location's longitude. Common methods used to estimate the longitude difference included comparing the local timing of a lunar eclipse versus the timing in Mecca, or measuring the distance of caravan routes; the Central Asian scholar Al-Biruni made his estimate by averaging various approximate methods. Because of longitudinal inaccuracy, medieval qibla calculations (including those using mathematically accurate methods) differ from the modern values. For example, while the Al-Azhar Mosque in Cairo was built using the "qibla of the astronomers", but the mosque's qibla (127°) differs somewhat from the results of modern calculations (135°) because the longitude difference used was off by three degrees.