In central Arabia, the course of the year was charted by the position of the stars relative to the horizon at sunset or sunrise, dividing the ecliptic into 28 equal parts corresponding to the location of the moon on each successive night of the month. On the eve of Islam, the Himyarites appear to have used a calendar based on the Julian form, but with an epoch of 110 bce. In South Arabia, some calendars apparently were lunar, while others were lunisolar, using months based on the phases of the moon but intercalating days outside the lunar cycle to synchronize the calendar with the seasons. In pre-Islamic Arabia, various other systems of measuring time had been used. Although all were solar calendars, and hence geared to the seasons and containing 365 days, each also had a different system for periodically adding days to compensate for the fact that the true length of the solar year is not 365 but 365.2422 days. Egypt used the Coptic calendar, with an epoch of August 29, 284 ce. Syria, which until the Muslim conquest was part of the Byzantine Empire, used a form of the Roman “Julian” calendar, with an epoch of October 1, 312 bce. The Sasanids, the ruling dynasty of Persia, used June 16, 632 ce, the date of the accession of the last Sasanid monarch, Yazdagird iii. Each of these calendars had a different starting point, or epoch. But Persia used a different calendar from Syria, where the caliphate was based Egypt used yet another. Correspondence with military and civilian officials in the newly conquered lands had to be dated. This was first of all a practical matter. In 638 ce, six years after the death of the Prophet Muhammad, Islam’s second caliph, ‘Umar, recognized the necessity of a calendar to govern the affairs of Muslims. Qur‘an 10:5 (English by Yusuf Ali) The Hijri calendar
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“It is he who made the sun to be a shining glory, and the moon to be a light (of beauty), and measured out stages for her, that ye might know the number of years and the count (of time).”