The measuring of time in Byzantium comprised the chronology, that is the determination of the number of the year from a given origin, the calendar, that is the division of the year, and the time, that is the division of the day. All three were calculated with the help of astronomy.
The Byzantine era counted its origin starting from the Creation (21 March 5508 B.C.). and used the Julian calendar, The astronomical tables were based on an era and calendar different from the Byzantine ones, therefore one of the major concerns of Byzantine astronomers was the chronological conversion, that is the correspondence between the tables dates and the Byzantine ones. The first chapters in aft Byzantine astronomical manuals were dedicated to this subject These manuals were based mainly on Ptolemaic astronomy, which followed the commentaries on Ptolemy by Theon of Alexandria, or on Persian astron¬omy and the School of Maragha. Rolemaic and Persian astronomical tables used a calendar of a 365-days year.
Byzantine astronomers used the astrolabe to determine time, an instrument considered as the most precise clock by Theodoros Meliteniotis. one of the greatest astronomers of the Paiaeologan period. Although only one Byzantine astrolabe has been preserved, there is a rich literature on the subject. Byzantines used both the equal hours -being in fact slightly unequal, as the astrolabe measures the real and not the main solar time- and the unequal ones – defined by dividing the night as well as the day by twelve.
