Ptolemy and Calendar Reform

Ptolemy did not directly contribute to calendar reform, but his work proved essential for the study of historical chronology, the establishment of the exact period of time elapsed between two dates, when those dates might be given in different calendar systems, as a consequence, for example, of calendar reform.

To achieve an absolute chronology, it is necessary to have dates in a local calendar that is continuous (without interruptions) and precisely known, tied to observations of astronomical events (such as lunar eclipses) to provide fixed points. Such circumstances existed in Mesopotamia and Egypt. In dating astronomical observations, Ptolemy employed the Egyptian administrative year. This was of unvarying length: twelve months of thirty days, and five extra days at the end to total 365. The numbers of individual years could be given with reference to a standard epoch (defining some particular event as occurring at year 1), or to the reign of a particular ruler. In the latter case, an unbroken list of kings would also be required. Ptolemy provided such a list, which was maintained and extended by the Byzantines well into the Middle Ages.

Thus, when calendars were reformed or new ones introduced, as long as a systematic relationship could be established with the old calendars, it would be possible to translate one date between systems and to maintain an absolute chronology. The calendars would be tied, ultimately, to objective astronomical events. The data from the Syntaxis provided just such a foundation for subsequent Byzantine, Islamic, and medieval calendars.

Recommended Reading

M.Hoskin (ed.) The Cambridge Illustrated History of Astronomy, Cambridge 1997

J.North The Fontana History of Astronomy and Cosmology, London 1994

G.J.Toomer "Ptolemy", pp.186-206 in The Dictionary of Scientific Biography, New York 1970

Full Bibliography