Manuscript from 813 correcting the Julian calendar discovered

The measurement system introduced by Julius Caesar in 46 BC had an average duration of the year slightly longer than the astronomical one, leading over the centuries to a progressive deviation of the official dates with respect to the equinoxes and solstices. Although this discrepancy was formally resolved only in 1582 with the Gregorian Reform of Luigi Lilio, the Florentine manuscript attests that a technical awareness of the phenomenon had already been achieved in the 9th century. “The calendar was found in a Sacramentary from the Opera del Duomo and although it had already been reported in 1757 by the Jesuit astronomer Leonardo Ximenes, its significance for the history of astronomy and the measurement of time had never been adequately valorised”, explains Francesco Vizza. “Historiography has traditionally maintained that only starting from the 13th century did a full awareness of the gap between the Julian calendar and the solar year develop”.

The document integrates both the conventional dates of the ecclesiastical calculation and those corresponding to the celestial phenomena detected. While the tradition established by the Council of Nicaea in 325 fixed the spring equinox on March 21, the calendar of 813 records the entry of the Sun into the sign of Aries on March 18, highlighting a gap of three days.

Regarding the impact of this finding on scientific historiography, Vizza observes: “Historiography has traditionally maintained that only starting from the 13th century did a full awareness of the gap between the Julian calendar and the solar year developIn addition to the astronomical value, the manuscript fits into a wide-ranging cultural context, presenting analogies with the famous Prüm calendar of 840. This proximity in time and content suggests the existence of a Reichskalender (empire calendar) or of a dense network of scientific exchanges between the computing centers of the Carolingian Empire, confirming a level of methodological accuracy in the measurement of time that significantly anticipates the scientific acquisitions of the late Middle Ages.