In the Laurentian Library of Florence, a manuscript with a calendar from the year 813 has come to light. So far, nothing exceptional, if it weren’t for a rather relevant detail: it quantifies and corrects a three-day gap between the Julian calendar and the solar year. A study by Francesco Vizza of the CNR, with the support of Giuseppe Giari, archivist of the Opera di Santa Maria del Fiore, demonstrated that that calendar placed the liturgical dates alongside those of real astronomical phenomena, indicating a difference of three days. This would be the oldest attestation of awareness of the error of the Julian calendar to date, 7 centuries before the issue was resolved with the Gregorian Reform of 1582.
The Julian calendar, developed by Sosigenes of Alexandria and introduced by Julius Caesar in 46 BC, attributed 365 and a quarter days to the year. It was inaccurate by about 11 minutes compared to the actual solar year. The error, consequently, accumulates over time: from the year 325, i.e. from when the Council of Nicaea had set the date of the spring equinox at 21 March, until 813, the difference had reached approximately three days.
The manuscript documents it: March 18th is noted as the day on which the Sun enters Aries, i.e. the true astronomical equinox, while March 21st bears the indication Equinoctiumthe date of the ecclesiastical calculation. The same three-day advance appears for all other equinoxes and solstices. The compilers of the manuscript quantified it with a result in agreement with what modern astronomy calculates for the beginning of the 9th century.
The calendar was found in a Sacramentary from the Opera del Duomo, already reported in 1757 by the Jesuit astronomer Leonardo Ximenes, but never adequately valorised. The discovery dates back by four centuries the full awareness of the time gap of the calendar, until now attributed to the 13th century, and precedes by thirty years a similar calendar from the Prüm Abbey in Germany. The definitive correction would only arrive in 1582 with the Gregorian reform, implemented by Luigi Lilio, which gave rise to the calendar we still use today.
