1582: the calendar reform

On 24 February 1582 Pope Gregory XIII promulgated the calendar reform with the bull Inter gravissimas. The measure concluded a long process of proposals, consultations and checks, in which the project of Luigi Lilio played a fundamental role.

Why intervention was necessary

The discrepancy accumulated by the Julian calendar had moved the spring equinox away from the conventional date of 21 March. The lunar computation also needed correction. The reform addressed both problems, acting on the solar calendar and on the rules for calculating Easter.

From 4 to 15 October

In the territories that applied the reform in October 1582, Thursday 4 October was followed by Friday 15 October. Ten dates were omitted, while the sequence of the days of the week remained regular.

The initial correction was accompanied by rules intended to limit the recurrence of the discrepancy in the following centuries.

A gradual adoption

The introduction of the new calendar did not take place everywhere at the same time. The decisions depended on the political and religious authorities of the different territories.

For this reason, when studying letters, registers and astronomical observations, a date must be interpreted by considering the calendar actually in use in the place and at the time of writing.

The history of the Gregorian reform thus brings together mathematics, astronomy, religion and politics: it shows how a scientific solution becomes a shared practice through a long historical process.