Calendars have always been complicated affairs. In 46 BC, Julius Caesar implemented one of the most successful. A key feature was the creation of an intercalary day every four years, which was achieved by doubling February 24. These “leap years” worked reasonably well, except that by the 1500s they had created problems with the date of Easter.
In AD 325 the First Council of Nicaea had ruled that Easter should fall on the Sunday after the full moon after the Spring equinox. At the time of the council, the equinox fell on 20 or 21 March. However, by the 1500s, things had drifted so that the equinox was falling in the wrong place.
A Neapolitan astronomer named Luigi Lilio approached Pope Gregory...
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