Page "Pope Gregory XIII" Paragraph 10

Pope Gregory XIII is best known for his reformation of the calendar, with the aid of Jesuit priest / astronomer Christopher Clavius, who is credited as the calendar's chief architect.

The reason for the reform was that the average length of the year in the Julian calendar was too long – it treated each year as 365 days, 6 hours in length, whereas calculations showed that the actual mean length of a year is slightly less ( 365 days, 5 hours and 49 minutes ) As a result, the date of the actual vernal equinox had slowly ( over the course of 13 centuries ) slipped to 10 March, while the computus ( calculation ) of the date of Easter still followed the traditional date of 21 March.