Pope Gregory XIII issued the papal bull Inter gravissimas on the 24th of February 1582 to replace the Julian calendar. The Catholic Church needed to fix a drift that had pushed the vernal equinox away from its traditional date of the 21st of March. This shift threatened the calculation of Easter, which depends on the spring equinox falling near that specific day. By AD 325, the First Council of Nicaea had set rules for celebrating Easter based on the equinox. Over the next twelve centuries, the Julian calendar accumulated an error of about ten days. In October 1582, Thursday the 4th of October was immediately followed by Friday the 15th of October to correct this gap. Scholars like Bede in the eighth century and Roger Bacon later noted the growing discrepancy between the calendar and observed reality. Dante Alighieri also wrote about the need for reform before the project stalled until the late fifteenth century. Pope Sixtus IV invited Regiomontanus to Rome in 1475 to address the issue but the mathematician died shortly after arriving. The urgency grew as astronomical observations became more precise toward the end of the 1500s.
Design And Mechanics
The Gregorian system modifies leap year rules to create an average year of 365.2425 days. A normal year has 365 days while a leap year adds one day to February making it 29 days long. Every year divisible by four is normally a leap year except for years divisible by 100 unless they are also divisible by 400. This means 1800 and 1900 were not leap years but 1600 and 2000 were. The rule reduces the number of leap years from 100 to 97 every four centuries. Christopher Clavius expanded on the proposal made by Calabrian doctor Aloysius Lilius in an 800-page volume. Lilius suggested deleting ten Julian leap days over forty years to restore the equinox to the 21st of March. The reform also adjusted the lunar cycle used to calculate Easter because new moons occurred four days earlier than calculated tables predicted. A 19-year cycle required revision with corrections applied once every 300 or 400 years. The calendar repeats completely every 400 years which equals 146,097 days. These cycles contain exactly 20,871 weeks.