POPE GREGORY XIII 1502-1585

In solving another scientific problem, the rest of the world lagged far behind while the Catholic Church took the lead and a pope justly took the credit. Papal astronomers noted with alarm that the date of the vernal equinox, the first day of spring, kept changing according to the Julian Calendar implemented by Julius Caesar in 45 B.C. In A.D. 325, the date of the vernal equinox was March 21st. Over time, the date kept moving gradually backward until, by 1582, it fell on March 11th. The Pope, Gregory XIII, grasped the problem and seized the solution.

After consulting the brilliant priest-astronomer Christopher Clavius, Pope Gregory recognized that the Julian Calendar contained an innaccuracy of 11 minutes per year, which, after the passage of 1,257 years, amounted to an error of 10 days. He therefore undertook a two-part reform of the Julian Calendar. On February 24, 1582, he issued the papal bull Inter gravissimas, in which he “prescribed and ordered” all Catholic monarchs to cancel 10 days immediately following Thursday, October 4, 1582, and to re-designate what would have been Friday, October 5th, to be, instead, Friday, October 15th.Furthermore, to correct the Julian Calendar’s annual 11-minute error, Pope Gregory reduced the number of leap years observed every 400 years from 100 to 97. Only those century years (ending in 00) divisible by 400 were to be leap years; the other century years were not. Thus, the year 1600 was a leap year, but 1700, 1800, and 1900 were not. Likewise, the year 2000 was a leap year, but 2100, 2200, and 2300 will not be; 2400 will be; 2500, 2600, and 2700 won’t be; and so forth. Otherwise, leap year was still to be observed every fourth year by adding 1 day to February, giving it 29 days and thereby giving the leap year itself 366 days.