Dirk Hesse

Credit to Pope Gregory’s Jesuit priests for figuring out the Julian calendar needed updating - before telescopes, and while Christendom was sure the Sun orbited Earth. Fifty years later, Jesuits would prosecute Galileo for saying (and flaunting) that Earth orbits the Sun.

By 1582, the Julian calendar, with a Leap Day every four years, had accumulated TEN extra days relative to Earth’s orbit. So Pope Gregory jump-started his new and exquisitely accurate calendar by canceling 10 days that year, in which October 4 was followed by October 15.

The 1582 Gregorian update to the Julian Calendar prescribes: The century years 1600 1700 1800 1900 & 2000 are all divisible by four, but 1700 1800 1900 were not leap years, yet 1600 & 2000 were. And 2400 will be. On February 29, 2000 most had no idea how rare that Leap Day was.

The Leap Day alone overcorrects the calendar, requiring we remove a Leap Day every century year. But that under-corrects the calendar, requiring we put a Leap Day back in, every four centuries. Behold the Gregorian Calendar.