Gregorian Calendar Adopted

The Gregorian calendar (named after Pope Gregory XIII, who decreed its production) was initially designed as a Christian calendar to keep better track of Easter. Luigi Lilio, an Italian doctor, mathematician and astronomer was commissioned in the 1570s to find a way to fix the deficiencies in the Julian calendar used by the Romans. These studies uncovered the fact that the Romans had been performing leap years at the wrong times (against the command of Julius Caesar, the calendar’s namesake). Each of the calendars attempted to track the time from the supposed year of the birth of Jesus Christ (presumably 1AD) and are given the abbreviation AD or CE for anno Domini (meaning “the year of our Lord”) or common era. Easter is then calculated by most Christians to be the fourteenth day of the lunar month that falls on or after the twenty-first of March. Ideally this day would be the venial equinox or the day that the sun is directly over the equator.

The Gregorian calendar skipped ten days to realign the dates with the seasons and continued the leap year sequence of the Romans every fourth year. The mathematics of the calendar can be staggering to think about. The day is the basic unit of time and it was shown that the solar calendar repeats completely every 146,097 days or 400 years. This number also happens to be divisible into 20,871 seven-day weeks. Of the 400 years, 303 have 365 days and 97 are 366 day leap years. The average year length is exactly 365.2425 days or 365 days, 5 hours, 49 minutes and 12 seconds.

The Gregorian calendar was almost never published. Six years after Luigi died, his brother, Antonio, presented it to the pope. After a few modifications by the highly respected German Jesuit Christopher Clavius (a Catholic scholar and mathematician), a papal bull was sent out that all Catholic people would switch to using the new calendar. Only eight countries adopted the calendar in the first year. Most countries now use the Gregorian calendar, China being one of the last to adopt it in the 1920s. There are a few debates and mistakes that arise from time to time, primarily surrounding the idea that no one really knows when Jesus was actually born, but the calendar seems to be here to stay. Some countries and religions maintain a second calendar that tracks their historical dates and things like the new year and famous celebrations, but these are regional and do not affect the world as a whole.

Luigi Lilio (also Aluise Lilio, left), Pope Gregory XIII (center) & Christopher Clavius (right) contributed to the publishing of the Gregorian calendar in an attempt to better track the official date for Easter based on when they thought that the First Council of Nicaea had celebrated in the year 325AD.