Gregorian calendar

See also:

adoption of the Gregorian calendar
eras used with the Gregorian calendar

The Gregorian calendar is a version of the Julian calendar, altered a bit to reduce an intolerable difference between lengths of the solar year and the Julian calendar year. The Julian calendar (introduced 46 bce) assumes the solar year is 365.25 days long, but even in Caesar's time the solar year was actually closer to 365.2422 days, about 11 minutes 4 seconds shorter (it is now about 11 minutes 14 seconds shorter). In 384 years the annual eleven minute error adds up to about 3 days, and in a millennium to about 7.8 days.

By the 8th century people were already noticing that the vernal equinox was coming too early. By the 13th century the error was more than 7 days and Roger Bacon was urging calendar reform on the pope.¹ Three hundred years later Pope Gregory XIII (elected 1572) decided to do something about it.

The principal creator of the Gregorian calendar was Luigi Giglio (Latinized as Aloysius Lilius, ?-1576), a lecturer in medicine at the University of Perugia. In its early years the calendar was sometimes called the Lilian calendar.

Lilius died before the reform occurred, but after his death his brother Antonio presented to Pope Gregory a manuscript titled Compendiuem novae rationis restituendi kalendarium (Compendium of the New Plan for the Restitution of the Calendar) that Aloysius had written. The Pope forwarded it to the calendar reform commission he had created. The commission, which included the noted astronomer Christopher Clavius (1537-1612) was broad-minded enough to recognize the plan's virtues.

Acting on the commission's recommendations, by a papal bull of February 24, 1582,² Gregory made the following reforms:

Ten days of the accumulated error were removed by making the day following Thursday, October 4, 1582, Friday October 15. The days were subtracted from October because that month had the fewest religious feasts, and the commissioners also felt it was a slow month for trade.

This change was not enough to return the vernal equinox to March 25, where it had been in Caesar's time, but rather to March 21, where it was during the Council of Nicea in 325 ce. That Council had fixed the date of Easter in relation to the vernal equinox (the first Sunday after the 14th day of the ecclesiastical full moon that occurs on or after March 21).³

To prevent the equinoxes from drifting in the future, Gregory and his advisors added a new rule: to be a leap year, a century year must be evenly divisible by 400. So 1600 is a leap year and 1700 is not, even though 1700, like all century years, is evenly divisible by 4. This rule leads to alternately eight and seven fewer leap days per millennium than in the Julian calendar, and made the calendar agree with earth's revolution about the sun within one day in 3,323 years.

In modern times an additional rule has been suggested: years evenly divisible by 4,000 (4000 ce, 8000 ce, etc.) will not be leap years. Such a change would make the Gregorian calendar correct to one day in 20,000 years.⁴

1. Roger Bacon.
De reformatione calendaris.

2. Pope Gregory XIII.
Inter gravissimas...
Papal bull of February 24 1582.
Kalendarium Gregorianum perpetuum.
1582.

3. “In order therefore to restore the vernal equinox to its former place, which the Fathers of the Nicene Council put at XII Calend.* Aprilis, we prescribe and command as concerning the month of October in the year 1582 that 10 days inclusive from III Nones* to the day before the Ides be taken away.”

*See Roman days of the month.

4. E. R. Hope.
Further adjustment of the Gregorian calendar year.
The Journal of the Royal Astronomical Society of Canada. Part I, volume 58, number 1, pages 3-9 (February, 1964). Part II, volume 58, number 2, pages 79-87 (April 1964).