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 8^{th} century people were
already noticing that the vernal equinox was coming too early. By the 13^{th}
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 14^{th} 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).

See also source 1, below.

1

(632.) The Gregorian rule is as follows:— The
years are denominated from the birth of Christ, according to one chronological determination
of that event. Every year whose number is not divisible by 4 without remainder, consists of
365 days; every year which *is* so divisible, but is not divisible by 100, of 366;
every year divisible by 100 but not by 400, again of 366. For example, the year 1833, not
being divisible by 4, consists of 365 days; 1836 of 366, 1800 and 1900 of 365 each; but
2000 of 366. In order to see how near this rule will bring us to the truth, let us see
what number of days 10000 Gregorian years will contain, beginning with the year 1. Now,
in 10000, the numbers not divisible by 4 will be ¾ of 10000, or 7500; those
divisible by 100 but not by 400, will in like manner be ¾ of 100, or 75; so that,
in the 10000 years in question, 7575 consists of 366, and the remaining 2425 of 365,
producing in all 3652425 days, which would give for an average of each year, one with
another, 365^{d}.2425. The actual value of the tropical year (art. 327) reduced into a
decimal fraction, is 365.24224. so the error of the Gregorian rule on 10000 of the
present tropical years is 2.6, or 2^{d} 14^{h} 24^{m}; that
is to say, less than a day in 3000 years; which is more than sufficient for all human purposes,
those of the astronomer excepted, who is in no danger of being led into error from this cause.
Even this error might be avoided by extending the wording of the Gregorian rule one step farther than its contrivers probably thought it worth while to go, and declaring that years divisible by 4000 should consist of 365 days. This would take off two integer days from the above calculated number, and 2.5 from a larger average; making the sum of days in 100000 Gregorian years, 36524225, which differs only by a single day from 100000 real tropical years, such as they exist at present.

Sir F. W. John Herschel.

*A Treatise on Astronomy*

Philadelphia: Lea & Blanchard, 1851.

Pages 383-384.

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Last revised: 4 May 2010.