Easter and The Gregorian Calendar


gregory_xiii.jpgHappy Easter! Since Easter played a key role in moving away from the Julian calendar, it seems fitting to talk about the adoption of the Gregorian calendar today.

As mentioned earlier, the Roman calendar had been having all sorts of problems until the time of Julius Caesar, who right before his death got everything back on track by adding a leap day (a second February 24th) every four years. This worked really well except for one small problem: the solar year isn’t exactly 365.25 days long, it’s about 11 minutes/year less than that. But 11 minutes is hard to notice, so everything seemed hunky dory for a very long time.

By the 1500s, those 11 minutes had added up to over 10 days, however, and that was a big problem for getting Easter right. Easter is the first Sunday after the first full moon after the vernal equinox, so if you’ve messed up the vernal equinox then Easter is all wrong and that is not good. For that matter, the vernal equinox itself hasn’t always been agreed upon — The Church of Rome said it was March 25 and the Church of Alexandria said it was March 21. But no one said it was March 11, which is about where it was falling by the mid-1500s.

In 1563 the Council of Trent agreed on how to fix the calendar: an extra day would be added in years that were divisible by 4, except in years divisible by 100, except except in years divisible by 400 (so 1700, 1800, 1900 didn’t have leap years but 2000 did). The exact moments of the equinox and full moon were set by ecclesiastical tables (although some folk like Giovanni Battista Benedetti thought it made more sense to use the actual positions of the sun and moon, what with all the concern over exactness and all.) This reform is typically attributed to Christopher Clavius, and since the fix was decreed by Pope Gregory XIII, who looks a little bit like Santa Claus in the picture above, the calendar was named the Gregorian calendar.

To make this change, the 10 extra days that the calendar had accumulated needed to be skipped. October didn’t have as many feast days as other months (and you wouldn’t want to skip a feast day!) so Thursday, October 4, 1582 was followed by Friday, October 15, 1582. And all was well.

But of course it wasn’t that simple. Only Spain, Portugal, the Polish-Lithuanian Commonwealth, and most of Italy followed those instructions. France adopted the new calendar in 1582, but waited until December to skip the days. Likewise, Spanish and Portuguese colonies took a little longer to switch calendars because they didn’t get the fax telephone call message in time.

The general population was upset because they were concerned about paying a full month’s rent in a drastically shortened work-month. Protestant countries weren’t thrilled at having to change calendars just because the Catholic pope thought it was a good idea. Even though Holland and Zeeland switched at about the same time as France, most Protestant countries held out. Denmark and Norway got rid of half of February in 1700, but used Kepler’s tables rather than the ecclesiastical tables to calculate Easter for most of the 1700s.

Sweden just decided to skip 11 leap days in the first half of the 1700s (because by this point the two calendars were 11, not 10, days off), thus guaranteeing that their calendar matched no other country in the world during that time period. They dropped 1700’s leap day, but forgot to skip the next two leap days and decided just to go back to the Julian calendar. Because they’d skipped that leap day in 1700, however, they were a day off, so they added a February 30 in 1712 to get back on the Julian track, and finally switched to the Gregorian calendar in 1753 by letting March 1 follow February 17.

Britain changed in September 1752, but they also declared that beginning in 1753 the tax year would start on April 5 rather than March 25, so that even the 1752/1753 tax year had 365 days. (They later changed it to April 6 because of the missed leap day in 1800, but didn’t bother to change it because of the missed leap day in 1900. We’ll see what happens in 2100.)

The fact that Spain and Britain changed in different years leads to the following question:

Question: Authors William Shakespeare and Miguel Cervantes both died on April 23, 1616. Who died first?

Answer: Cervantes died about ten days before Shakespeare: Spain was using the Gregorian calendar and Britain was using the Julian calendar (so Shakespeare’s April 23 death actually corresponded to May 3 in Spain).

Likewise, in 1688 William Henry of Orange left the Dutch Republic on November 11, sailed across the Channel, and got to England several days later on November 5. Now that’s speedy traveling.

Since the Gregorian calendar was adopted in Britain before the Revolutionary War, the United States has always used the Gregorian calendar. But just to make things fun, George Washington was born while Britain and its colonies were still using the Julian calendar, so figuring out his birthday isn’t a completely simple matter. Take a look at the following February 14, 1790 letter from Tobias Lear (Washington’s secretary) to Clement Biddle:


which reads:

In reply to your wish to know the Presidents birth day it will be sufficient to observe that it is on the 11th of February Old Style but the almanack makers have generally set it down opposite to the 11th day of February of the present age — how far that may go towards establishing it on that day I dont know — but I could never consider it any otherways than as stealing so many days from his valuable life as the difference between the old and the new Style.

Not that anyone wants to steal any days from Washington’s valuable life, but official sources say he was born on February 22, which is what his February 11 Old Style (i.e. Julian) birthday would have been in the Gregorian calendar. [The year is another story entirely, but I’ll talk about that on March 25.]

Russia was one of the last countries to switch from the Julian to the Gregorian calendar, in February 1918. This had two consequences. First, Alaska used the Julian calendar until it was purchased by the U.S in 1867. It switched to the Gregorian calendar later that year, which would normally have been a change of now-twelve-days but only eleven days were skipped because, at the very same time, the International Date Line was moved from the eastern to the western edge of Alaska. This Date Line quirk also meant that Alaska had two Fridays in a row: Friday, October 6 was followed by Friday, October 18.

Second, the Bolshevik Revolution began on October 25, 1917 in the Julian calendar and is known as the October Revolution, although the date in the Gregorian calendar is November 7.


I took this photo in Budapest in December 1990: Hungary had just moved away from Communist Rule, and was in the process of changing all its street names, so the Communist November 7 Tér (square) was renamed Oktogon after its shape.

One final tidbit: although most countries now use the Gregorian calendar, the Orthodox churches never adopted it. Many of the Orthodox churches still celebrate Christmas on January 7, which corresponds to December 25 in the Julian calendar.

Credits: Most of this information I got from Wikipedia’s entry on the Gregorian calendar, although I tried to verify it by looking at other sources. The Pope Gregory portrait is from Wikimedia and is in the public domain . The photo of the letter didn’t claim to be copyright free but given its age and that it seems to be government property, I believe it’s legal to include the image here. You can see a larger version at the Library of Congress site. The Budapest photo is one I took, all rights reserved etc, and I think it’s so cool to have been there at that time that I have a big picture of it in my office.

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4 Responses to “Easter and The Gregorian Calendar”

  1. Happy New Year (Again!) « 360 Says:

    […] 1. Even though that’s the same date that many people use today, its adoption (like that of the Gregorian Calendar) wasn’t completely […]

  2. Todd Says:

    Congratulations on an extremely terse, yet exhaustive explanation of the Gregorian year’s place in history and in our very lives.

    I hadn’t known about Russia’s late arrival on the Gregorian scene and, therefore, my puzzlement over the October/November Revolution differential is finally cleared.

    Excellently written!

  3. Ξ Says:

    Thanks Todd!

  4. Cinco de Mayo Math « 360 Says:

    […] del año in 1584, which wasn’t so much about math as about how to deal with the fact that switching from the Julian to the Gregorian calendar involved skipping ten days, and several books that look at military mathematics and formations. In […]

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