From Julian to Gregory, and the correction of 10 days

On February 24, 1582, Pope Gregory XIII corrected mistakes on the Julian calendar, effectively putting into place a New Style, or what we call Gregorian, calendar. These corrections and change into a new calendar effectively meant that the day after October 4, 1582 would be followed by October 15, 1582, a difference of 10 days. 

The Julian calendar, first implemented by Julius Caesar in 46 B.C., miscalculated the length of the solar year by 11 minutes. This doesn’t seem like much, but over time, the calendar eventually fell out of sync with the seasons. Why was Pope Gregory concerned? Because this meant that Easter, traditionally observed on March 21, fell on the calendar further and further away from the spring equinox with every passing year. Additionally, the reckoned Moon used to compute Easter was fixed to the Julian year by a 19-year cycle, building up an error of one day every 310 years. By the 16th century, this meant the lunar calendar was out of phase with the real Moon by four days.

European scholars had been aware of the calendar drift since the early medieval period. Pope Sixtus IV attempted a reform, inviting Regiomontanus to the Vatican for this purpose. Unfortunately, the project was interrupted by Regiomontanus’ death shortly after his arrival in Rome. By the end of the 15th century, however, astronomical knowledge and the precision of observations had increased, making the question more pressing.  

Originally, Gregory’s reformation of the calendar had no power beyond the Catholic Church, although Catholic countries, including Italy and Spain, quickly adopted this new calendar for their civil affairs. European Protestants, however, felt this change was too heavily tied to the papacy; it wasn’t until 1700 that Protestant Germany changed to the new calendar and England held out until 1752.

Stories have been told about the riots in England in 1752, when an act of Parliament advanced the calendar overnight from September 2 to September 14, 1752 and people demanded that the government “give us our 11 days.” However, these stories have never been verified. The website notes that the claims of civil unrest may have stemmed from a misinterpretation of a painting by William Hogarth, “An Election Entertainment,” which refers to the elections of 1754. In the painting, a campaign banner states “Give us our Eleven Days,” and the Tories can be seen outside the window, demonstrating.  Another fun fact: in the fiction book Slammerkin by Emma Donoghue, the main character’s father is involved in riots over these lost 10 days.

If you would like to read more about the change from the Julian to the Gregorian calendar, we suggest: The Improvement of the Gregorian Calendar by Alexander Philip and/or The World's Work and the Calendar: Telling the Story of the Evolution of the Julian-Gregorian Calendar, of How It Failed to Anticipate the Needs of A Changed Civilization, and of the International Undertaking to Improve It by Meredith Newcomb Stiles.

And a few facts to consider:

  • According to, the Gregorian calendar differs from the solar year by 26 seconds per year. This barely seems like it would have an impact, but if you calculate from the time Gregory introduced his calendar in 1582, this means that a discrepancy has been building. It is calculated that by the year 4909, the Gregorian calendar will be a full day ahead of the solar year.
  • Gregory has been the name of 16 Roman Catholic popes and 2 Antipopes.  If you’re thinking of the Gregory connected to Gregorian chant, that would be Pope Gregory I.  If you’re thinking of the Gregorian Reform, that would be Pope Gregory VII.  If you’d like to learn more about the history of the Popes, we suggest starting with An Illustrated History of the Popes, Saint Peter to John Paul II by Michael J. Walsh or A History of the Popes,From Peter to the Present by John O'Malley.
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