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—— ON THIS DAY ——
OCTOBER 15, 1582
Rome, Papal States
444 years ago
The Gregorian calendar, introduced by Pope Gregory XIII, which came into force on October 15, 1582, and is now the civil calendar used by almost the entire world.
In much of Catholic Europe, Thursday, October 4, 1582, was followed directly by Friday, October 15. The ten days in between never happened. The days of the week ran on without a break, but the dates jumped, by order of a papal decree issued the previous February. Anyone born on October 9 in Rome or Madrid had no birthday that year. The Spanish mystic Teresa of Avila died during that very night, which is why her feast is kept on October 15.
The reason was a small error that had been accumulating for sixteen centuries. The Julian calendar, introduced by Julius Caesar in 45 BCE, assumed that the solar year lasts exactly 365 and a quarter days and so added a leap day every fourth year. The true figure is about eleven minutes shorter. That tiny surplus added up to a full day roughly every 128 years, and by the sixteenth century the calendar had drifted ten days out of step with the seasons.
—— MARQUEE EVENT ——
Pope Gregory XIII, who issued the bull Inter gravissimas in February 1582, ordering the calendar reform that bears his name.
The Church cared because of Easter. The date of Easter is tied to the spring equinox, which early councils had fixed at March 21. By the 1500s the actual equinox was falling around March 11, and the most important feast of the Christian year was sliding slowly toward summer. Scholars had complained for centuries. Gregory XIII finally appointed a commission, which adopted a scheme devised by an Italian physician named Aloysius Lilius and worked out in detail by the Jesuit mathematician Christopher Clavius.
The fix had two parts. Dropping ten days put the equinox back on March 21. To keep it there, the leap year rule was adjusted: years ending in 00 would be leap years only if divisible by 400. So 1700, 1800, and 1900 were ordinary years, while 1600 and 2000 kept their leap day. That removes three days every four centuries, and it is accurate enough that the calendar gains only about one day in three thousand years.
Because the reform came from the pope, Protestant and Orthodox countries refused it, some for a very long time. For generations a traveler crossing Europe had to change the date at the border. Britain and its American colonies held out until 1752, by which time eleven days had to be dropped, and September 2 was followed by September 14. George Washington, born on February 11 under the old calendar, thereafter celebrated on February 22. Russia switched only after the Bolshevik Revolution in 1918, which is why the October Revolution is commemorated in November. Greece followed in 1923. Several Orthodox churches still use the Julian calendar for their feasts.
—— WHY THIS MATTERS ——
The Gregorian calendar is the closest thing the world has to a universal standard. Whatever other calendars people use for religious or cultural purposes, international business, travel, science, and diplomacy run on the dates set out in a papal bull of 1582.
It was a triumph of applied astronomy. The reformers produced an elegant rule that keeps the calendar aligned with the seasons for millennia, using observations made before the invention of the telescope.
Its slow adoption is a lesson in how politics shapes even technical questions. Everyone agreed the old calendar was wrong. It still took 341 years for the correction to cross Europe, because accepting it meant accepting the authority of the institution that issued it.
—— THE TAKEAWAY ——
On October 15, 1582, a good part of Europe woke up ten days later than it had gone to sleep. The pope's astronomers had fixed an eleven-minute error that dated from Julius Caesar. The mathematics was settled at once. The politics took until 1923.
—— QUOTE OF THE DAY ——
“Give us our eleven days!”
— Slogan traditionally associated with English objections to the calendar change of 1752
—— OUR QUIZ OF THE DAY ——
How much do you know about the Gregorian calendar, the error in the Julian calendar it corrected, the missing ten days of October 1582, and the centuries it took other countries to adopt it?
