A detail on Pope Gregory XIII’s tomb, carved by Camillo Rusconi, shows the Pope being presented with a plan for what would become the Gregorian Calendar. (Image: Terfili/WikiCommons CC BY-SA 3.0)

The Pope tends to be a powerful guy. He can start global conversations, rock all kinds of colors, and sometimes perform miracles.

But only once in history has a pope made time literally disappear.

In 1582, thanks to then-Pope Gregory XIII, October 6th—along with the 5th, 7th, 8th, 9th, 10th, 11th, 12th, 13th, and 14th—simply didn’t exist. Ten days, gone.

Why? Because of that perpetual thorn in Christianity’s side: the Sun. In the 16th century, much of Europe was on the Julian calendar, the result of an earlier temporal restructuring, in 46 B.C., when Julius Caesar gathered up Rome’s best mathematicians and astronomers and decided to turn what had been a 355-day lunar year into a 365 1/4-day solar one. The republic gritted its teeth through one 445-day year (known as “annus confusionis,” Latin for ”year of confusion”), and after that everything went mostly smoothly, with a leap day added every 4 years to account for the 6-hour surplus. No one touched the Julian calendar for nearly 17 centuries—except Caesar Augustus, who, legend has it, stole one day from February and added it to August, so that his month would be as long as his great-uncle’s.

But there was one small problem. The Earth doesn’t take 365.25 days to go from equinox to equinox—it actually takes 365.24219 days. With each cycle of the Julian calendar, human time fell behind celestial time an additional 11 minutes and 14 seconds. As the years piled up, the difference increased, and after 1600 years of deploying the same, slightly flawed algorithm, this small margin of error had multiplied into about 10 days of lag time.

The cause of all the trouble. (Image: Tau’olunga/WikiCommons CC0 1.0)

Everyone realized what was going on from the beginning, but for the first dozen or so centuries of the calendar’s deployment, it was an end-of-the-list kind of problem. As the mismatch got worse, though, it became more urgent, particularly to Christian religious leaders.

Way back in 325 AD, the church had held a big meetup, The First Council of Nicea, to standardize the dates of major holidays, particularly Easter. “What could be more beautiful and more desirable than to see this festival, through which we receive the hope of immortality, celebrated by all with one accord, and in the same manner?” wrote Emperor Constantine, the Council’s convener. (It was particularly important to him not to celebrate it on any Jewish feast days.) They decided on the first full moon after the vernal equinox, which, in 325 A.D., fell on March 21st.

The Council of Nicea, as portrayed in a modern Eastern Orthodox church. (Image: Migel Sances Huares/WikiCommons CC BY-SA 3.0)

They set Easter as the first Sunday after March 21st, clocked the other saint’s days and holidays off of that, and called it a year. A millennium later, the whole calendar was out of whack with the equinoxes, and therefore the seasons—as one modern historian put it, Easter “was slowly working its way toward becoming a summer holiday,” and Christmas was sneaking into spring.

By the end of the 15th century, church leaders agreed that celebrating Easter on the wrong day was “a scandal.” By this time, the discrepancy was also affecting sailors, merchants, farmers, and others who wanted to keep their datebooks in sync with the lunar cycles and the climate. The best hope for fixing things came from the church, who (at least compared to the fragmented governments of Europe) was more likely to act impartially. Pope Sixtus IV was the first to try, in the early 1470s, but the effort screeched to a halt when his chosen astronomer died (depending on who you ask, he was either assassinated or died in a plague).

Francesco Villamena’s 1606 illustration of Christopher Clavius, head astronomer for the calendar switcheroo. (Image: Smithsonian Libraries/Public Domain)

A hundred years later, Pope Gregory XIII rolled up his sleeves and went for it in earnest. After a call for suggestions, he he was brought a gigantic manuscript. This was the life’s work of physician Luigi Lilio, who argued for a “slow, 10-day correction” to bring things back into alignment, and a new leap year system to keep them that way. This would have meant that years divisible by 100 but not by 400 (e.g. 1800, 1900, and 2100) didn’t get the extra day, thereby shrinking the difference between the solar calendar and the Earthly calendar down to a mere .00031 days, or 26 seconds.

The new lead astronomer on the project, Jesuit prodigy Christopher Clavius, considered this and other proposals for five years. He ended up agreeing with most of Lilio’s ideas, but thought the extra ten days should be made up in one fell swoop. So in February of 1582, Pope Gregory issued “Inter Gravissimus,” a papal bull explaining how he was changing time. And half a year later, people in the more religiously obedient sections of Europe went to sleep on Thursday, October 4th, 1582, and woke up on October 15th. (It was still Friday, though. Nobody wanted to skip Friday.)

William Hogarth’s “An Election Entertainment,” from 1750s-era London, features an anti-Gregorian banner in the bottom left corner that says “Give us our Eleven Days.” Voltaire wrote that the British “preferred their calendars to disagree with the Sun than to agree with the Pope.” (Image: WikiCommons/Public Domain)

The disappeared days caused a certain amount of turmoil. Some people worried that Heaven had missed the memo, and that prayers timed to particular saints’ days wouldn’t go through. Rents and wages needed to be recalculated, and birthdays and anniversaries moved ten days later in order to be accurate. There were riots. Plenty of countries couldn’t or wouldn’t make the switch for months, years, or even centuries. By the time British Parliament came around, citing “divers inconveniences” too bad to ignore, it was 1752 and they (and their colonies) had to skip 11 days. They had riots too, and to this day, people aren’t quite sure where in February to pin George Washington’s birthday—the 11th or the 22nd.

As the changes cascaded through Europe, crossing borders from Julian to Gregorian countries was basically time-traveling. The last country to switch from Julian to Gregorian was Greece, which finally skipped its 12 days in March of 1924.

A Russian marriage certificate from 1907 features both Julian and Gregorian dates—November 23rd and December 6th. (Image: WikiCommons/Public Domain)

Since then, things have been pretty steady. But the march of time is inexorable, human systems remain flawed, and by the year 4909, we will have outstripped the solar year by an entire day yet again. Here’s hoping future popes, tyrants, or alien overlords have the presence of mind to make the necessary adjustments, and to never, ever skip a Friday.