10 days that shook the world

From the names and number of days in each month, to the year itself, almost every aspect of our calendar has been messed with by the powers that be. Including when 10 days were simply erased from October. Here’s why.

Libras must have been so furious. It’s the beginning of Autumn, and your birthday is coming up soon. You’re excited, and you’ve been dropping hints to your loved ones about the perfect gift for you – you know, maybe a silver ear wax scoop, or a Pyrenean boar spear, or a dozen live quails in a cage. And then you hear the news. In the Year of our Lord 1582, your birthday has been cancelled. One third of October has been cancelled. Today is Thursday the 4th, and tomorrow is Friday the 15th. To a Libra like you, this is dangerously unbalancing. And it’s all the pope’s fault.

This wouldn’t be the first time a higher power was messing with the calendar. Calendars, in a sense, have always been the playthings of high priests and kings. Roman emperors named months after themselves and fiddled with the number of days in each: one more day for Julius in July, another one for Augustus in August. In fact, the Julian calendar, instituted under the original Caesar, has given us a last four months that are two places off: September means seventh, not ninth; December means tenth, not twelfth.

Fast forward to 1789, and the rationalizing revolutionaries of the First Republic would behead a king but walk firmly in the emperors’ footsteps. They cut and pasted from the inconsistent old calendar to create an all-new decimalized version that divided newly named months into three 10-day weeks (with an extra 5 or 6 days tacked on at the end of each year). But while their decimalizing élan succeeded with distance (the metre) and mass (the gram), it didn’t make a lasting change when it came to time – Napoleon said adieu to the Republican calendar shortly after the beginning of his Imperial reign.

This could all sound simply capricious, but deep down many of these high-powered calendar reformers were trying to solve a conundrum as old as time: How to make a simple, usable, and consistently accurate calendar when our annual trip around the sun just doesn’t line up with simple maths?

Nature resists easy quantification. The solar year is 356 days, 5 hours, 48 minutes, and 46 seconds – or 356 days and 6 hours if you round up. And for the first millennium-and-a-half after Caesar instituted the Julian calendar, simply rounding up is what Europe did. But as Pope Gregory XIII discovered, an acceptable rounding error of 11 minutes and 14 seconds per year makes a huge difference over many centuries.

By 1582, the calendar was a full 10 days out of sync with the astronomically fixed spring equinox (as observable in the Northern Hemisphere). This meant that, by the complicated maths of fixing the dates of movable feasts, Easter was threatening to arrive in the summer. And so, on advice of his learned counsellors, Gregory set out to set the calendar to rights – by giving the good people of 1582 a 10 days’ leap into the future.

The new rules of the “Improved Calendar”, since adopted throughout most of the world as the Gregorian Calendar, stipulates there shall be 97 leap years every 400, creating a simple repeatable system that anchors Easter – and all our other holidays – in roughly the same place.

While some unlucky Libras might have bemoaned their cancelled birthdays, surely many were relieved to discover that Gregory’s papal pronouncement officially leapt over 10 days of rent and debt payments that year. Adoption throughout Catholic countries like Spain, Italy and France, was swift.

After centuries of messing with it, there may be much that is still wrong with our calendar. But it took the bold move of erasing 10 days from it in the 16th century to help us keep getting so much more right.