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Why Do We Use the Gregorian Calendar?

Updated: Apr 21, 2023

We don't really think about the calendar — it's just kind of there as means of organizing our time. Well, since today is New Year's Eve, we're taking a break from discussing digital and, instead, look at how we ended up with the calendar we currently use.


The name of the calendar we're all familiar with is the Gregorian calendar, which is now 438 years old.

Back in 1582, Pope Gregory XIII decreed that the 10 days following October 4 simply wouldn’t exist. The next day after October 4 would be October 15. Thereafter, a new calendar would come into effect that would better align the months with Earth’s journey around the sun. This would correct a mismatch in the old Roman calendar, first set up by Julius Caesar, that was causing the months to fall steadily out of line with the seasons.

The Gregorian calendar, with its intricate dance of leap days and leap years, seems utterly banal to those of us in the Western world today. But it’s worth pausing to reflect on what an odd system it is — and how we actually got here in the first place.

How the Gregorian calendar saved the seasons

The fundamental problem that anyone making a calendar has to grapple with is the fact that it takes just a shade more than 365 days for Earth to make a full trip around the sun. More precisely, it takes 365.24219 days.

So if you construct a calendar with only 365 days, the seasons will fall ever so slowly out of whack with the months. This dilemma was grasped early on by astronomers in Alexandria, Egypt, who helped Julius Caesar devise a new calendar in 46 BC. Until that point, the Roman calendar was a messy hodgepodge, with extra days tacked on in February every now and again based on the whims of politicians. Caesar wanted a steadier, more reliable way to mark the dates.

But the new Julian calendar that resulted was still flawed. It had a leap day every four years, which turned out to be an over-correction. The average year now had 365.25 days in it — just a shade more than 365.24219.

By the 1570s, those slight differences had added up. The calendar was now out of sync with the solar year by about 10 days.

So, in 1577, Pope Gregory XIII appointed a commission, led by physician Aloysius Lilius and astronomer Christopher Clavius, to solve the problem. It took them five years, but they came up with a fix: First, let’s just eliminate those extra 10 days and get back on schedule. Gone!

Next, let’s tweak the system of leap years. We’ll have leap years every four years except on centennial years that aren’t divisible by 400. So there’s a leap year in 2000, but not in 1900 or 1800 or 1700.

This changed the length of the average year to 365.2425 days. Still not perfect, but close enough. Gregory also moved the New Year from March 25 (the Feast of Annunciation) to January 1.

Not everyone adopted the Gregorian calendar right away

Since he was pope, Gregory was able to persuade Catholic countries like Italy, Spain and Portugal to adopt the new calendar immediately. But Protestant countries were wary of this new popish initiative, seeing it as a suspicious Catholic intrusion.

Great Britain and the American colonies didn’t actually switch over until 1752. (When they finally did, they had to erase 11 days.) Sweden only made the change in 1753. Turkey was split between the Julian and Islamic calendars until 1917.

During the French Revolution, meanwhile, leaders in France decided to purge their calendars of any religious overtones. The new French Republican calendar, adopted in 1792, had 12 identical months of 30 days. Weeks had 10 days in them. And there were five or six extra days at the end of each year for holidays. The calendar also renamed the months, with monikers like Brumaire or Thermidor. That funky calendar, alas, was abandoned in 1805 and only revived briefly by the Paris Commune in 1871.

Nowadays, the Gregorian calendar has largely conquered the world, and most countries now follow it for coordination purposes. Saudi Arabia recently switched over in 2016 from the Islamic calendar.

But countries don’t always follow the Gregorian schedule to celebrate New Year’s. That holiday is often based on lunar cycles and doesn’t necessarily fall on January 1. The Persian New Year in Iran, for instance, is determined by the Northern Hemisphere’s vernal equinox.

Here’s a guide to New Years’ celebrations in 2021:

The Eastern Orthodox Church, meanwhile, still follows a version of the Julian calendar — which means that Christmas falls 13 days later than in the Gregorian calendar.

Even the current Gregorian calendar isn’t perfect

As noted above, the Gregorian calendar is just a tad out of sync with Earth’s trip ’round the sun. The difference isn’t huge — we’ll have an extra day to deal with by 4909 — but it’s enough to bother some persnickety calendar mavens.

The Gregorian calendar also has a bunch of oddities. Unlike the French Republican calendar, our months are uneven, some 31 days, some 30, plus the monstrosity that is February. What’s more, each year dates fall on different days of the week.

Some reformers have suggested we change the calendar to correct these blemishes. At Johns Hopkins University, Steve Hanke and Richard Henry have proposed the Hanke-Henry Permanent Calendar, which would have 364 days every year plus a "leap week" tacked on every five or six years to adjust for the error. The advantage here, they say, is that the calendar would be the same every year. October 4 would always fall on a Wednesday, for instance. No need for confusing updates to calendars.

But probably we’ll just muddle along with the Gregorian calendar. If worst comes to worst, we can always convene again in 4909 and remove that pesky day.

Earth’s rotation is also troublesome — which is why we have to add leap seconds

By the way, leap days aren’t the only hassle for timekeepers. We also have leap seconds to contend with.

It technically takes Earth a bit longer than 24 hours to complete a full rotation: 86,400.002 seconds rather than 86,400. So in order to keep our clocks matched up with solar noon, when the sun is highest in the sky, a leap second gets added every few years.

Since the practice began in 1972, 27 out of 44 years have included leap seconds. What makes this tricky is that timekeepers can’t just add these on a predictable schedule, because the Earth’s rotation is constantly speeding up and slowing down in ways that are tough to foresee. Over the long term, tidal friction is gradually lengthening the time it takes Earth to make one full rotation. But other factors like glacial melt, wind, storms, and more also affect the length of the day.

Will there be a leap second this year?

Nope! World timekeepers have decreed there will be no leap second on December 31, 2020.

Happy New Year, everyone!

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