Back in 1582, Pope Gregory XIII decreed that the 10 days following October 4 simply wouldn’t exist. The next day 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.
his 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 months based on the cycles of the moon and 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.
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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 Gregorian 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 just switched over this month from the Islamic calendar, since the elimination of 11 days would help save money on civil servant pay for the budget-crunched kingdom.
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 2016.
Read more on site http://www.vox.com/2016/10/4/13147306/434th-gregorian-calendar-anniversary-google-doogle