Gregory Conquers Julius Caesar

Pope Gregory XIII with the calendar that was to make his name
Pope Gregory XIII with the calendar that was to make his name

by Ray Setterfield


October 4, 1582Julius Caesar famously came, saw and conquered and was a brilliant Roman general. But he wasn’t very good at sums. And the calendar that he devised in 46 BC – named the Julian calendar in his honour – was flawed, even though it was to last for 1,600 years.

The problem was Caesar had calculated that a year lasted for 365 days and six hours. But this did not properly reflect the actual time it takes the Earth to circle once around the Sun, known as a tropical year.

In fact it is only 365 days, five hours, 48 minutes and 45 seconds. Not much difference, but enough, 1,600 years later, to have put the world astray by a whole week.

So it was on this day that Ugo Buoncompagni, an Italian better known as Pope Gregory XIII, introduced a new calendar – the Gregorian calendar – which would iron out the Julian discrepancies, eventually become widely accepted and is the calendar in use today across much of the world.

Gregory needed to lose a few days so under his new system October 4, 1582 was followed the next day by 15 October. And he decreed that New Year’s Day should be moved from 1 April to 1 January.

Then there was the question of leap years – those containing 366 days and necessary to keep the calendar in alignment with the Earth’s revolutions around the Sun. Gregory calculated that if we didn’t add a leap day on February 29 nearly every four years, we would lose almost six hours off the calendar every year. After only 100 years it would be astray by 24 days!

But he also modified Caesar’s concept of a leap year precisely every four years, which is too many. The Gregorian calendar uses a much more accurate method for calculating leap years and stipulates that century years, even though divisible by four, are not leap years. The exceptions are those that can be divided by 400. Thus, 2000 was a leap year, but 1900 was not.

The Gregorian calendar was to become accepted worldwide, even though some countries stuck out for Julian. The UK did not accept Gregory until 1752, whereas Greece held out until 1923. The last convert was Turkey, which finally accepted the Gregorian calendar in 1927.

Published: August 23, 2019

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