Thread by @culturaltutor

Why the hell does February sometimes get an extra day? And why is it shorter than the other months, anyway?

It's all because of what Julius Caesar did 2,070 years ago in 46 BC.

What's special about 46 BC? It had 445 days, making it the longest year in history...

But the story begins in 753 BC, when Rome was founded by the mythical Romulus.

Romulus was credited with creating the first Roman calendar.

It had ten months, each of 30 or 31 days, beginning in Martius and ending in December.

What about the time between December and Martius?

Early Rome was fundamentally agricultural. With little work to do in Winter it was simply a long, dark, and undated stretch of time.

But as Roman society grew more sophisticated this situation became troublesome...

The story goes that Numa Pompilius — the second of Rome's legendary seven kings — fixed it.

He introduced two new months, Ianuarius and Februarius, to cover the time between December and Martius.

So the Romans had a twelve month calendar, but they kept the original names — including December as the "10th month" even though it had become the 12th.

Hence September, October, and November, meaning 7th, 8th, and 9th month respectively, also remained... and still do, somehow.

This calendar was lunar — that means it was based on the cycles of the moon rather than the sun.

The Romans borrowed it from the Greeks and Mesopotamians, who had calculated that each lunar cycle lasted 29.5 days.

So twelve of them, forming the lunar year, lasted 354 days.

But things were complicated by Roman superstitions regarding even numbers — they were considered unlucky.

Roman society was deeply religious and it was the priests who were in charge of organising the calendar, filled as it was with festivals, rites, and sacrifices.

So, to avoid unlucky even numbers, they subtracted one day from each of the previously 30 day months and added a single day to the lunar year to make it an odd number: 355 days.

That left 57 days to split between the two new months, January and February.

They gave January 29 days, leaving February with 28 — maths dictated that one of the twelve months simply had to have an even number of days.

Why they chose February we can only speculate. Some believe it is because Roman festivals of the dead were held in that month.

But the trouble with Numa's calendar was that, at 355 days, it fell ten days short of the 365 day solar year.

This is a problem for all lunar calendars because, over time, they "drift" away from the solar year and so the dates of festivals and seasons fall out of sync.

To fix this and keep the calendar synchronised the Roman priests were tasked with adding an "intercalary" month of 22 or 23 days, called Intercalaris, every two years.

In these leap years Intercalaris was added to the end of February, and those years lasted 377 or 378 days.

But this system was open to abuse, not least because Roman officials served annual terms and the priests in charge of Intercalaris were also politicians.

Thus they made years shorter or longer depending on whether their enemies or allies were in power.

Politics!

By the 1st century BC decades of civil war and political strife had caused this calendar to fall completely out of sync with the solar year.

So when Julius Caesar defeated Pompey and became "Dictator for Life" he decided to fix the old dating system once and for all.

With help from leading mathematicians and astronomers he created what we now call the Julian Calendar.

A year was now 365 days long, with an extra day added once every four years to make up for the real length of the solar year, which had been calculated as 365.25 days.

Caesar used these extra ten days to make every month 30 or 31 days.

Apart from February, which was left with its traditional 28 days — and in leap years February was chosen to receive the extra day.

That decision over 2,000 years ago has stuck, hence the date today!

To make sure his new calendar started in sync with the solar year Caesar had to add three extra months to 46 BC, the year of his reform.

And so 46 BC was 445 days long; the longest year in history.

No wonder it was called "annus confusionis" — the year of confusion.

In 44 BC the month Quintilis was renamed in honour of Julius Caesar, becoming our modern July.

And thirty years later Sextilis was renamed in honour of Augustus, the heir to Caesar and Roman Emperor, becoming our August.

But this system had a flaw — the real length of the solar year is 365.2422 days, not 365.25.

So the Julian Calendar drifted by one day every 128 years.

That seems insignificant, but it was used for over 1,500 years — by the 16th century Easter was 10 days out of alignment.

So in 1582 Pope Gregory XIII commissioned a reform to correct the Julian Calendar, which was done by removing one leap year every four centuries.

The calendar was also reset, resulting in a bizzare event — Thursday 4th October 1582 was followed by... Friday 15th October!

And this "Gregorian Calendar" is the one we still use today.

It is a palimpsest whose systems are, deep within their coding, thousands of years old, going right back to the very dawn of human civilisation in Ancient Mesopotamia...

...including the Roman choice to make February both the shortest month and the one that would have a day added every leap year.

So, even in the age of computers and nuclear bombs, the way we measure time is still shaped by decisions made thousands of years ago.