Julius Caesar and the longest Leap Year in history

When it came to taking charge of chaotic situations, Julius Caesar did not mess about. The stories surrounding his courage on the battlefield, his talent for strategic thinking and his downright tenacity are countless, but did you know that tackling the hopelessly disorganised Roman calendar and introducing the concept of the Leap Year was also among Caesar’s claims to fame?

Picture the scene. You’re a farmer in the 1st century BC and – according to the calendar, which circled around the ritual of state religion – you ought to be doling out ripe vegetables ready for the festivals of plenty. Yet to you and any of your slave-labourers, for whom the passage of the seasons are essential, it is clear that those harvests are months away from fruition. How did this end up happening? Well, the Roman calendar had become so out of sync with astronomical reality that annual festivals were starting to bear little resemblance to what was going on in the real world. Something had to be done.

Julius Caesar wanted to fix the mess but this was no mean feat: to shift the entire Roman empire and all its provinces onto a calendar that was properly aligned with both the rotation of Earth on its axis (one day) and its orbit of the Sun (a year). Caesar’s solution created not only the longest year in history, adding months to the calendar during that year, it also anchored the calendar to the seasons and brought us the leap year. It was a phenomenal task. We are in 46BC, otherwise known as “the year of confusion”.

Centuries prior to Caesar’s intervention, the early Roman calendar was drawn up according to the cycles of the Moon and the agricultural year. The origins of the calendar being focused on agriculture gave rise to the phenomenon of a calendar with only 10 months in it, starting in spring, with the tenth and final month of the year roughly equivalent to what we now know as December. Six of the months had 30 days, and four had 31 days, giving a total of 304 days. So what about the rest? Well, this is where it gets really weird. For the two “months” of the year when there was no work being done in the fields, those days were simply not counted. The Sun continued to rise and set but – according to the early Roman calendar, no “days” officially passed. As far back as 731BC people realised that this was a little unhinged, and King Numa, the second King of Rome tried to improve the situation by introducing two extra months to cover that dead winter period. He added 51 days to the calendar, creating what we now call January and February, and this extension brought the calendar year up to 355 days.

If you think that 355 days seems like an odd number, you’d be right. The number took its starting point from the lunar year (12 lunar months), which is 354 days long. However, due to Roman superstitions about even numbers being unlucky, an additional day was added to make a nice non-threatening 355. At the same time, and for the same reason, the months of the year were arranged in such a way that they all had odd numbers of days, except for February, which had 28. February, as a result, was considered to be unlucky and became a time during which the dead were honoured as well as a time of ritual purification.

This all looks like good progress, but it was a situation that still left the Romans around 11 days out from the Solar year and even with all the improvements made, it remained inevitable that the calendar would gradually become more and more out of sync with the seasons, which are controlled by the Earth’s position in relation to the sun. By the 2nd century BC things had got so bad that a near-total eclipse of the Sun was observed in Rome in what we would now consider to be mid-March, but it was recorded as having taken place on 11th July.

Increasingly unable to escape the problem, the College of Pontiffs in Rome resorted to inserting an additional month called Mercedonius on an ad-hoc basis to try to realign the calendar. This did not go well, since public officials tended to pop the month in whenever it suited them best politically, without sufficient focus on the goal of re-aligning the calendar with the seasons. According to Suetonius, if anything it made the situation worse: “the negligence of the Pontiffs had disordered the calendar for so long through their privilege of adding months or days at pleasure, that the harvest festivals did not come in summer nor those of the vintage in the autumn”.

During 46BC – Caesar’s year of confusion – there was already an extra Mercedonius month planned for that year. But Caesar’s Egyptian astronomical advisor Sosigenes, warned that Mercedonius wasn’t going to be enough this time and that things were getting drastic. On the astronomer’s advice, Caesar therefore added another two extra months to the year, one of 33 days and one of 34, to bring the calendar in line with the Sun. These additions created the longest year in history: 15 months, lasting 445 days. Caesar’s drastic intervention brought the calendar back in line with the seasons, meaning that the practice of the ad hoc extra month of Mercedonius could be abandoned.

Of course, getting the calendar to line up with the Sun is one thing; keeping it that way is quite another. As an astronomer, Sosigenes was well aware of the problem. The issue arises from the inconvenient fact that there aren’t a nice round number of days (i.e. Earth rotations) per year (i.e. Earth orbits of the Sun). The number of Earth rotations on each of its trips around the Sun is – I am reliably infomed – roughly 365.2421897. Hence the problem and hence the need for a leap year. The Earth fits in almost an extra quarter-turn every time it does a full orbit of the Sun. Sosigenes therefore calculated that adding an extra day every four years – in February – would help to fix the mismatch. It doesn’t completely solve the problem forever, but it was a jolly good stop-gap.

Photo by Dan Meyers on Unsplash