Cold Open

It’s a warm, still day in February, 47 BCE in the Nile Delta of Egypt.

By the banks of the river, 52-year-old Roman general Julius Caesar watches as the last of his men march across a wooden bridge they’ve built over the water. The makeshift structure wobbles under the strain, but it holds until the legionaries make it to the other side, and Caesar smiles with satisfaction. The bridge has served its purpose, enabling his army to quickly cross the water and sneak up on his enemy.

Four months ago, Caesar sailed to Egypt in pursuit of a political rival from Rome, only to find that he’d already been killed. Caesar prolonged his stay, however, after he began an affair with the 22-year-old Queen of Egypt, Cleopatra. Cleopatra is locked in a bitter civil war with her brother, Ptolemy and Caesar has thrown his support behind his lover’s cause. Now, the Romans have cornered Ptolemy’s forces between two branches of the River Nile, and Caesar hopes to strike a decisive blow.

But as the last of his soldiers cross the river, there’s a sudden cry of alarm. Caesar’s bodyguards draw their swords and form a protective wall around their general. Caesar peers over their shields and spots dozens of enemy soldiers approaching - archers on horseback.

Arrows soon dart through the air, plunging downwards towards the Romans.

Caesar scowls because he knows his legionaries are in a vulnerable position -out of formation after crossing the river, and the Egyptian mounted archers can loose their arrows from a safe distance and dash away before the swords and spears of the Romans can touch them.

But then, a second group of mounted soldiers gallops into view and charge for the enemy. Caesar recognizes them as his own Roman cavalry. They crossed at a ford several miles upriver, and they’ve arrived just in time.

The two groups of cavalry clash and exchange blows. The Egyptian riders wheel their mounts around, but their momentum is gone—and Caesar spies an opportunity.

He shouts an order, and his signaler blows a horn. The legionaries charge forward, their swords drawn. And pinned in place by the Roman cavalry, the Egyptian horsemen are unable to flee. Within minutes, Caesar’s army has won yet another victory.

The Roman army will follow up their triumph with a successful attack on Ptolemy’s camp. Ptolemy himself will drown as he tries to flee across the river and Caesar will place his lover Cleopatra on the Egyptian throne. But the Battle of the Nile will have consequences far beyond deciding who rules Egypt. It will lead to a fundamental reform of the Roman calendar that will last long into the future, adding an extra day almost every four years, as it will on February 29th, 2024.

Introduction

From Noiser and Airship, I’m Lindsay Graham and this is History Daily.

History is made every day. On this podcast—every day—we tell the true stories of the people and events that shaped our world.

Today is February 29th, 2024: The Leap Year.

Act One

It’s February 47th, BCE in Alexandria, Egypt; a few days after the Battle of the Nile.

Julius Caesar lifts a cup of wine and drinks deeply. He feels he deserves a moment of celebration. As dictator, his grip on power back in Rome is unrivaled—and thanks to his relationship with Cleopatra, Egypt is now under his sway too. Caesar catches his lover’s eye across the banquet room, and she smiles flirtatiously back at him.

But Caesar’s attention is drawn away by one of his officers clearing his throat. Caesar turns to see the man standing with a well-dressed Egyptian. The officer introduces the man "Acoreus" , a man famed as one of Egypt’s greatest thinkers. Caesar invites Acoreus to sit with him and bombards the philosopher with questions about how Roman and Egyptian societies differ. Their conversation ranges widely, from how Egyptian society functions with an autocratic ruler, to how the Egyptian calendar is used to predict the annual floods of the Nile River.

For centuries, the Roman calendar has consisted of 12 months with a total of 355 days. But that calendar is shorter than the actual time it takes the earth to rotate around the sun. And soon after the system was first introduced, the months fell out of sync with the seasons. The Romans recognized this was a problem and came up with a solution. Now, every three years or so, an extra month of 27 or 28 days is inserted between February and March. But this procedure has proved to be complicated and open to abuse. The officials responsible for organizing the calendar have often been bribed or threatened. Powerful Romans have demanded extra days in the year to prolong the rule of a political ally, or they have tried to shorten a year in which their opponents were in office. Now, as Caesar consolidates his hold on Rome, he wants to remove any means by which his opponents might loosen his grip on power—and that includes reforming the calendar.

As Caesar listens intently, Acoreus explains how the Egyptian calendar differs to the Roman one. It also has twelve months, but each is a fixed period of 30 days. An extra five-day month is then added at the end to bring the total for the year to 365 days. But Acoreus explains that the Nile floods don’t always happen at the same point on the calendar. Since the solar year is 365 and a quarter days long, the Egyptian calendar also gradually loses sync with the seasons. Acoreus explains to Caesar that Egyptians must take account of this so-called “wandering year” when they plan the best time to plant and harvest their crop, because a month that coincides with the floods today may have been harvest time in the past.

Caesar is fascinated by what the philosopher tells him, and he doesn’t forget it when he leaves Egypt. Upon his return to Rome the following year, Caesar summons the empire’s best philosophers and mathematicians and gives them the ambitious task of coming up with a new calendar. They return to Caesar with a proposal that combines the old Roman months with the fixed 365-day length of the Egyptian calendar. But to ensure that this  new calendar doesn’t wander steadily off-season even like the Egyptian one does, this new Roman calendar will have an extra day every four years, a leap year when February 24th will be followed by another February 24th.

Caesar enthusiastically agrees to the proposed reforms and releases a decree that states the new calendar will begin on January 1st, 45 BCE. To some in Rome, it’s more proof of Caesar’s growing megalomania - even time itself is being bent to his whim. And that’s not the end of the political tampering with the dating system in Rome. A year later, the calendar is tweaked once again - this time in honor of its creator. The seventh month, previously known as Quintilis, is renamed “Julius” - or July. And a few decades later, the eighth month is renamed in honor of Caesar’s successor, the first Emperor of Rome, Augustus. After that, the Julian Calendar will remain largely unchanged for centuries with one exception: medieval scholars will gradually abandon the Roman practice of having two February 24ths. In its place, every four years, they will instead add an extra day to the end of the month.

The Julian Calendar with the addition of February 29th will continue as the accepted means of recording dates in Europe for more than a thousand years - until a new problem with Caesar’s reforms will be discovered, and a new calendar will be created to fix it.

Act Two

It’s spring 1577, in a library in Rome, Italy, more than 1600 years after the creation of the Julian Calendar.

Christopher Clavius, a German mathematician in his late 30s, shifts uncomfortably in his stiff-backed chair. Christopher is sat at a large, ornate table, flanked by a panel of the top scientists and astronomers of the Catholic Church. The men come from all different religious orders and nations, brought together by Pope Gregory XIII with one purpose - to reform the Julian calendar.

The Julian Calendar has been in use for centuries. But scientists have long known that the ancients got a crucial fact wrong. The solar year is not precisely 365 and a quarter days long - it’s a fraction of a day shorter than that. Back in Caesar’s time, the effects of this error were not obvious. But every 128 years since then, the Julian calendar has been getting one day more inaccurate. This has become especially problematic for the Catholic Church. Easter is supposed to be celebrated on the first Sunday after the first full moon after March 21st. That date should be fixed to the astronomical spring equinox, when the sun appears directly above the equator and the length of day and night are the same all across the world. But the error in the Julian Calendar means that March 21st is no longer aligned with the equinox, and, as a result, Easter is happening at the wrong time of year. So, Pope Gregory XIII has ordered his most esteemed scholars to advise on the creation of a new calendar, one that can correct the old error and return Easter to its proper place in the solar year. And as the best mathematician in Rome, Christopher Clavius has been placed in charge of the effort.

In the library, shadows lengthen. Servants circle noiselessly through the gloom with tapers, lighting candles, and Christopher creases his brow in exhaustion as one of the older men beside him shuffles through his papers and begins to speak, laying out his position yet again. His croaked, slow Latin echoes feebly through the chamber. Christopher is tempted to cut him short, to end the day’s work here. But Christopher is also determined to finish the job he has been given.

The commission has agreed that the new calendar should be based on the work of a distinguished professor of medicine at the University of Perugia. Aloysius Lilius labored over this model for a decade and came up with an ingenious mathematical solution to the problem of the wandering calendar. It involves skipping three Julian leap days every four hundred years. But there remains disagreement over how and when this new system should be introduced.

After months of debate, the Pope grows frustrated with the efforts of the commission. And in an attempt to break the deadlock, Pope Gregory XIII takes it out of the scientists’ hands and sends out the calendar proposal to all the Catholic kingdoms in Europe for their feedback and approval. For the Pope, the opinion of princes matter more than the arguments of mathematicians. And the Pope soon finds that princes are in favor of reform, but they also cannot come to a consensus on how to introduce the new calendar.

Three years later, by 1580, there’s still no agreement in sight. In the end, the mathematician Christopher Clavius takes charge of the final draft of the proposed almost single-handedly. His recommendations are published in September 1580. They run to 800 pages of densely argued text, but Christopher’s comprehensive work is enough to convince the Pope, and Gregory XIII decides to push forward the proposal. In March 1582, another year and a half later, a Papal edict announces that the new calendar will come into effect in the October of that year.

But the Pope only has power over the Catholic Church. For the new Gregorian Calendar to be used more widely, it must be approved and introduced by the civil authorities in every country. The likes of Spain and France are ruled by Catholic kings who are quick to embrace the change. But Christianity in Europe is bitterly divided. Many non-Catholics are suspicious of anything approved by the Pope. Protestant countries like England and Sweden suspect this new Gregorian Calendar is some sort of plot against them - and they refuse to introduce it.

All this means is that in the late 16th Century, half of Europe starts to operate on one calendar and half remains on another. It’s a confusing state of affairs. But reality cannot be denied forever. In the countries which do not adopt the Pope’s reforms, the calendar and the seasons will keep drifting further apart, until at last, it will be recognized in even the most anti-Catholic of nations that something will have to be done.

Act Three

It’s the evening of September 2nd, 1752 in Endon, a village in Staffordshire, England; 170 years after the introduction of the Gregorian calendar.

At a corner table in the White Horse pub, a farmer named William Willett nurses a glass of beer. He’s trying to read a newspaper, but the other villagers are making it hard for him. William owes money to several of his neighbors, and all evening they’ve been threatening him for repayment. But William is no fool, and as he looks over the newspaper, the day’s date catches his eye - and gives him an idea.

More than a year ago, Parliament passed legislation introducing the Gregorian calendar in Great Britain. It’s a move that will finally bring Britain in line with most of Europe, which shifted to the new calendar more than a century earlier. To get back in sync, Britain and its colonies are going to omit eleven days from the year. September 2nd on the old Julian Calendar is to be followed directly by September 14th on the new Calendar. And in a quiet Staffordshire pub, the farmer William Willett hopes to use this fact to make a quick profit.

William rises from his seat and walks around the pub, challenging other patrons to a wager that he can dance non-stop for twelve days. The pubgoers laugh and shake William’s hand, eagerly betting that he can’t do it. But William just smiles.

Later that night, just before the clock strikes midnight, William begins a spirited jig. He keeps it up for an hour until he runs out of energy. The witnesses heckle William as he stops, saying they’ll collect their winnings from him the following day. But when William returns the next evening, he carries the latest edition of a newspaper under his arm. He parades around the pub pointing out the date on the paper. It’s September 14th. William shouts out the facts, he started dancing before midnight on September 2nd and ended in the early hours of September 14th — he must have danced for twelve days. The patrons grudgingly pay up, and William is soon spending his winnings at the bar. This is one night at least, where he won’t have to settle for just a single pint all evening.

But the introduction of the Gregorian calendar does more than give one Staffordshire farmer a windfall — it corrects a fundamental error in the calendar, and eases confusing barriers to trade and communication between Britain and the rest of Europe. Over the centuries that follow, Britain and its colonies will be joined in the use of the new calendar by Japan and Egypt, by Russia and Turkey, China and Greece, until almost the entire world will measure its days in the same way. Thanks to Julius Caesar, to Aloysius Lilius, Christopher Clavius and Pope Gregory XIII, our months are kept in sync with our seasons, and, almost every four years, an extra day is added to the calendar, as it is today, February 29th 2024.

Outro

Next on History Daily. March 1st, 1854. The transatlantic steamer City of Glasgow leaves England with approximately 480 passengers and crew but is never seen again.

From Noiser and Airship, this is History Daily, hosted, edited, and executive produced by me, Lindsay Graham.

Audio editing by Muhammed Shahzaib.

Sound design by Gabriel Gould.

Music by Lindsay Graham.

This episode is written and researched by Scott Reeves and William Simpson.

Edited by Dorian Merina.

Managing producer, Emily Burke.

Executive Producers are William Simpson for Airship, and Pascal Hughes for Noiser.