Julian calendar

Based on Wikipedia: Julian calendar

The Calendar That Ruled for Sixteen Centuries

Julius Caesar was stuck in Egypt, besieged by an army, and apparently thinking about calendars.

It was 48 BCE, and the Roman dictator had sailed to Alexandria chasing his rival Pompey, only to find himself trapped in the palace district with Cleopatra while Egyptian forces surrounded them. During a banquet amid this siege, Caesar reportedly announced his intention to create a calendar "more perfect than that of Eudoxus"—the Greek astronomer who had calculated the year's length centuries earlier.

What emerged from this unlikely moment would become one of the most successful administrative technologies in human history. The Julian calendar, named for its creator, would govern time across the Western world for more than 1,600 years. It still determines when millions of Orthodox Christians celebrate Easter, and it quietly ticks along in traditional Amazigh (Berber) communities of North Africa.

The Problem Caesar Was Solving

To understand why Caesar bothered redesigning time itself, you need to understand just how broken the old Roman calendar had become.

The pre-Julian Roman calendar was a mess. It had twelve months totaling only 355 days—about ten days short of an actual solar year. To fix this drift, priests called pontifices were supposed to insert an extra month every few years, jamming 22 or 23 additional days between February and March.

This could have worked. It didn't.

The problem was politics. The pontifices weren't just religious figures—they were politicians. And a Roman magistrate's term of office lasted exactly one calendar year. So if you controlled when extra months got added, you controlled whose political career got an extension and whose got cut short. A pontifex could stretch the year when his allies were in power and conveniently forget to do so when his enemies held office.

By Caesar's time, this abuse had thrown the calendar completely out of alignment with the seasons. The month that was supposed to mark spring planting might arrive in what was actually winter. Farmers couldn't trust the official date to tell them when to sow crops. Festivals meant to celebrate the harvest occurred when fields were bare.

Learning from Egypt's Mistakes

Caesar didn't invent the solution from scratch. He borrowed it.

The Egyptians had long used a simple 365-day calendar—no intercalary months, no priestly manipulation. But this simplicity came at a cost: their calendar drifted against the sun by one day every four years. Over a human lifetime, the Egyptian calendar would slip by almost three weeks. Over centuries, their New Year wandered through all the seasons.

In 238 BCE, about two centuries before Caesar's visit, Egyptian priests had tried to fix this by adding an extra day every four years. The reform failed—people simply didn't adopt it.

Caesar saw both the elegance of the Egyptian approach and its flaw. He also knew that Greek astronomers, particularly Eudoxus, had calculated the year's length at 365 and one-quarter days. The solution was obvious: take Egypt's fixed calendar, add a leap day every four years to account for that quarter-day, and remove the whole system from human manipulation.

No more deciding whether to add extra months. No more political calendars. The sun itself would keep the calendar honest.

The Longest Year in History

Implementing this reform required solving a significant problem: the old calendar was so far out of alignment that simply switching to the new system would perpetuate the error. Caesar needed to reset the clock.

His solution was to make 46 BCE extraordinarily long.

The year had already been extended from the usual 355 days to 378 by inserting a regular intercalary month. Then Caesar added two more extraordinary months between November and December, stretching the year to an astonishing 445 days. Contemporary sources called these additions "Intercalaris Prior" and "Intercalaris Posterior"—the first intercalary month and the second. Some later writers jokingly called this "the last year of confusion."

When January 1st of 45 BCE finally arrived, the calendar was properly aligned with the seasons, and the new system took effect.

How the Julian Calendar Works

The mechanics are beautifully simple.

Each year has 365 days divided into twelve months. Every four years, without exception, an extra day is added. This creates a repeating cycle: three regular years of 365 days, then one leap year of 366 days. The average year length works out to exactly 365.25 days.

Caesar added ten days to the old 355-day year to reach 365. He distributed these extra days carefully: two days each went to January, August (then called Sextilis), and December. One day each went to April, June, September, and November. February kept its traditional 28 days in normal years, receiving the leap day when needed.

The month lengths Caesar established—31 days for January, March, May, July, August, October, and December; 30 days for April, June, September, and November; 28 or 29 for February—are exactly what we use today, more than two thousand years later.

The Bissextile Day

The Romans had a peculiar way of counting days. Rather than numbering them forward from the start of each month, they counted backward from three fixed points: the Kalends (the first day), the Nones (roughly a week in), and the Ides (roughly mid-month). The famous "Ides of March" when Caesar was later assassinated was simply the 15th of March by our reckoning.

When adding a leap day, the Romans didn't just tack it onto the end of February. Instead, they inserted it by doubling the sixth day before the Kalends of March. In Latin, this was "ante diem bis sextum Kalendas Martias"—literally "the sixth day before the Kalends of March, doubled." This is where we get the term "bissextile" for leap years.

This created an odd situation: a 48-hour "day" that was legally counted as a single unit. Roman jurists debated for centuries which half of this doubled day was the "real" February 24th and which was the inserted addition. The question mattered for legal purposes—contracts, court dates, the beginning and end of terms of office. The debate was never fully settled, with different authorities taking different positions well into the medieval period.

Why It Worked So Well

The Julian calendar succeeded where previous reforms had failed for one crucial reason: it removed human discretion.

Before Caesar, keeping the calendar aligned with the seasons required someone to decide when to add extra time. That decision could be made poorly, whether through incompetence or corruption. The Julian system needed no decisions at all. Every fourth year had an extra day. Period. No priests required, no politicians involved.

This proved useful almost immediately. Just eight years after the reform, the scholar Varro was able to calculate fixed calendar dates for the start of each season—something that would have been impossible under the old chaotic system. A century later, the Roman naturalist Pliny could casually note that the winter solstice fell on December 25th, treating the stability of the calendar as an unremarkable fact of life.

Spreading Across the Empire

Caesar's reform technically applied only to Rome itself. But over the following decades, the Julian calendar became a template for the entire Mediterranean world.

Local and provincial calendars across the Roman Empire gradually aligned themselves to the Julian system. They kept many of their old features—different month names, different New Year dates, different positions for the leap day—but they all adopted the fundamental 365-day structure with an extra day every four years. This created a network of compatible calendars that could be converted between each other using reference tables called "hemerologia."

The Alexandrian calendar in Egypt adopted the Julian structure. So did the ancient Macedonian calendar, which developed into two variants: one used in Roman Syria and one used in the province of Asia (roughly modern Turkey). The Asian version, reformed in 8 BCE, actually began its year on September 23rd—the birthday of Emperor Augustus—and renamed its first month "Kaisar" in his honor.

Not everyone adopted the reform. The Gauls kept their traditional Celtic calendar, as did the Greeks in Macedonia and parts of the Balkans. Jewish communities in Palestine maintained their own lunisolar calendar. But for administrative purposes across most of the Roman world, the Julian calendar became the standard.

The Flaw That Took Centuries to Matter

The Julian calendar has one small problem: the year isn't actually 365.25 days long.

The true length of a tropical year—the time it takes Earth to complete one cycle of seasons—is approximately 365.2419 days. That's about 11 minutes shorter than the Julian year. It seems negligible. It isn't.

Those 11 minutes add up. Every 129 years, the Julian calendar gains one full day against the actual position of the sun. Over centuries, the calendar slowly drifts forward through the seasons.

By the 1500s, the spring equinox—which was supposed to fall around March 21st—was occurring on March 11th. The calendar had drifted ten full days. This was more than an astronomical curiosity. The date of Easter depends on the spring equinox, and the Church was increasingly alarmed that Christianity's most important holiday was being calculated from a calendar that no longer matched the heavens.

The Gregorian Correction

In 1582, Pope Gregory XIII promulgated a reformed calendar that bears his name.

The Gregorian calendar made one elegant adjustment to Caesar's system. Instead of having a leap year every four years without exception, it skips the leap year in century years—unless that century year is divisible by 400. So 1700, 1800, and 1900 were not leap years. But 1600 and 2000 were.

This simple modification changes the average year length from 365.25 days to 365.2425 days—much closer to the true tropical year. The Gregorian calendar still drifts, but only by about one day every 3,236 years instead of one day every 129 years. For most practical purposes, it stays aligned with the seasons indefinitely.

Gregory also needed to fix the accumulated drift. He decreed that October 4, 1582 would be followed directly by October 15, 1582, erasing ten days from the calendar to bring the spring equinox back to its proper date.

A Calendar Divided

Not everyone accepted Gregory's reform.

Catholic countries—Spain, Portugal, Italy, and Poland—adopted the new calendar immediately. France followed within months. But Protestant nations resisted what they saw as papal interference with time itself. The Protestant German states didn't switch until 1700. Britain and its American colonies held out until 1752, by which point the gap between the Julian and Gregorian calendars had grown to eleven days.

Orthodox Christian countries resisted even longer. Greece didn't adopt the Gregorian calendar for civil purposes until 1923. Russia switched only after the Bolshevik Revolution, which is why the "October Revolution" of 1917 actually occurred in November by the Gregorian calendar.

Even today, some Orthodox churches calculate religious holidays using the Julian calendar. This is why Orthodox Christmas falls on January 7th in the Gregorian calendar—it's December 25th by Julian reckoning. The gap is now 13 days and will reach 14 days in 2100 when the Gregorian calendar skips a leap year that the Julian calendar keeps.

The Living Calendar

Twenty centuries after Julius Caesar stood besieged in Alexandria, contemplating the measurement of time, his calendar remains alive.

It may seem like a historical curiosity—a superseded technology, like sundials or mechanical clocks. But millions of people still organize their religious lives by it. Orthodox Christians, Amazigh communities in North Africa, and others continue to count their days by Caesar's innovation.

And even those of us using the Gregorian calendar are really using a modified Julian calendar. The twelve months, their names, their lengths, the basic structure of the year—all of this comes directly from the reform that Caesar implemented in 45 BCE. Gregory didn't replace the Julian calendar; he refined it.

There's something remarkable about that. An administrative decision made by a Roman dictator more than two thousand years ago, influenced by Egyptian practice and Greek astronomy, still shapes how humanity organizes time. The Julian calendar outlasted the Roman Republic that created it, outlasted the Roman Empire, outlasted the medieval kingdoms that rose from its ashes, and continues into an era its creators could never have imagined.

Every time you check the date, you're using a system that can trace its lineage back to a besieged palace in Alexandria, where a general thought about the stars.