The Julian Calendar

Catching the Drift

By the middle of the first century BCE, time reckoning in the Western world had descended into chaos. The Roman calendar had proven to be woefully inadequate for keeping in sync with the tropical year. Over time, it had become badly misaligned with the seasons.

To remedy this, the Roman general Julius Caesar ordered a sweeping calendar reform. The old Roman lunisolar calendar was to be replaced by a solar calendar, with the express aim of keeping it in line with the seasons.

In 45 BCE, the Julian calendar came into force.

A Tropical Year plus Eleven Minutes

Today, we know that it takes Earth about 365 days, 5 hours, 48 minutes, and 45 seconds (365.242189 days) on average to complete one full orbit around the Sun. Commonly called the “solar year,” astronomers refer to this time span as the tropical year, and our modern calendar is pretty good at approximating it.

Sosigenes of Alexandria, however, didn’t have that luxury. He is the Greek astronomer whom Caesar is believed to have tasked with devising the new Julian calendar.

According to the wisdom of the time, an average tropical year lasted approximately 365.25 days, which translates to 365 days and 6 hours.* Today, we know that’s 11 minutes and 15 seconds too long.

It’s a remarkably small error, given the lack of modern measuring instruments and knowledge. However, it was big enough to ultimately doom the calendar system.

Introducing Leap Days

Sosigenes ran with it. He devised a completely new intercalation system that utilized leap days. If that sounds familiar, that’s because we still use that basic system up to the present day.

At the time, however, that was a revolutionary invention.** What’s more, it was mathematically elegant: by inserting one extra day every four years, the Julian year would last precisely 365.25 days on average, perfectly reflecting the assumed length of the tropical year in one simple formula.

Julian Calendar Structure

First, however, the length of the common year (non-leap year) had to be adjusted to align as closely as possible with the tropical year’s assumed length of 365.25 days. A common year in the Roman calendar had lasted only 355 days, so Sosigenes had to add ten days to the calendar, bringing the total length to 365 days.

He did so by distributing them fairly evenly across the twelve months, resulting in the month lengths we are used to today.

• The months of January, August, and December were extended by two days each—from 29 to 31 days.
• April, June, September, and November gained one day each—from 29 to 30 days.

To achieve an average year length of 365.25 days, a leap day was added every four years without exception. In the initial version of the calendar, Leap Day fell on February 24,*** in contrast to today’s Leap Day, which happens on February 29.

Started Drifting from Day One

Once the new calendar system had been adopted, it seemed to work like a charm. It was now much more closely tied to the seasons, and it was immune to the ad-hoc adjustments that had been common in the Roman calendar. As a result, dates became predictable years in advance.

Under the hood, however, with that slight error hardcoded into its leap year rules, it started drifting from day one. The following numbers are based on the average length of a tropical year:

• The seasons occurred 5 hours, 48 minutes, and 45 seconds later each common year;
• Without leap days, the deviation would have grown to 23 hours and 15 minutes over the course of four common years.
• However, each time an additional leap day—lasting 24 hours—was added in the fourth year, this deviation was overcompensated, so the seasons occurred 45 minutes earlier, on the calendar, than on the previous leap day.
• This translates to the previously mentioned deviation of 11 minutes and 15 seconds per year on average.

In other words, with a leap day added every four years without exception, the calendar had too many leap days to correctly reflect the length of the tropical year over long periods of time.

At first, of course, a deviation that small didn’t really matter for everyday life. But the error kept accumulating at a rate of 1 day every 128 years. Eventually, something needed to be done to stop the drift.

Felled by the Catholic Church

On February 24, 1582, Pope Gregory XIII issued a papal bull , instituting a new, more accurate calendar system: the Gregorian calendar we still use today.

By the mid-16th century CE, the Julian calendar’s deviation had grown to ten days. This posed a particular problem for the Catholic Church: It threw the calculation of the Easter date into disarray.

From the early days of Christianity, Easter had been closely linked to the March equinox—the vernal equinox in the Northern Hemisphere. Crucially, however, the Church used a fixed calendar date as an approximation of the equinox when determining the Easter date: March 21. This was the date of the equinox when the method of calculating the Easter date was developed.

Now, with the calendar having drifted away from the astronomical seasons over the centuries—the vernal equinox now fell as early as March 11—so had the date of Easter. Specifically, Easter came too late because the calendrical equinox, March 21, now fell ten days after the vernal equinox.

This was increasingly unacceptable to the Church, and the clergy now faced a choice: Change the Computus, the rules governing the calculation of the Easter date, or initiate a calendar reform. In 1545, it was decided to do the latter.

Dropping Some Leap Years

37 years later, in 1582, the Gregorian calendar came into force in some European countries, and ten days were dropped in the calendar to realign it with the seasons.

However, it took more than 300 years for it to be adopted around the world. For example, in the British colonies of North America that eventually became the United States and Canada, it wasn’t introduced until September 1752. By then, the offset had grown by another day, so 11 days had to be omitted as the new system took effect.

The Gregorian calendar retained its predecessor’s structure but introduced rare exceptions to the leap year rule to better reflect the tropical year over time.

Even so, the Gregorian calendar is not perfect either: It drifts by one day every 3236 years.

The Julian Calendar Is Still in Use

Even today, some Eastern Orthodox Churches, such as the Russian and Serbian Orthodox Churches, use the Julian calendar to calculate the dates of religious holidays.

That is why they celebrate the fixed (non-movable) holidays 13 days later than their Western counterpart—it is simply the current deviation between the Gregorian and Julian calendar systems. For example, they celebrate Christmas on December 25 in the Julian calendar, which corresponds to January 7 in the Gregorian calendar.

In the year 2101, the difference will have increased to 14 days, so Orthodox Christmas will fall on January 8 (Gregorian).

A Different Kind of Julian

The term ‘Julian’ is also used in other calendar-related contexts:

• Most confusingly, a calendar showing ordinal numbers for each day of the year is often colloquially referred to as a “Julian calendar”—even if the calendar itself is Gregorian. See an example.
• In that context, the term “Julian date” refers to a date expressed as the day number within a year. For example, ‘2026-123’ is the 123rd day of 2026, or May 3, 2026.
• The “Julian period” is a time reckoning system devised by the French Scholar Joseph Justus Scaliger in 1583 to provide astronomers with a single system of dates spanning millennia. The Julian period started on January 1, 4713 BCE, in the proleptic Julian calendar , and lasts for 7980 years.
• Based on the Julian period system, each day is assigned a “Julian day number,” allowing astronomers to express dates without having to worry about things like calendar reforms or BCE and CE.

* About a century before the reform, the Greek astronomer Hipparchus achieved a much more accurate measurement. It is unknown if Sosigenes was unaware of it or deliberately chose the mathematically simpler value of 365.25 days.

** The Ptolemaic calendar was a version of the Egyptian calendar that introduced leap days as early as 238 BCE. However, the calendar was never widely adopted, and it is unknown if Sosigenes was aware of it.

*** According to some sources, February 24 was actually repeated during leap years, in effect resulting in a 48-hour calendar day.

Topics: Calendar