The Gregorian Calendar: Even Better than the Julian Calendar

In last month’s post, I wrote about the Julian Calendar, a huge advance in timekeeping technology that was introduced in 45 B.C. It was far more accurate than its predecessor, the Roman Calendar, which was based on a lunar year with extra days added in whenever a committee decided they were needed. The Julian Calendar wasn’t the first solar calendar ever made, but it was the first to be widely adopted across Europe. It had a standard year of 365 days, with a leap year of 366 days every fourth year.

That sounds an awful lot like the calendar we use today, doesn’t it? Well, almost, but not quite. It turns out that one leap year every four years is just a smidge too much. A tropical year—the time it takes the earth to complete exactly one revolution around the sun—is 365 days, 5 hours, 48 minutes, and 46 seconds. This means that, once standard and leap years have been averaged together, the Julian Calendar year is longer than an actual tropical year by around 0.0077 days, or 11 minutes.

That may not seem like much time, but it adds up. Over the course of a thousand years, the Julian Calendar will drift almost 8 days ahead of the tropical year.

And so it did. By the late 16th century, the calendar used by most of the Western world was 10 days at variance from the actual orbit of the earth.

It was time to make a change to a more accurate solar calendar. The motivating force behind the switch was Pope Gregory XIII, acting on concerns about errors in calculating the date for Easter. A reform commission was assembled, and after considerable study and consultation with expert mathematicians, its members agreed on a new calendar with a small but significant change. Unlike the Julian Calendar, the Gregorian Calendar has a leap year every four years except for years which are divisible by 100 but not by 400. Those years are common years of 365 days.

The reason most people in the Western world today aren’t aware of these centennial common years is that we’ve never experienced one. The most recent centennial common year was in 1900, and the next one will be in 2100. The year 2000 was a leap year, because 2000 is divisible by 400.

The Gregorian Calendar went into effect in October of 1582 by means of a papal bull, which had no authority beyond the Roman Catholic Church and the papal states. That same year, Philip II of Spain decreed that the switch be made in his dominions, comprising not only Spain but also Portugal and much of Italy. The Polish-Lithuanian Commonwealth also adopted the new calendar.

By Aloysius Lilius - Biblioteca del Vaticano

Protestant, Eastern Orthodox, and Oriental Orthodox Churches continued with the Julian Calendar, as did Protestant nations, who resisted what they saw as a Catholic innovation. The British ultimately found a work-around with their Calendar (New Style) Act of 1750, which established a system for the computation of Easter that achieved the same result without referencing Pope Gregory XIII. Britain and the British Empire, which at that time included the eastern part of the future United States, adopted the new calendar in 1752. Sweden followed in 1753.

Alert readers familiar with American history will notice that 1752 was just a little bit before the French and Indian War. If you've done any reading about that conflict, you've probably come across some parenthetical adjustments between the date systems used in different parts of Europe. During this period, the Julian Calendar was 11 days at variance from the Gregorian. So the birthdate of George Washington, who was born under the Julian Calendar, can be written as February 11, 1731/32 (Old Style, depending on whether the writer is using the English or Continental start to the new year), or as February 22, 1732 (New Style).

At various times throughout the following centuries, other nations in Europe and worldwide adopted the Gregorian Calendar, with Saudi Arabia being the most recent addition in 2016. Ethiopia continues to use its own calendar of 13 months, which is 7-8 years behind the Gregorian and has a new year that falls in the Gregorian September. Nepal uses a lunisolar calendar 56-57 years ahead of the Gregorian calendar, with a new year in mid-April.

Today’s world is far more tightly interconnected than that of the sixteenth century. A uniform system of global timekeeping has clear advantages across many sectors, including international finance and trade, transportation and logistics, telecommunications and internet, science and research, military and defense, emergency and disaster response, and education and academia. The Gregorian Calendar didn't altogether unite the world in a global timekeeping standard, but it went a long way toward streamlining operations and ensuring clarity in planning, record-keeping, and communications.

Kit Hawthorne makes her home in south central Texas on her husband’s ancestral farm, which has been in the family for seven generations. When not writing, she can be found reading, drawing, sewing, quilting, reupholstering furniture, playing Irish pennywhistle, refinishing old wood, cooking huge amounts of food for the pressure canner, or wrangling various dogs, cats, horses, and people. Visit her at https://kithawthorne.com/.

The Julian Calendar: Technological Breakthrough of the Past

by Kit Hawthorne

We talk a lot about how technology has transformed the modern world, and for good reason. The 20th century saw the rise of automated production, the digital revolution, and global connectivity—and things have only sped up since then. Starting in the 21st century, computational power has doubled approximately every two years, leading to snowballing advances in automation, data analysis, and artificial intelligence.

But a lot of the everyday conventions and devices that we take for granted were huge innovations when they were first introduced (and, of course, provide the foundation for later advancements). Arabic numerals, alphabetic writing, libraries, indexes, concordances, classification systems for living organisms, measurement systems, navigation, cartography, standardized coinage, postal services, mechanical clocks, codified systems of law—these intellectual technologies didn’t just make it easier for people to perform physical tasks. They revolutionized the way people thought. Every one of them has its own complex and fascinating history, often reaching back to the ancient world.

But the intellectual technology that I want to drill down on today is calendars—specifically, the Julian Calendar, which made its first appearance in 45 B.C.

The Tusculum Portrait of Julius Caesar

We deal with calendars in some form or other just about every day but rarely pause to think about how very useful they are or how marvelous it is that we have them at all. Agricultural planning, religious and cultural ceremonies, navigation and timekeeping, economic and administrative coordination, political stability, and the keeping of historical records would be impossible in the absence of an accurate shared calendar.

A 2007 Wall Calendar by Claudio Elias

The Julian Calendar was a big improvement over its predecessor, the Roman Calendar, which was based on a lunar year. It’s easy to see the trouble with a purely lunar calendar. A period of twelve lunar months is roughly 354 days, 11-12 days shorter than a solar year, which meant that the Romans had to keep fiddling with theirs to correct the inevitable drift with respect to astronomical events like vernal equinox and winter solstice. The adding and removing of days was managed by a committee within the College of Pontiffs, a priestly group that had a lot of other duties to attend to.

Julius Caesar decided that Rome could do better. Acting with characteristic pragmatism and dispatch, he consulted an Alexandrian astronomer called Sosigenes and developed a solar calendar with a common year of 365 days. In reality, it takes approximately 365 days, 5 hours, 48 minutes, and 45 seconds for the earth to complete a full orbit around the sun. To make up the difference, the Julian calendar added an extra day every four years.

The Julian calendar was soon adopted throughout the Roman Empire and by various Christian churches. It is still used by the Berber people of North Africa and by some Orthodox churches, which employ it to calculate the dates of movable feasts, but in most of the world it was eventually replaced by the even more accurate Gregorian Calendar, the calendar with which most of us are familiar today.

The monastic community at Mt. Athos still uses the Julian Calendar.
Photo of Agiou Pavlou Monastery by Explorer1940.

At this point, you might be thinking, “Hold on. A calendar with three 365-day common years, followed by one leap year of 366 days—isn’t that what we use now?” Well, almost. The Gregorian Calendar is very similar to the Julian Calendar, with one small but important difference. Getting it adopted as the international standard was a wild ride that took 345 years…and, Lord willing, that’ll be the subject of my December post.

Kit Hawthorne makes her home in south central Texas on her husband’s ancestral farm, which has been in the family for seven generations. When not writing, she can be found reading, drawing, sewing, quilting, reupholstering furniture, playing Irish pennywhistle, refinishing old wood, cooking huge amounts of food for the pressure canner, or wrangling various dogs, cats, horses, and people. Visit her at https://kithawthorne.com/.