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The History of Calendars, in 7 Facts (or why we have leap years)

As we celebrate the first day of 2024, consider it a testament to the longevity of one of civilization's oldest staples: the calendar. The earliest means of measuring days and weeks dates back 10,000 years, and timekeeping techniques adopted by the ancient Babylonians, Egyptians, and Romans slowly evolved into the calendar we use today. Yet the emphasis here is on “slowly.” The evolution from charting moon phases to separating seasons to measuring fiscal years was one of controversy and chaos across centuries. Still, humans never stopped working to perfect how we mark the passage of time. Here’s a brief look at the fascinating history of calendars, just in time to start a new one.

The First Known Calendar Is From Prehistoric Scotland

In 2013, British archaeologists discovered what they consider the world’s oldest calendar, dating back to around 8000 BCE. The prehistoric calendar, located at Warren Field in Scotland, consists of 12 pits believed to have contained wooden posts representing months of the year. Positioned to chart lunar phrases, the pits are aligned with the southeast horizonand were likely used by hunter-gatherer societies to track seasons. The site precedes Stonehenge by several thousand years.

Stonehenge Used Celestial Bodies to Track Time

Though the exact purpose of Stonehenge remains a mystery, archaeologists believe that in addition to a burial site, the prehistoric monument may have been a solar calendar. Keeping track of months and years in ancient societies relied on charting the stars, sun, and lunar patterns (and the subsequent weather they’d bring). Stonehenge, built around 3000 BCE, consists of stones and archways arranged in a circular pattern, and experts believe time was measured by the way the sun, moon, and stars lined up with these markers. Archaeological evidence suggests lunar and solar eclipses were also observed, as were the winter and summer solstices.

The Babylonian Calendar Had a 12-Month Year and Seven-Day Week

The Babylonians, a civilization that began in the Fertile Crescent of ancient Mesopotamia (modern-day Iraq) around 4000 BCE, used lunar patterns to determine the length of the year. They divided the calendar into 12 monthsbased on the cycle of the moon, starting with the first sighting of the crescent moon. This put the year at 354 days, slightly shorter than a solar year. (The moon cycle is actually 29.5 days long, so there are 13 lunar phases each year.) To sync the lunar and solar calendars, the Babylonians adopted a system known as intercalation, which allowed them to add an extra month when necessary. They also divided each month into seven-day weeks, based on (and named for) the seven celestial bodies they were able to observe: the sun, the moon, and the five nearest planets. 

The Ancient Egyptians Had Three Seasons

Like many ancient agricultural societies, the ancient Egyptians measured time according to the planting and harvest seasons. The Nile river flooded each year between the months of what is now May and August, and this annual flooding was crucial to Egyptian society, as it determined the success of that year’s crops. As a result, the Egyptians divided the year into three seasons, each related to the river’s status and the agricultural phases associated with it: Akhet (translated as Flood or Inundation), Peret (Emergence or Going Forth), and Shemu (Low Water, Deficiency, or Harvest). 

Our Modern Calendar Was (Partially) Created by Julius Caesar

In 46 BCE, Julius Caesar introduced the Julian calendar as a means of reforming the existing and flawed Roman timekeeping system. The Roman Republic’s calendar (which referred to the first day of each month as kalends, the origin of the word “calendar”) not only miscalculated the length of a solar year, but also could be manipulated to control political power, which created further inaccuracies. Caesar’s response was a new system based on calculations by the Greek  astronomer Sosigenes of Alexandria, who determined each year consisted of 365.25 days. To account for that .25, the Julian calendar added an extra day every fourth February: leap year. Though the calendar was still imperfect, much of what Caesar put in place remains in use today — including, of course, the name of the seventh month, July.

The Gregorian Calendar Fixed Caesar’s Math

For all its improvements, the Julian calendar still miscalculated the length of a solar year by 11 minutes. That may not seem like much, but the discrepancy added up over time, and by 1582 CE, the months of the calendar and the seasons were misaligned. This concerned Pope Gregory XIII, who noticed that Easter was becoming increasingly out of sync with the spring equinox — a problem for the Catholic Church. In response, the pope launched the Gregorian calendar on October 4, 1582, which set the following day as October 15, jumping the calendar ahead 10 days. Chaos ensued: Catholic countries adopted the reformed calendar, but many Protestant nations waited more than a century to get on board. The British Empire, including its colonies in America, waited until 1752 to switch to the Gregorian calendar, and Russia didn’t adopt the new calendar until as late as 1918, after the Russian Revolution.

France Had a Calendar With 10-Day Weeks During the Revolution

In 1793, during the French Revolution, the revolutionary government attempted to create a new secular calendar for the French Republic, keen to do away with Christian associations as part of its break from the ruling elite. Known as the French Republican calendar, it set each month at 30 days, divided into three weeks consisting of 10 days each. The calendar never took off, however.  It was abandoned for its Gregorian counterpart on January 1, 1806, after Napoleon Bonaparte became emperor.

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