Why Does the Calendar Begin on January 1?

modern calendar originated in rome

By Jonny Lupsha, Wondrium Staff Writer

Where did the modern calendar come from? Before Julius Caesar, the existing calendar year looked far different than it does today. The Romans revolutionized timekeeping.

Close up of Roman sundial
Although Romans used a sundial to measure the passage of time, seasonal differences of the amount of daylight during a day would mean that an hour in Rome could be longer or shorter depending on the time of year. Photo by Inked Pixels / Shutterstock

The way that humanity keeps time is a complex but relatively efficient division and subdivision of equal amounts, 365 days in a year—with an exception for leap year once every four years—24 hours in a day, and so on, down to fractions of a second. How did we decide to keep time this way? Much like cement and aqueducts, the Julian calendar is a Roman invention.

How did the Romans develop modern timekeeping? In his video series The Rise of Rome, Dr. Gregory S. Aldrete, Professor of Humanistic Studies and History at the University of Wisconsin, Green Bay, dives into the rich history of the calendar year.

How Did the Romans Keep Time?

“For the Romans, each day was divided into two periods: The time when it was light outside, and the time when it was dark,” Dr. Aldrete said. “Each of these periods was then subdivided into 12 hours. Superficially, this sounds a lot like our modern system of 24-hour days. However […] Roman hours were not of a fixed length because they were equivalent to the amount of light or darkness on a given day divided by 12.”

Since the length of daylight varies so vastly from winter to summer, an hour in Rome could be longer or shorter depending on the time of year. Dr. Aldrete said a Roman hour could be anywhere from 40 to 90 minutes. Meetings could only be scheduled very approximately and both parties would simply have to guess when they would meet.

“One of the most common tools that the Romans employed to measure the passage of time, the sundial, was based on this daily motion of the Sun,” he said. “A sundial might consist of nothing more complex than a stick placed in the ground to crudely track the progress of the Sun, or it could be an elaborate marble shell with curved paths carved into it that corresponded to the progress of the gnomon’s shadow.”

Why Are There 365 Days in a Year?

The Romans divided the year into 12 months like we do today. However, they counted just 355 days per year. Now, we know there are 365.25 days in an astronomical year. Before this was common knowledge, though, the Roman calendar would get out of sync with the natural seasons after just a few years. This would cause massive disruption for farming—a vital industry in every society. If farmers stuck to the calendar in use, they would end up planting crops at the wrong time of year, leading to low yields.

“The solution that the Romans came up with was that every so often, the priests would declare an intercalary month, termed Mercedonius or Intercalaris, which was inserted in between two existing months in order to bring the months back into line with the natural seasons,” Dr. Aldrete said.

This problem compounded so much during the reign of Julius Caesar that multiple intercalary months had to be added into the year, one time. To ensure this never happened again, Caesar added 10 days to the calendar year, plus an extra day every four years, giving the world leap year and the modern calendar.

The current-day English names of the calendar months derived from the Roman influence, too. Several of the names of the first five months came from names of Roman gods, such as January for Janus. The next five names came from numbers. Additionally, the start of the Roman calendar year began on March 1.

“Later in Roman history, the fifth and sixth months were renamed Julius and Augustus to honor Julius Caesar and the Emperor Augustus.”

The Rise of Rome is now available to stream on Wondrium.

Edited by Angela Shoemaker, Wondrium Daily