45 BCE

Julian Calendar

Julius Caesar (100–44 BCE)

Like other past civilizations that were tuned in to the skies, the Romans had developed a calendar system that had strong astronomical connections. The calendar system that they originally designed in the eighth century BCE was the source of constant confusion, however, partly because it was cobbled together from pieces borrowed from the Greeks and others before them. For example, the year had 10 months of 30 or 31 days for a total of 304 days—the remaining 61-plus days needed to make up an actual trip around the Sun were brushed under the rug as “winter.” A later change added two new winter months (January and February) but still came to only 355 days total per year. To keep the calendar lined up with the seasons, a leap month was occasionally added in by the high priests, but the decision to add extra days to a given year was often arbitrary and politically motivated. The situation got so muddled that many ordinary Romans had no idea what day, year, or month it was.

In fact, the Roman calendar system was such a confused mess when Julius Caesar came to power in 49 BCE that he ordered a reform that would align the calendar more with the motions of the Sun rather than with the affairs of men. Days were added to some of the 12 months to bring the total number of days in a year to 365, and he decreed that every fourth year an extra leap day would be added to the end of February, making the average length of the year 365.25 days, which is close to the actual length of a solar year—365.242 days. Caesar’s reform of the calendar took effect on January 1 in the year 45 BCE (or, to the Romans, 709 years after the founding of Rome), after the priests had to make the year 46 BCE 445 days long to try to fix all the problems that had accumulated before the reform.

The Julian calendar worked well for a long time because it was only 0.008 days (about 11 minutes) per year different from a true solar year. By the sixteenth century, however, those 11 minutes per year had added up to a significant shift between the calendar year and the solar year, and so a further tweak, called the Gregorian Calendar reform, was needed to resync the calendar with the seasons.

Ancient Roman calendars were sometimes carved into stone blocks, as in this marble version that shows day names and astrological symbols for the months of April through September.