Britannica
Sources used in this answer
How did Julius Caesar reform the Roman calendar?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
Julius Caesar
Julius Caesar, bronze sculpture in Rome.
By the 1st century BCE, the Roman republican calendar had become significantly misaligned with the solar year due to its reliance on lunar cycles and political manipulation by officials. To resolve this, Julius Caesar implemented a major reform in 46 BCE, advised by the Alexandrian astronomer Sosigenes.123
Key aspects of the reform included:
- Solar Alignment: Caesar adopted a 365-day solar calendar, estimating the year at 365 1/4 days.13
- Leap Years: To account for the fractional day, an extra day was added every four years. Due to a misunderstanding of inclusive counting, this was initially applied every three years before being corrected during the reign of Augustus.3
- Calendar Adjustment: To synchronize the existing civic calendar with the seasons, Caesar added days to 46 BCE, resulting in a year of 445 days.13
- Structure: The year was divided into 12 months of 30 or 31 days, with February containing 28 or 29 days.1
This new system, known as the Julian calendar, served as the standard until the introduction of the Gregorian calendar in 1582.13