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How did Julius Caesar reform the Roman republican calendar?
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calendar
First complete printed title page for the Kalendarium (“Calendar”) by Regiomontanus, 1476.
In the mid-1st century BCE, the Roman republican calendar had become significantly misaligned with the solar year due to its reliance on lunar cycles and the inconsistent application of intercalary months by the Pontifices. To address this, Julius Caesar consulted the astronomer Sosigenes of Alexandria to implement a more accurate system.123
The reform involved several key steps:
- Abandoning the Lunar Calendar: Caesar replaced the lunar-based system with a solar calendar modeled after the Egyptian system, setting the year at 365.25 days.14
- Correcting the Discrepancy: To realign the calendar with the seasons, 46 BCE was extended to 445 days through the addition of 90 days, including two intercalary months inserted between November and December.13
- Establishing Leap Years: Caesar introduced a leap year system, adding an extra day every fourth year. Due to the Roman practice of inclusive counting, this was initially misinterpreted as every three years, an error later corrected by Augustus.13
- Standardizing Months: The lengths of the months were adjusted to 30 or 31 days, with February serving as the variable month.14