According to legend, the Roman calendar began with Romulus, cofounder of Rome, who had a calendar of 10 months, or 304 days. His successor, King Numa, added 2 months for a total of 354 days and then added another day to avoid the Roman superstition against even numbers. Still, this did not meet the accurate tropical year of 365.242 days.
To fix the inaccuracy, priests added an extra month every 2 years, which came to a total of 366.25 days a year. They also tried to add an intercalary month every 8 years, as a Greek version of the calendar did. This added up to roughly 365 days, but priests often forgot to add extra months when needed. The priests and aristocrats kept the calendar a secret so they could use it to their advantage, adding days or subtracting days to hasten or stall elections. In 304 BCE, Creius Flavius posted the codes to force the priests to publish the calendar as a public document, although the priests still held the power to add extra months.
Julius Caesar became pontifex maximus in 63 BCE, and undertook to reform the Roman calendar. While on campaign in Egypt, Caesar had seen the Egyptians’ effective and simple calendar system. By the time he came to power, the republican calendar had drifted more than 3 months out of line. When elected, Caesar recruited Sosigenes and Alexandrian astronomers, along with many other philosophers and mathematicians, to aid in the task of calendar reform.
The first stage of reform began in 46 BCE by adding 2 extra months to bring the calendar back into alignment with the vernal equinox on March 25. In total, the year 46 BCE had 445 days, becoming known by some as the year of confusion, and by Caesar as the last year of confusion. The year 45 BCE then became the first year of the new system, and the date for the beginning of the year moved from March 1st to January 1st.
The length of the new calendar year included an extra 10 days, totaling 365 days instead of 355. The lengths of months were organized starting with January having 31 days and February having 29 days (except every 4th year, or leap year, when February had 30). From February on, months alternated between 31 and 30 days, beginning with March having 31 days. The dates of festivals and holidays stayed the same along with the old system of numbering days according to calends (1st day of the month), nones (5th/7th day), and ides (13th/15th day). Although most of the months kept their traditional names, the senate renamed the month Quinctilis as Julius (July) in Caesar’s honor.
The calendar helped foster a middle class of traders, bureaucrats, soldiers, lawyers, money lenders, and craftsmen who now could measure time in the civic world of the empire. Because the new calendar was based on science, none could tamper with it for politics. With the new idea of a leap year, the calendar held 365.25 days compared with the actual tropical year of 365.242. This new calendar became the most accurate in the world and lasted until the Gregorian reform in 1582.
Alexandrian astronomers such as Hipparchus and Ptolemy later noticed that the calendar ran 6 hours slow. Roger Bacon also noted this at a later date. The Romans had no concept of a minute, so they could divide the day only into simple fractions for astronomical purposes. But for Caesar, a calendar year’s being off by a few minutes or hours meant nothing compared with the former calendar’s being off by days and months for the previous 2 centuries. After the death of Caesar, pontiffs began to confuse the leap year rule. Since Romans counted inclusively, they began adding a leap year every 3rd year instead of every 4th year.
To correct the leap year error in the calendar, Augustus Caesar left out all leap years between 8 BCE and 8 CE. To honor him, the senate renamed Sextilis as Augustus and took a day from February to add to his month so his would not be shorter than Julius Caesar’s. They then adjusted the lengths of the remaining months by switching the lengths of September through December to maintain a constant alternation. As amended by Augustus, the calendar continued in use until the Gregorian reform.
See also Gaius Julius Caesar; Astronomical Calendar; Egyptian Calendar; Gregorian Calendar; Roman Calendar; Ancient Rome; Measurements of Time
Bellenir, K. (1998). Religious holidays and calendars. Detroit, MI: Omnigraphics.
DeBourgoing, J. (2001). Calendar: Humanity’s epic struggle. New York: Harry N. Abrams.
Duncan, D. E. (1998). The calendar: History, lore, and legend. New York: Avon.