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Julius Caesar: Father of the Leap Year Posted by on Mar 2, 2016 in Roman culture

First and foremost, hello everyone and Happy Leap Year!


Julius Caesar was behind the origin of leap year in 45 BC. The early Romans had a 355 day calendar and to keep festivals occurring around the same season each year- a 22 or 23 day month was created every second year.

Roman calendar


The calendar was  regulated by the movement of the moon, and this had made it a bit of mess and confusing. Caesar replaced this calendar with the Egyptian calendar, which was regulated by the sun. He set the length of the year to 365.25 days by adding an intercalary/leap day at the end of February every fourth year.

Bust of Julius Caesar. Courtesy of WikiCommons.

Bust of Julius Caesar. Courtesy of WikiCommons.

Furthermore, in order to bring the calendar into alignment with the seasons, he decreed that three extra months be inserted into 46 BC (the ordinary intercalary month at the end of February, and two extra months after November).

For a post more about the calendar, check it out here and here!

This is known as the Julian Calendar which started on on 1 January 45 BC. Also, this calendar is almost identical to the current Western calendar.

Months (Latin) Lengths before 45 BC Lengths as of 45 BC Months (English)
Ianuarius 29 31 January
Februarius 28 (in common years)
In intercalary years:
23 if Intercalaris is variable
23/24 if Intercalaris is fixed
28 (leap years: 29) February
Mercedonius/Intercalaris 0 (leap years: variable (27/28 days)
or fixed)
Martius 31 31 March
Aprilis 29 30 April
Maius 31 31 May
Iunius 29 30 June
Quintilis(Iulius) 31 31 July
Sextilis (Augustus) 29 31 August
September 29 30 September
October 31 31 October
November 29 30 November
December 29 31 December

I should note that the actual calculation for the calendar were made by Caesar’s astronomer, Sosigenes.

Pope Gregory XIII

Pope Gregory XIII

In 1582, Pope Gregory XIII further refined the calendar with the rule that leap day would occur in any year divisible by 4.

So there you have it a condensed and concise overview of the leap year! Well I hope you enjoyed


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About the Author:Brittany Britanniae

Hello There! Please feel free to ask me anything about Latin Grammar, Syntax, or the Ancient World.