Why Do We Have A Leap Year?

Sammy Rich on Feb 24, 2020 9:52:00 AM

Why Do We Have A Leap Year_

When it comes to the 12-month calendar, February is the odd one out. It is the shortest, for starters, but it is also the only month that can have an extra day tacked onto it every four years. February 29th, the leap day-- arguably the worst day to have a birthday on-- has an interesting backstory behind it, which all revolves around trying to find order with the imperfect aspects of the natural world.

A full calendar year is one cycle around the sun, which is known to be 365-ish days long. The ‘-ish’ part is where things become complicated. Because a full rotation around the sun isn’t 365 days exactly, there is approximately an extra quarter of a day leftover. To compensate for this extra amount of time, a ‘leap day’ is added to the calendar to re-sync it with the solar calendar. If we didn’t have a leap year, over time the calendar would not align with the rotation of the sun.

This usually happens every four years-- unless the year is divisible by 100, then there is no extra day. But if the year is divisible by 400, then there is an extra day. For example, 2100 will not be a leap year, but 2400 will be. Yeah, it’s a little confusing.

And there are other questions that remain: why have the leap day be in February, of all months? Wouldn’t it make more sense to add the leap day in January or December and have 32 days for one of those months? As with most things that don’t make much sense, we can say thanks to our ancestors for coming up with the idea. Namely, Julius Caesar.

Egyptians were the first to discover the need for a leap year, but the European year didn’t adopt the leap year until Caesar came into power. Before Caesar, the Roman calendar was based off of the lunar cycles that frequently required an additional month to keep in sync with the moon. There were only 10 months with an unnamed winter period, with the year beginning in March.

With the help of astronomer Sosigenes, Caesar revamped the Roman calendar in 46 BCE to be 365 days long and 12 months long. Known as the “Julian Calendar”, it compensated for the extra .25 day by adding a leap day every four years. While Caesar’s model helped realign the Roman calendar, it had one issue.

Since the solar year is only .242 days longer than the calendar year and not an even .25, adding a leap year every four years leaves an extra 11 minutes yearly. This discrepancy means that the Julian Calendar de-synced with the sun’s cycle by one day every 128 years, and by the 14th century, it had strayed 10 days off the solar year. To fix the error, Pope Gregory XIII created the revised “Gregorian Calendar” in 1582.

Caesar also made all odd numbered months 31 days and all even numbered months 30 days, making the last month February 29 days in non-leap years and of 30 days every leap year. Until 8 BCE, the eighth month Sixtilis used to be only 30 days. When Sixtilis was renamed after Augustus, it is believed that the emperor knocked off one day from February to make Augustus a 31-day month, leaving February with 28 days during non-leap years.

For all these reasons, February 29th is a unique day. Might as well enjoy it, along with the trial and error it took to get it right!


Andrews, E. (2018). Why do we have a leap year? History.com, https://www.history.com/news/why-do-we-have-leap-year.

Sabur, R., Macphail, C., & Eysenck, J. (2016). Leap Year 2016: Why does February have 29 days every four years? The Telegraph, https://www.telegraph.co.uk/news/uknews/12177017/Leap-Year-2016-Why-does-February-have-29-days-every-four-years.html.




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