MEMPHIS, Tenn. (WMC) - If you haven’t checked your calendar, you will notice there is an extra day this month. 2020 is a leap year, meaning there is one extra day in February.
Why exactly do we add this leap day? In this episode of the breakdown, we will explain why the 29th is added to the calendar every four years.
A leap day keeps the modern-day Gregorian calendar in alignment with Earth’s revolutions around the sun.
While most people think it only takes 365 days to revolve around the sun, that exact number is closer to 365.242189 days, or 365 days, five hours, 48 minutes and 45 seconds to make one full circle. This is considered a tropical year, and it starts on the March equinox.
While our modern-day calendar only has 365 days in a typical year, so if we didn’t add the leap day on February 29 almost every four years, each calendar year would start nearly six hours before the earth finishes its revolution around the sun.
This means leap days fix the error by giving our Earth a little additional time it needs to complete that full loop around the sun.
While it’s ingrained in our head that ever four years we add that day, this statement isn’t entirely true. There are instances that we don’t add the day.
According to timeanddate.com, the deviation between the common year and the tropical year is a bit less than six hours. The Gregorian calendar addresses this by employing a bit more complicated set of rules to keep us align and in sync.
While it’s a bit complicated set of rules, and its not perfect, but the rules help keep the deviation very small. These three rules are considered to identify leap years.
1. The leap year must be evenly divisible by four.
2. If the year can also be evenly divided by 100, it is not a leap year.
3. This is unless the year is also evenly divisible by 400, then it is a leap year.
So that means when we start a century, leap years will include 2000 and 2400 but will not include 1800, 1900, 2100, 2200, 2300 and 2500.
This whole idea of leap years was invented by Julius Caesar. His Julian calendar stated that any year evenly divisible by four would be a leap year.
This created too many leap years, causing the calendar to drift away from the tropical year. Which was not corrected until the Gregorian calendar was created some 1500 years later.
Another fun fact, the Roman Calendar added a whole extra month every few years to keep the seasons in sync, like a Chinese leap month.