• Post published:February 29, 2020

No this is not a day to dress as a frog and go leaping around your town. This is a day of science!

What is a Leap Year?

In short it is how we keep our calendars in sync with the seasons.

Leap years are years where an extra day is added to the end of the shortest month, February. February 29th is generally referred to as Leap Day. Today!

Leap years have 366 days as apposed to the usual 365 days.

Leap years occur almost every four years.

Now let’s get technical….

Why do we have Leap Years?

The extra day keeps our modern-day Gregorian calendar in correct alignment with Earth’s orbit around the Sun.

In actuality, it takes the Earth about 365.242189 days, or in words that we can understand, 365 days, 5 hours, 48 minutes and 45 seconds, to complete a full orbit around the Sun. This is what we call a tropical year.

As the Gregorian calendar only has 365 days a year in it each calendar year would begin about 6 hours before the Earth completes its revolutions around the sun if we didn’t have a leap day!

The different types of year according to calendars and astronomy.
The different types of year according to calendars and astronomy.


Why only Almost every Four Years?

If a tropical year was precisely 6 hours longer than a 365-day year then the old Julian calendar could be used. This calendar added a leap day every 4 years without exceptions.

However, due to the small deviation of 11 minutes and 55 seconds… the Gregorian calendar was put in place which has three rules for defining which years are leap years:

  1. The year must be evenly divisible by 4
  2. If the year can also be evenly divided by 100 it is not a leap year


  1. The year is also evenly divisible by 400. Then it is a leap year.

 So according to those rules the years 2000 and 2400 are leap years BUT 1800,1900,2100,2200,2300 are not leap years.

Fact: Julius Caesar was the first to introduce leap years.

Roman General Julius Caesar. ©bigstockphoto.com
Roman General Julius Caesar. ©bigstockphoto.com


What would happen if we didn’t have a Leap Year?

Within 100 years of not adding a leap year to our calendars the seasons would shift by about 24 calendar days. Allowing this to happen for a few centuries and soon the Northern Hemisphere would be celebrating Christmas in the middle of the summer!

Have a good Leap day!

Enjoy this wonderful day that we have because of science! Brag about how you know all about it to your friends.  Finally, to those of you with a birthday today, enjoy it while you can you wont have one for another few years!

Izzy David

On board Naturalist/Biologist for SpringTide Whale Watching & Eco Tours


Tropical year- Aka: a solar year or astronomical year is 365 days, 5 hours, 48 minutes and 45 seconds long (365.24219 days).

Common year- 365 days a year. A common year has 52 weeks and one day.

Leap year- Leap years are years where an extra, or intercalary, day is added to the end of the shortest month, February. They happen almost every 4 years.

Gregorian calendar- 12 months of irregular length with a leap year nearly every 4 years.

Julian calendar- 12 months of irregular length with EVERY 4th year being a leap year with 366 days in it.

Close Menu