How often is leap year?

What is a leap year?

By definition, a leap year is a year that has 366 days in the calendar.

It has an extra day in February. February 29th.

On the other hand, years that are not leap years (common years) have 365 days.

As an example, 2020 is a leap year. So it had an extra day in February.

Conversely, 2019 or 2021, are common years and hence not leap years.

Having leap years helps us to keep time effectively.

Let me get into that a bit more.

Why do we have leap years?

Do we need them at all?

We use calendars to track and organise time and days.

And much like calendars, leap years are a human construct to assist us in tracking time.

In fact, they have been around for over 2000 years.

First added by Julius Caesar in 46BC at the advice of Sosigenes, an astronomer, leap years have proven to be quite useful.

Here’s the run down of why we have them.

A year means the period of time it takes for the Earth to rotate around the sun.

A day means the period of time it takes Earth to rotate around its axis once.

And now comes the important detail. The Earth rotates just a little over 365 times per year!

365.242375 times to be more exact. In other words, about 365 and a quarter days. It is this quarter of a day extra which means we occasionally need to add in an extra day to the year.

So, 365 days is a bit short and 366 days too long to account for a year. 

We could say the difference is minimal and for a few years, you could get away without leap years. That’s true.

However, as time passed, our calendar would lose touch with the Earth’s position in the Solar system. This would throw us out of whack, for example affecting timing of seasons we are all used to. Summer in January in the Northern Hemisphere or snowing during Christmas in Australia?

And this is where the leap years come in handy – to keep our calendar in sync long term.

For the purposes of this post, note that we are using and describing their use in the Gregorian calendar, which is the usual calendar used worldwide.

How often does a leap year occur?

As we said above, in basic terms, a leap year occurs every 4 years.

And this is pretty much going to work well in general. If you remember one thing from the article, that’s it.

But let’s take a look at it closely as there are some exceptions.

The exceptions are there only in order to get the calendar to be just right when looking at the Earth and the solar system.

This is the “formula” to figure it out fully:

Here are some example questions with answers to work through.

Use the formula above to help you answer the questions if you are confused.

Is 2020 a leap year?

2020 is a leap year. 2020 is divisible by 4 and cannot be exactly divided by 100. So it is.

Is 2021 a leap year?

2021 is not a leap year (and so it is called a common year). 2021 is not divisible by 4.

Is 2024 a leap year?

2024 is a leap year. It is divisible by 4 and not divisible by 100.

Is 2100 a leap year?

2100 is not one. 2100 is divisible by 4 and 100 but not 400.

What about year 2400?

Yes, 2400 will be a leap year. 2400 can be divided exactly by 4 and 100. But also with 400, meaning the exception does not apply as per the formula above.

How can I know which years are leap years?

In addition to the rules above, some easy tips I have found to remember leap years are:

The Olympics – the Olympics are on every 4 years and tend to occur on leap years. Well, except in 2020 (due to the Coronavirus pandemic). And with a few exceptions as above (year 2100 will be one for example).

USA elections – another event that occurs every 4 years and on leap years generally are the USA presidential elections.

Trivia and interesting facts

Born on February 29 – a person has a 1 in 1461 chance of being born on February 29. Pretty rare and unique. These individuals are often called leapers.

If you are a leaper, you will have to figure out how to celebrate your birthdays in the common years. Or just ignore them and stay younger :D.

An extra day in the year – if you get paid on a strictly annual wage, you may end up working one day per leap year for free. It also affects prisoners on year or multiple year sentences, needing to serve an extra day.

As long as the Earth keeps spinning on itself and around the sun, we will have to occasionally welcome February 29 to our calendars.

Do you have any tips to remember how often the leap years are?

Or do you disagree with anything we have written here?

Let me know in the comments below.