What Is Leap Year? And Why Was 1900 a Non-leap Year?

A calendar with the date 'February 29th' circled in red. — The concept of a leap year is both fascinating and confusing. mbbirdy / Getty Images

We all know that February is a funny month — every four years it has one extra day (February 29) instead of the normal 28 days. When February has 29 days, we call it a leap year.

The year 2000 was a leap year. But 1900 was not. And neither 1800 nor 1700 were leap years. But 1700, 1800, 1900 and 2000 are all divisible by 4, so why aren't they all leap years? What is leap year, exactly, and why do we have leap years in the first place?

Contents

What Is a Calendar Year?
Why We Have Leap Years
Leap Year Rules
When Is Leap Day?

What Is a Calendar Year?

Let's start with the concept of a year: We define a solar year, or tropical year, to be the amount of time it takes for the Earth to make one complete orbit around the sun. The reason we care about our orbital position around the sun is because of the seasons.

In the Northern Hemisphere, we expect summer weather to occur around June, July and August, and winter weather to occur in December, January and February.

Why We Have Leap Years

We have leap years to account for the difference between a calendar year and a solar year.

Today, most of the world uses the the Gregorian calendar, created by Pope Gregory XIII in 1582 as an update to Julius Caesar's Julian calendar. In the Georgian calendar, a typical calendar year has 365 days.

However, Earth's orbit around the sun actually takes 365.242199 days [source: Encyclopedia Britannica]. By adding one extra day to every fourth year, we get an average of 365.25 days per year, which is fairly close to the actual number.

Without leap days, the Gregorian calendar would slowly drift out of step with Earth's revolutions, and calendar dates would no longer reflect the changes in the seasons.

Leap Year Rules

To get even closer to the actual number, every 100 years is not a leap year, but every 400 years is a leap year. That brings the average length of the year to 365.2425 days, which is very close to the actual number.

Putting all of these rules together, you can see that a year is a leap year not only if it is divisible by 4 — it also has to be divisible by 400 if it is a centurial year. So 1700, 1800 and 1900 were not leap years, but 2000 was.

This is related to the Y2K or Year 2000 problem, because many computer programs would have calculated the leap year incorrectly in the year 2000.

When Is Leap Day?

Leap day is February 29. It occurs approximately once every four years. A person born on leap day will usually celebrate their birthday on February 28 or March 1.

Even though leap day only comes around once every four(ish) years, it's not the least common birth date. On a list of the most popular birthdays, it ranks No. 347 out of 366.

Leap Year FAQ

How often is a leap year?

A leap year occurs almost every four years.

Is a leap year every four years?

In order for a year to qualify as a leap year, it must be divisible by 4. If it is a centurial year, it must also be divisible by 400.

What exactly does leap year mean?

It takes the Earth about 365.25 days to orbit the Sun. This number is rounded off to 365. Making up for the missing day by adding an extra one to the calendar nearly every four years, we have what is called a leap year.