What is a leap year, and why wasn't 1900 one?

We all know February behaves unusually—it gets an extra day, February 29th, every four years, instead of the usual 28. When this happens, it's called a leap year.

The year 2000 was a leap year, but 1900 was not. Similarly, 1700 and 1800 weren't leap years either. Despite all these years being divisible by 4, why weren't they all leap years? What is a leap year, and why do we have them in the first place?

What Is a Calendar Year?

Let’s begin with the idea of a year: A solar year (or tropical year) refers to the time it takes for Earth to orbit the sun once. Our interest in this orbit is tied to the changing seasons.

In the Northern Hemisphere, we typically associate summer with the months of June, July, and August, while winter is expected from December through February.

Why Do We Have Leap Years?

Leap years are introduced to adjust for the discrepancy between the length of a calendar year and the actual solar year.

Today, the majority of the world follows the Gregorian calendar, introduced by Pope Gregory XIII in 1582 as a modification of Julius Caesar's Julian calendar. A typical year in this calendar has 365 days.

In reality, Earth’s orbit around the sun takes about 365.242199 days [source: Encyclopedia Britannica]. By adding an extra day every four years, we average 365.25 days per year, which is very close to the actual duration.

Without the addition of leap days, the Gregorian calendar would gradually become misaligned with Earth’s orbit, causing the dates to no longer match the changing seasons.

Leap Year Rules

To refine the calendar even further, every 100 years is not a leap year, but every 400 years is. This adjustment brings the average year length to 365.2425 days, which closely matches the actual solar year.

When you combine all the rules, you can see that a year is considered a leap year if it's divisible by 4, but it must also be divisible by 400 if it’s a century year. Therefore, 1700, 1800, and 1900 weren’t leap years, but 2000 was.

This is connected to the Y2K or Year 2000 problem, as many computer programs would have miscalculated the leap year for the year 2000.

When Is Leap Day?

Leap day is February 29, which occurs roughly once every four years. People born on leap day typically celebrate their birthday on February 28 or March 1.

Although leap day only happens once every four years, it’s not the rarest birthday. On the list of the most popular birthdays, it ranks No. 347 out of 366.

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