# What Is a Leap Year?

We use leap years to keep our calendar in sync with the seasons. How do leap years work, and how often do they occur?

## Leap Years Have an Extra Day

Leap years are years where **an extra day is added** to the end of the shortest month, February. This so-called *intercalary* day, February 29, is commonly referred to as leap day.

Leap years have 366 days instead of the usual 365 days and occur **almost every four years**.

## Is 2024 a Leap Year?

Yes, **2024 is a leap year**. The 2024 leap day will fall on February 29, 2024. The next one is February 29, 2028.

## Leap Year Rules: How to Calculate Leap Years

In our modern-day Gregorian calendar, three criteria must be taken into account to identify leap years:

The year must be evenly divisible by 4; | |

If the year can also be evenly divided by 100, it is a leap year;not | |

unless... | |

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

According to these rules, the years 2000 and 2400 are leap years,

while 1800, 1900, 2100, 2200, 2300, and 2500 are *not* leap years.

## Why Do We Have Leap Years?

Leap days keep our calendar **in alignment with Earth's revolutions around the Sun**. It takes Earth approximately 365.242189 days, or 365 days, 5 hours, 48 minutes, and 45 seconds, to circle once around the Sun. This is called a tropical year, and it starts on the March equinox.

However, the Gregorian **calendar has only 365 days in a year**. If we didn't add a leap day on February 29 almost every four years, each calendar year would begin about 6 hours earlier in relation to Earth's revolution around the Sun (see illustration).

As a consequence, our time reckoning **would slowly drift apart** from the tropical year and get increasingly out of sync with the seasons. With a deviation of approximately 6 hours per year, the seasons would shift by about 24 calendar days within 100 years. Allow this to happen for a while, and Northern Hemisphere dwellers will be celebrating Christmas in the middle of summer in a matter of a few centuries.

**Leap days fix that error** by giving Earth the additional time it needs to complete a full circle around the Sun.

Exact timings for spring, summer, fall, and winter

## Why Don't We Add a Leap Day Every 4 Years?

If the tropical year was *precisely* 6 hours longer than a calendar year with 365 days, we could use the Julian calendar, which adds a **leap day every 4 years without exception**. The deviation would grow to exactly 24 hours over 4 years, and Earth would need exactly one day to catch up to the position in its orbit where it was 4 years prior.

However, the deviation between the common year and the tropical year is a little *less* than 6 hours. The Gregorian calendar addresses this by employing a **slightly more complicated set of rules** to determine which years are leap years. It's still not perfect, but the resulting deviation is very small.

## Special Leap Year 2000

The year 2000 was the **first time the third criterion was used** in most parts of the world since the transition from the Julian calendar to the Gregorian calendar, which began in 1582.

The number 2000 is evenly divisible by 400, so **it was a leap year** even though it can also be evenly divided by 100.

Of course, the **same can be said about the year 1600**. However, only a handful of countries had adopted the Gregorian calendar then, and the rest of the world was **still using the Julian calendar**.

The **next time** the third criterion takes effect will be the **year 2400**.

## Who Invented Leap Years?

Leap years in the western calendar were first introduced over 2000 years ago by Roman general **Julius Caesar**. The Julian calendar, which was named after him, had only one rule: any year evenly divisible by four would be a leap year.

This formula produced too many leap years, causing the Julian calendar to drift apart from the tropical year at a rate of 1 day per 128 years. This was not corrected until the introduction of the Gregorian calendar more than 1500 years later, when a number of days were skipped to realign our calendar with the seasons.

## Leap Months

The ancient Roman Calendar added an extra month every few years to stay in sync with the seasons, similar to the Chinese leap month.