What Is a Leap Year?

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

Illustration image — A common year (non-leap year) is shorter than a tropical year, which is the time it takes the Earth to orbit the Sun. A leap year is a little longer.

Leap Years Have an Extra Day

Leap years are years wherean extra day is added to the end of the shortest month,February. This so-calledintercalary day, February 29, is commonly referred to asleap day.

Leap years have 366 days instead of theusual 365 days and occuralmost every four years.

What is a leap second?

Is 2026 a Leap Year?

No,2026 is not a leap year. The lastleap day fell onFebruary 29, 2024. The next one isFebruary 29, 2028.

Leap Year Rules: How to Calculate Leap Years

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

	The yearmust be evenly divisible by 4;
	If the year can also beevenly divided by 100, it isnot a leap year;
	unless...
	The year is alsoevenly divisible by 400. Then itis a leap year.

According to these rules, the years2000 and2400 are leap years,
while1800,1900,2100,2200,2300, and2500 arenot leap years.

Why Mars has more leap years than Earth

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Why Do We Have Leap Years?

Leap days keep our calendarin 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 atropical year, and it starts on theMarch equinox.

However, the Gregoriancalendar 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 reckoningwould slowly drift apart from the tropical year and get increasingly out of sync with theseasons. 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, andNorthern Hemisphere dwellers will be celebratingChristmas in the middle ofsummer 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 wasprecisely 6 hours longer than a calendar year with 365 days, we could use theJulian calendar, which adds aleap 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 littleless than 6 hours. The Gregorian calendar addresses this by employing aslightly more complicated set of rules to determine which years are leap years. It's still not perfect, but the resulting deviation isvery small.

Special Leap Year 2000

The year2000 was thefirst time the third criterion was used in most parts of the world since thetransition from theJulian calendar to the Gregorian calendar, which began in 1582.

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

Of course, thesame 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 wasstill using the Julian calendar.

Thenext time the third criterion takes effect will be theyear 2400.

Who Invented Leap Years?

Leap years in the western calendar were first introduced over 2000 years ago by Roman generalJulius Caesar. TheJulian 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 theGregorian calendar more than 1500 years later, when a number of dayswere skipped to realign our calendar with the seasons.