Leap Year has 366 days instead of 365.
Lisa Gardiner/Windows to the Universe

# Leap Year: What is it and when does it happen?

The calendar used in most parts of the world today, known as the Gregorian calendar, has 365 days in most years.  However, the length of an actual year is 365.2425 days.  That's the time it takes for the Earth to make one trip around the Sun. Through a little simple math, you can figure out that there is approximately an extra quarter of a day.  If the calendar didn’t have a way to take this into account, then each year the calendar would get a little more out of rhythm from the cycle of the seasons.

But we do have a way to take that portion of a day into account. Approximately every four years in the Gregorian calendar, February has 29 days instead of the usual 28 days. This type of year is called a leap year

It is actually a bit more complex than every four years. In fact, the extra day is added to every year that can be evenly divided by the number four unless that year can also be evenly divided by 100, in which case it is only a leap year if it can also be evenly divided by 400.

Let’s review what that means for a few recent years:

The year 2008 is evenly divisible by 4
2008/4=502
So it is a leap year.

The year 2006 is not evenly divisible by 4.
2006/4=501.5
So it was not a leap year.

The year 2000 is evenly divisible by 4.
2000/4 = 500
But it is also divisible by 100.
2000/100=200
Is it also divisible by 400? Yes it is!
2000/400=5
Thus the year 2000 was a leap year.

Other calendars also have ways to account for this quarter day of difference.  The Revised Julian Calendar adds an extra day to February in certain years. It has a slightly different formula than the Gregorian calendar for calculating which years get this extra day. The Ethiopian calendar adds an extra day at the end of every forth year. The Chinese calendar adds an extra month for leap year. The Hebrew calendar adds an extra month in seven out of every 19 years.

## Cool It! Game

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

## Albedo

This picture of the Earth surface was taken from high above the planet in the International Space Station. In this view from above, we can see that there are lots of different things that cover the Earth....more

## Desert Birds

Like the other creatures of the desert, birds come up with interesting ways to survive in the harsh climate. The sandgrouse has special feathers that soak up water. It can then carry the water to its...more

## The Desert Biome

Deserts are full of interesting questions. How can anything survive in a place with hardly any water? Why is it so dry to begin with? You can find at least one desert on every continent except Europe....more

## Desert Insects and Arachnids

You can find insects almost anywhere in the world. So it should be of no surprise that there are plenty of insects in the desert. One of the most common and destructive pests is the locust. A locust is...more

## Desert Mammals

There are several species of mammals in the desert. They range in size from a few inches to several feet in length. Like other desert wildlife, mammals have to find ways to stay cool and drink plenty...more

## Biomes and Ecosystems

Biomes are large regions of the world with similar plants, animals, and other living things that are adapted to the climate and other conditions. Explore the links below to learn more about different biomes....more

## Temperate Forests

The temperate forest biome is found in regions where winters are cold and summers are warm. Regions with this climate are common in the mid-latitudes, far from both the equator and the poles. Tropical...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information.