Every four years, during a leap year, the imperfect match between the length of a calendar year and the Earth's orbit results in an interesting calendar adjustment known as a leap day.

The length of a year depends on how long it takes a planet to orbit the Sun. It takes the Earth about 365.2422 days to make one revolution around the Sun. This is about six hours longer than the 365 days we usually include in a calendar year. As a result, we have about 24 extra hours that we add to the calendar as leap days at the end of February every four years.

Without the leap day, the dates of annual events such as the equinoxes and solstices would gradually shift to later in the year and the date of each season would change. Without the leap day, summer would not start until mid-July, just a century later!

Image description:

The western hemisphere of the Blue Marble, created in 2002. North America is visible, clouds frame the continent, and they swirl over a part of South America that can be seen in the frame. The Earth is surrounded by space.