February 29 (Leap Day)

calendar · astronomy · time

What is leap day?. February 29 is an extra day added to the calendar every four years. It exists because Earth takes about 365.25 days to orbit the sun, not exactly 365 days.

Why we need it. Without leap day, our calendar would slowly drift out of alignment with the seasons. After about 100 years, summer could start in the wrong month if we didn't add this extra day.

Leap year babies. People born on February 29 are called leap year babies or leaplings. They only get a real birthday every four years, though most celebrate on February 28 or March 1 in other years.

The quarter day problem. The reason leap day exists is that Earth's year is 365 days and 6 hours long. Every four years, those extra 6-hour chunks add up to one whole day that needs to go somewhere.

Two calendar systems. Leap day appears in both the Julian calendar (an older system) and the Gregorian calendar (the one most of the world uses today).