Leap Day, Explained

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To mark our extra day this Leap Year, we asked Jackie Faherty, a research scientist at the Museum, to explain how Leap Days came to be and what purpose they serve. 

February is the dead of winter for the Northern Hemisphere, meaning most people are thrilled that it's also the shortest month of the year. But once every four years or so, February gets a one-day extension: Leap Day. This extra day serves to sync the  world’s most widely used civil calendar with the time it actually takes for the the Earth to complete a full orbit around the Sun.

Leap day

Leap Days make February longer and help keep the civil calendar on time.

Image courtesy of 123freevectors.com


If you ask anyone on the street how long a year is, you would probably get an answer of 365 days. In reality, though, it takes the Earth 365 days plus 5 hours, 48 minutes, and 45 seconds to make one trip around the Sun. This measurement is known as the “tropical year.”

While a difference of 5 hours between the civil and tropical calendars might not seem like no big deal in the course of a year, those differences add up. Without making an adjustment to bring the two in line, you would have Santa coming down chimneys in the middle of summer after a few centuries.

Timekeepers have long known about this discrepancy, and different calendar systems have used different approaches to deal with this extra time. Until the 16th century, the 365-day Julian calendar was commonly used, which simplistically added 1 day every four years to account for the discrepancy in timing.  But 5 hours 48 minutes and 45 seconds surplus each year does not add up to exactly a quarter day; it is just shy at 0.2421875 days.

Earth and Sun

The Earth circles the Sun once every 365 days...plus enough hours to cause a problem for calendar makers.

© NASA


Hence, every year under the Julian calendar had a surplus of 11.25 minutes. Because the Julian calendar was widely used for almost a century, by the 1400's the calendar was misaligned from the tropical year by 10 days—and getting worse!

In 1582, Pope Gregory XIII introduced a modification that would eventually be adopted globally. Dubbed the "Gregorian calendar" after its namesake, the approach to leap years was a considerable advancement. 

For the modern day Gregorian calendar, here are the three simple rules that determine if a year is a Leap Year:

  • YES—If that year can be evenly divided by 4
  • NO—If that year can be evenly divided by 100
  • UNLESS—the year is also evenly divisible for 400.  

So while the year 2000 was a Leap Year like 2016, the years 1800, and 1900 were not.  If you are looking ahead, the year 2100, 2200, 2300, and 2500 are NOT leap years. Something to keep in mind if you are planning for something special on the 29th of February in, say, 84 years.   

Jackie Faherty is a Research Associate in the Department of Astrophysics at the American Museum of Natural History. Her research is at the forefront of understanding the atmospheres of extrasolar worlds.