February 29, 2016

February 29 is a day that does not exist in most years. The year 2016 is a leap year, a phenomenon that occurs once every four years. Most years have 365 days. Leap years have 366. The extra day is added to the end of the shortest month, February.

February 29 occurs once every four years, on leap years. Credit: © Shutterstock

A year is the amount of time it takes Earth to orbit (circle around) the sun. We use leap years because Earth does not take exactly 365 days to complete one orbit. A real year is a bit longer—about 365 days, 5 hours, 48, minutes, and 46 seconds, to be precise. Every four years, those extra 5 hours and 48 minutes add up to about an extra day. Over many decades and centuries, those extra days can throw off a calendar if the years all have just 365 days. Leap years help reset the clock, ensuring that the months correspond to the same seasons year after year.

However, leap years do not solve all of the calendar’s problems. An extra day every four years would bring the calendar into balance if Earth took exactly 365 days and 6 hours to orbit the sun. But in fact, the planet takes about 12 minutes less than that. To prevent those missing 12 minutes from adding up and throwing off the calendar, leap years are not celebrated in century years—that is, years ending in “00″—with the exception those divisible by 400. The years 2100, 2200, and 2300 will not be leap years. But the year 2000 was a leap year, and the year 2400 will be one as well.

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