February 29 is Leap Day.

What is a Leap Day?

It is an intercalary day, which means it is added to the common year. Humans first added this extra year in about 46 B.C. This extra day is meant to synchronize our calendar with the solar year. While our calendars measure a year at 365 days, Earth’s rotation around the sun actually takes 365.2422 days.

Julian Calendar

Ancient Egyptians were the first to notice the disparity between the calendars and the seasons. Julius Caesar created the Julian calendar, a solar calendar, which replaced the lunar calendar used in the Roman Empire. With the help and advice of Alexandrian astronomer Sosigenes, the Julian calendar reflected the seasonal year with more precision than the lunar calendar. It included a Leap Day to keep the calendar as accurate as possible.

Originally the Leap Day fell between February 23 and 24 and did not have its own date. Later it was moved to the end of the month and given its own number.

Gregorian Calendar

In 1582, the Gregorian calendar, which we still use today, was created by Pope Gregory XIII with the assistance of a German mathematician and astronomer, Christopher Clavius. Clavius was tasked with reforming the Julian calendar to halt the drift of Catholic Church feast days and holidays, which were moving in relation to the seasons. Within three years he proposed the Gregorian calendar which was adopted by Catholic countries in 1582; this is also the internationally accepted civil calendar used throughout the world today.

Doing the Math

The Gregorian calendar also includes the Leap Day every four years, but adds a little extra confusion to account for the fact that the solar system is messy and the solar year is actually 365 days 5 hours 49 minutes 12 seconds long—10 minutes and 18 seconds shy of 6 hours, which would be a quarter of a day. Thereby, while every four years a Leap Day is added to the calendar, there are 3 leap days dropped every 400 years. The extra rule states:

“Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.”1

Extra Day

Despite the rarity of it, Leap Day isn’t a holiday in the traditional sense, though it can be a day to celebrate.

For people born on February 29, called leaplings, the Leap Year is the only time they get to celebrate their actual birthday. The rest of the time they either celebrate the day before or after, or sometimes both.

Check out the resources below for more information on the day as well as some fun ideas for making this Leap Day special, even if it’s not your birthday.

Resources:

What is a Leap Year?: quick 4-minute video that discusses this phenomenon in greater detail

The Year of Confusion: Highlights article on the creation of Leap Day and the creation of the Julian calendar, a solar calendar

29 Things to Do With Your Kids on Leap Day: from math pages or a reading day to a game of leap frog or a backyard obstacle course, this list has some great ideas for making Leap Day a celebration

29 Fun Things to Do on Leap Day: from Mother Nature Network

Honor Society of Leap Day Babies: group dedicated to accumulating knowledge about Leap Day and the way people view/celebrate it, while also advocating for leaplings in various arenas

Kids’ Page: collection of interesting Leap Year Day information, poems, and games for children

Free Leap Year Activities for Kids: from Examiner.com

21 Fun Leap Day Activities: reading and cute, frog-inspired snack ideas for Leap Year Day from How Does She?

1 “Introduction to Calendars,” United States Naval Observatory, accessed February 17, 2016.

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