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Leap Year 2016 puts Feb 29 back on the calendar. Here's why

Scott Sutherland
Meteorologist/Science Writer

Tuesday, January 26, 2016, 12:37 PM - In 2016, February 29 is making a return after a four year hiatus, marking this as a Leap Year. So, what's the reason for this periodic addition to our calendar?

A "year" is the length of time it takes the Earth to travel through one orbit around the Sun. We typically define this as a period of 365 days, and this is reflected in the modern day Gregorian calendar that's in use around the world.

While both of those definitions for a year are correct, both actually describe a different type of year.

The length of time it takes Earth to orbit the Sun once is called a "solar year," or "tropical year."

Astronomers set the solar year as the period from one Spring Equinox to the next. While that does equal 365 days, there's also (roughly) an extra 5 hours, 48 minutes and 45 seconds tacked on.

Why the discrepancy? Because the length of time it takes for Earth to revolve once on its axis - which we use to define what a "day" is - does not match up perfectly with how long it takes Earth to travel around the Sun.

Once you add up all of the calendar year's 24 hour days (which do not even accurately represent one revolution of the Earth, since that - the "sidereal day" - is only 23 hours, 56 minutes and 4 seconds long), you're left with that extra 5+ hours of time.

Let this go without compensating for it, and you run into the problem of your calendar slowly becoming out of sync with the seasons as we know them, by roughly a month every 125 years.

How do you solve this problem?

It really doesn't matter when you make up this "lost" time, as long as you account for it somehow. Of all the options available for balancing the accounting, the easiest is to just gather all that extra time and use it up all at once.

This is where February 29 comes into play.

Once every four years, ever since the Julian calendar was introduced by the Romans, the accumulated extra time has been used up in the form one extra day - the leap day - which was added on to the shortest month of the year.

Unfortunately, it's not quite that simple, though. 

While the Roman plan worked, it actually overcompensated by 11 minutes and 15 seconds per year. 

To round everything off just right, or at least as close as possible, the every-four-years rule has had one exception, which was factored into the "new" Gregorian calendar: the years that are evenly divisible by 100 are not leap years, unless that year is also divisible by the number 400.

Thus, ever since the Gregorian calendar was introduced in 1582, the year 1600 was a leap year, 1700, 1800 and 1900 were not leap years, and 2000 was a leap year. Going forward, the years 2100, 2200 and 2300 will not be leap years, but the year 2400 will.

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Why is it called a Leap Year?

The name "leap year" comes directly from the addition of the "leap day." The leap day is named due to its effect on how days of the year line up with the days of the week.

Going from one non-leap year (or "common" year) to the next, the same day of the year will advance by one day of the week.

For example, June 10 fell on a Tuesday in common year 2014, and it fell on a Wednesday for common year 2015. In 2016, however, June 10 will be on a Friday, "leaping" over Thursday due to February 29.

Sources: TimeandDate.com | Dictionary.com

Don't Miss: This animation from NASA shows us every phase of the Moon from 2016, February 29 included!

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