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Leap Year 2016: The Story Behind the Extra Day Every Four Years


Posted on February 24, 2016 by Delaware Valley University.

This year is a Leap Year, meaning February 29 is on the calendar again after four-year hiatus. But what does that really mean?

In short, we add February 29 to the calendar every four years to ensure that the calendar stays in line with the Earth's movement around the sun. It helps us stay synchronized with the seasons. Unfortunately, the actual "why" behind it is a little more complicated than that.

The Science Behind Leap Year

A year can be counted in two different ways: the length of time it takes the Earth to travel through one orbit around the Sun, and 365 days. And while we consider both of these to mean a year, they actually signify two different types of year. 

The time it takes the Earth to orbit the Sun is called a solar year, which was set by astronomers based on the period from one spring equinox to the next. This equals 365 days, however, there's an extra five hours, 48 minutes and 45 seconds.

These five+ hours account for the time it takes the Earth to revolve on its axis – which we refer to as one day. However, it does not match up to the time it takes the Earth to travel around the sun. This time must be accounted for.

After adding up the entire calendar year's 24-hour days, the five hours of time remains. If it weren’t compensated for, over time, the calendar would not connect to the seasons. To avoid this, the extra time is gathered to create February 29. 

Julian Calendar to Gregorian Calendar

Leap days were first added to the Julian calendar in 46 B.C., and then this was changed when the Gregorian calendar was created. The Gregorian calendar's rule was that leap days shouldn't be added to years ending in "00" unless the year is divisible by 400. This rule was added to ensure that the calendar is synchronized with the Earth's orbit around the sun. On non-leap years, the calendar doesn't account for that extra quarter of a day. 

The last leap year was 2012, and in 2016, it occurs again. So there you have it! You can educate others in class on February 29 why the day exists.

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