February 29: What is a leap year?

Every four years, we include an extra day in our calendars in February. This is called a leap year , an adjustment that takes place so that the time it takes the Earth to orbit the Sun coincides with the way we have decided to measure time.

A solar year takes approximately 365.25 days . Therefore, we round the days in a calendar year to 365 . However, the remaining 6 hours (5 hours, 46 minutes and 48 seconds, to be precise) do not just disappear, so every four years we add one day to our calendar to make up for the missing partial day.

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Since every year we subtract one-quarter of a day, after four years, on leap year, we must add the missing time that together makes up a day.

Leap years

are important so that our calendar matches the solar year, which is the time it takes the Earth to orbit the Sun.

If we never recovered the missing time, then the hours would add up into days , weeks , and even months and eventually our whole calendar would be messed up with the seasons and nothing would make sense.

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Leap years can happen in other planets since they take place because a planet’s orbit around the Sun and rotation on its axis are not perfectly in line .

According to NASA , Mars has actually more leap years than regular years. A year on Mars is 668 sols , or Martian days. However, it takes 668.6 sols for Mars to go around the Sun. So, you would sometimes have to add a sol to help the calendar catch up. In a 10 year period , four of the years would have 668 sols and six of the years would be leap years with 669 sols .

Without leap day, the dates of annual events, such as equinoxes and solstices , would slowly shift to later in the year, changing the dates of each season . After only a century without leap day, summer wouldn’t start until mid-July.

But there are even more details to leap year! Since Earth takes a little less time than 6 hours to orbit the Sun, rounding up and inserting a 24-hour leap day every four years adds about 45 extra minutes to every four-year leap cycle. That adds up to about three days every 400 years . To correct for that, years that are divisible by 100 don't have leap days unless they are also divisible by 400 . If you do the math, you will see that the year 2000 was a leap year, but 2100 , 2200 and 2300 will not be.

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