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 don’t 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 bit.
Let this go and you run into the problem of your calendar slowly becoming out of synch 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, 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, the accumulated extra time has been used up in the form another day – the leap day – which was added on to the shortest month of the year.
Unfortunately, it’s not quite that simple, though. That plan actually overcompensates by 11 minutes and 15 seconds per year.
So, 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: if a year is evenly divisible by the number 100 but not by 400, we skip that year.
Thus, ever since the Gregorian calendar was introduced in 1582, the years 1600 and 2000 were leap years, since both are evenly divisible by 100 and 400. All other years since then that were evenly divisible by only 100 were not. Going forward, the years 2100, 2200 and 2300 will not be leap years, but the year 2400 will.
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 standard 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.