Why 2016 will have the earliest Spring Equinox since 1896

If you're ready to see blooming flowers and sunny skies, it may help to know that this year's spring equinox will be the earliest to arrive in 120 years, largely because of an old rule governing leap years, experts said.

There are two equinoxes (taken from the Latin words aequus for "equal" and nox for "night") each year, marking the start of spring and fall. On these days, there are nearly equal amounts of daylight and darkness.

Usually, the spring equinox happens on March 20 or 21. But the 2016 spring equinox happened during the very early hours of Sunday (March 20) at 12:30 a.m. EDT, making it the earliest spring equinox since 1896.

The reason why goes back to Pope Gregory XIII, who created the Gregorian calendar in 1582, according to the website EarthSky. Each year on Earth lasts 365.242 days, and the existing calendar in Gregory's time accounted for this fraction of a day by having most years be 365 days long, with leap years every four years, where were 366 days long.

But under this system, with one extra day every four years, the average length of a year was 365.25 days -- still a hair longer than the actual length of a year.

And so Pope Gregory XIII declared that years ending in "00" should not be leap years unless they're also divisible by 400, EarthSky reported. That means that the year 2000 was a leap year, but the years 1700, 1800 and 1900 were not, and 2100 won't be either.

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