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Why the Northern Hemisphere is about to have its earliest spring in 124 years

Denise Chow - NBC News

Justice for Punxsutawney Phil.

The famed groundhog predicted an early spring back in early February, and for much of the world, the changing season is indeed coming sooner than normal.

This year's vernal equinox, which marks the official start of spring in the Northern Hemisphere, will occur Thursday — the earliest spring equinox those north of the equator have seen in 124 years.

"In most years, the Spring Equinox is on the 20th or 21st. For the first time since 1896, it will occur this year on the 19th in all of the U.S. time zones," tweeted Tony Pann, a meteorologist for WBAL-TV in Baltimore.

The earlier-than-usual spring is the result of leap years and the subtle impacts that the extra days have on the timing of Earth's orbits.

Every year, as the Earth makes one full orbit around the sun, there are two equinoxes — one in March and the other in September — to mark the changing astronomical seasons.

It takes around 365.25 days for Earth to complete one rotation around the sun, according to NASA, but this figure can vary from year to year. As such, adjustments to the calendar — in the form of leap years — were built in to compensate for these variations.

Leap years are usually observed every four years, but there are additional conditions to keep the calendar synced with the solar year. Leap years must be evenly divisible by four, but if the year is also evenly divisible by 100, it is not a leap year — the years 1800, 1900 and 2100, for instance.

But, if the year is also evenly divisible by 400, it is then designated a leap year. In the year 2000, this third special exception was invoked, and that year had 366 days.

The addition of that extra day in 2000 meant that astronomical spring was very slowly occurring earlier and earlier. And this year, the equinox falls a day earlier in some time zones.

This year's vernal equinox occurs Thursday at 11:49 p.m. ET, signifying the start of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.

The difference in seasons owes to the way Earth rotates on its axis as it circles the sun. The planet's 23.5-degree tilt on its axis means Earth's two hemispheres are exposed to the sun's rays for different lengths of time throughout the year.

At the precise moment of the equinox, as the sun's rays sweep over the equator moving north, daytime and nighttime across the entire planet is roughly equal. From that point on, the Northern Hemisphere will experience earlier sunrises and later sunsets, while sunrises will occur later and daytimes will be shortened in the Southern Hemisphere.

(And here I thought climate change was the reason.)

Story copied from HERE.