Spring equinox 2017: What is an equinox, anyway?

For those in the Northern Hemisphere, March 20 marks the spring equinox, one of two points of the year when both hemispheres of our planet get the same amount of daylight.

The Spinning Enhanced Visible and Infrared Imager (SEVIRI) on EUMETSAT's Meteosat-9 captured this view of Earth from geosynchronous orbit on March 20, 2011. During the spring and fall equinoxes, the terminator is a straight north-south line, and the sun is said to sit directly above the equator.

Spring is here in the Northern Hemisphere.

The astronomical start of spring began on Monday morning at 6:28 a.m. Eastern time with the March equinox, the annual celestial alignment between the Earth and the sun. Like the year's second equinox, in September, the shift in sunlight accompanies a shift in seasons.

It’s unclear precisely when ancient civilizations became aware of this phenomenon. Humans have been creating structures for thousands of years to observe the changing positions of the sun and the sky in relationship to the seasons. But only when humankind gained the ability to look down on Earth from space could it see this phenomenon more clearly.

"Once we were able to put satellites into space we were able to see the extent of cloud cover, landmasses, bodies of water,” C. Alex Young, associate director for science in the Heliophysics Science Division at NASA's Goddard Space Flight Center, told Live Science. “It's given us a new perspective on this very complicated and very vital system.”

Both the spring and fall equinoxes occur when the Earth is in just the right place with respect to the sun, according to NASA. At these points in the Earth’s roughly 365-day journey, both the Northern and Southern Hemispheres receive the same amount of daylight, meaning there are almost equal amounts of daytime and nighttime in both halves of the Earth. Hence, the word, equinox, which means “equal night” in Latin.

On this Monday, as in every March 20, the spring equinox marks the continuation of longer days for the Northern Hemisphere and shorter days for the Southern Hemisphere. This trend then reverses during the solstice in June, slowly ushering in the dark days of winter for the northern half of the world, and the dog days of summer for the south. [Editor's note: An earlier version mischaracterized solstices and equinoxes.]

To observe how this phenomenon works, as well as learn more about it, NASA has 20 missions and 23 spacecraft – known collectively as the Heliophysics System Observatory (HSO) – studying the relationship between the Earth and the sun, as well as releasing images and videos of the Earth’s terminator – the dividing line between day and night – becoming vertical, according to Live Science.

So now that it’s here, what will spring bring to parts of the Northern Hemisphere?

In the United States, forecasters predict balmy weather for much of spring, but not before more winter weirdness hits parts of the Northeast and the Midwest. The Northeast will experience a cold snap starting Wednesday, with the mercury dropping into the 20s and teens, and a snowstorm possible in the Midwest, according to Accuweather.com.

Spring also came early in Washington D.C., tempting into bloom half of the pink-and-white cherry blossoms that typically draw 1.5 million tourists to the nation’s capital in early April. Some of the flowers bloomed early during a warm spell, but will likley die off as temperatures plunge again. 

This report contains material from Reuters. 

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