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Night and Day: The Vernal Equinox

Night and Day: The Vernal Equinox

On March 20, the vernal equinox signals the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. This spring equinox marks the moment when the sun crosses the equator going from south to north.

What does that mean? For those of us in the Northern Hemisphere, March heralds a beautiful change. The days are getting longer and the nights are getting shorter. Trees are budding, birds are singing, bees are buzzing, and gentle breezes are blowing. These are some of the delightful daily proclamations of spring’s arrival. Some changes are subtler. If you stop to look at the arc of the sun across the sky, you’ll notice that it’s shifting toward the north. And butterflies and birds are making their migrations toward the north as well, along with the path of the sun.

So, what brings about this glorious transformation of the earth?

Essentially, the Northern Hemisphere is getting more light and warmth from the sun. At the vernal equinox, the sun is directly above the equator. The equinox is an astronomical event, caused by the tilt of the Earth on its axis and its ceaseless orbital motion. Because the Earth is tilted 23.5 degrees on its axis, Earth’s Northern and Southern hemispheres trade places in receiving the sun’s light and warmth most directly.

“Twice a year, in the spring and fall, we have an equinox. This is when the tilt of the Earth’s axis and Earth’s orbit around the sun combine in such a way that the axis is inclined neither away from nor toward the sun. At the equinox, Earth’s two hemispheres are receiving the sun’s rays equally. The word equinox comes from the Latin aequus (equal) and nox (night).” [Earth Sky: Everything you need to know about vernal equinox 2016: http://earthsky.org/?p=68679] This “equal night” refers to the idea that, on the equinox, day and night are of (approximately) equal lengths everywhere on the planet.

The change in orientation and angles between the Earth and the Sun causes the changing seasons. The Earth’s axis is tilted AWAY from the Sun at the December solstice and TOWARD the Sun at the June solstice. This tilt spreads more or less light on each hemisphere depending on how far the Earth is tilted toward or away from the Sun. At the equinoxes, the Earth’s tilt is at a right angle to the Sun and the light (and consequently, warmth) is spread evenly. The Earth’s terminator (the dividing line between night and day) becomes vertical.

The video above was NASA's Astronomy Picture of the Day for March 19, 2014. They wrote:

When does the line between day and night become vertical? On the equinox, which happens on March 19 or 20, 2016, depending on your time zone. It’s the time of year when day and night are nearly equal. At an equinox, the Earth’s terminator — the dividing line between day and night — becomes vertical and connects the north and south poles. The above time-lapse video demonstrates this by displaying an entire year on planet Earth in twelve seconds. From geosynchronous orbit, the Meteosat satellite recorded these infrared images of the Earth every day at the same local time. The video started at the September 2010 equinox with the terminator line being vertical. As the Earth revolved around the sun, the terminator was seen to tilt in a way that provides less daily sunlight to the northern hemisphere, causing winter in the north. As the year progressed, the March 2011 equinox arrived halfway through the video, followed by the terminator tilting the other way, causing winter in the southern hemisphere — and summer in the north. The captured year ends again with the September equinox. 

Sources:

NASA Earth Observatory: http://earthobservatory.nasa.gov/IOTD/view.php?id=52248

NASA Astronomy Picture of the Day: http://apod.nasa.gov/apod/astropix.html 

Earth Sky: Everything you need to know about vernal equinox 2016: http://earthsky.org/?p=68679