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PRI's Environmental News Magazine

Happy Spring! The Vernal Equinox

Published: August 11, 2012


By Mary Bates


The relative position of the sun is shown for the two equinoxes and two solstices that earth experiences each year. (NASA)

Spring has sprung...So what happens when Spring springs?

Happy Spring! The vernal equinox marks the precise moment when the sun crosses the earth’s equator, going from south to north. This year, it will happen at precisely 1:14 a.m. EST on March 20, marking the start of the Nothern Hemisphere's spring. Since the sun is directly above the equator, it shines equally on both Earth’s hemispheres. This causes night and day to be approximately equal in length all over the world. In fact, the word equinox comes from the Latin for “equal night.”

There are two equinoxes each year – in March and September- when the equator faces the sun directly. The Earth wobbles on its axis as it goes round the sun. The Northern Hemisphere tilts 23.4 degrees away from the sun in the winter, so its rays arrive at a greater slant, resulting in lower temperatures. But in the summer, the Northern Hemisphere is titled towards the sun and gets more direct rays. If the Earth had no tilt, day lengths and temperatures would be the same year-round and we would have no seasons. This is the case at the equator, where the sun’s rays arrive perpendicularly. Night and day are roughly the same length throughout the year, and there is little seasonal change in temperature.

On March 20, 2012, for everyone on earth, the sun will rise exactly in the east, travel through the sky for 12 hours, and then set due west. In the coming months, the sun will continue to climb higher and higher in our sky, bringing longer days, until it reaches its highest point at the Summer Solstice. And solstice - again from the latin - means the sun stands still, as it seems to do before it starts the gradual descent that will bring the Northern Hemisphere another
winter.

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