What Is the Vernal Equinox & When Does it Occur?
The word "equinox" comes from the Latin, meaning "equal night". There are two equinoxes a year, usually on March 20 or 21 and Sept. 22 or 23.
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What is an Equinox?
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Twice a year, on the vernal equinox and the autumnal equinox, the sun shines directly on the equator, creating a nearly equal day and night on all parts of the globe.
Beginning of Spring
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The March equinox (often referred to as the spring or vernal equinox in the Northern Hemisphere) marks the beginning of spring in the Northern Hemisphere and the beginning of autumn in the Southern Hemisphere. For those in the Southern Hemisphere, the vernal equinox occurs in September.
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When It Occurs
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The date of the March equinox falls almost always on March 20 or 21, but occasionally on March 19 because of the leap-year cycle.
Nearly Equal Day and Night
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The vernal equinox has nearly equal lengths of day and night--but not exactly equal. The day when length of day and night are the same always falls a little before the vernal equinox, according to Geoff Chester of the U.S. Naval Observatory. Because of atmospheric refraction and the fact that the sun takes longer to rise and set in locations farther away from the equator, the exact moment of equal lengths of day and night is relative to where on Earth you live.
Why the Equinox is Special
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The two equinoxes are the only days of the year when you can see the sun pass directly overhead from the location of the equator. The March equinox signals opposite things to persons on the poles--it means the beginning of six months of light at the north pole and six months of darkness at the south pole.
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References
- Photo Credit Image by Flickr.com, courtesy of Steve Jurvetson
