From the ABC 7 Weather team

Autumnal equinox: Equal Hours of Daylight and Darkness? Or Not?

September 22, 2012 - 09:00 AM
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The autumnal equinox arrives on Saturday, September 22, 2012 at 10:49 a.m. EDT.  What exactly does the equinox mean?

Wait for it... 10:49am EDT Saturday, September 22, 2012 = autumn in the Northern Hemisphere! 

The equinox occurs when the sun passes directly over the equator.  It can also be understood that the tilt of the Earth's axis is neither away or toward the sun.  This occurs twice a year - on the spring and autumnal equinox.  The word equinox, in Latin, means "equal night", which signifies the equal parts of daylight and darkness.  Take a look at where the Earth is in relation to the sun during both the vernal (spring) and autumnal equinoxes. 

NASA

Since equinox means equal, the hours of daylight and darkness should be equal, right?  Here are the sunrise and sunset time for D.C. on the equinox (Saturday, Sept. 22): Sunrise: 6:56 a.m.  Sunset: 7:04 p.m..  So if there isn't equal parts of daylight and darkness on Saturday, why is it the equinox?  The day we have 12 hours of daylight and 12 hours of darkness actually falls on Tuesday, Sept. 25.  Sunrise: 6:59 a.m.  Sunset: 7:00 p.m.   Shouldn't Tuesday be the autumnal equinox?

Well, we have the atmosphere to thank for this oddity.  Also, the definition of sunrise and sunset.  Sunrise occurs the moment the tip of the sun can be seen on the horizon and sunset is the last minute the sun can be seen before it dips below the horizon.  Also, keep in mind our atmosphere refracts, or bends, light, which makes it appear as if the sun is rising or setting earlier. 

The true equinox occurs when the center of the sun's disk crosses the celestial equator and this occurs at 10:59 a.m. EDT on September 22.  At the same time the equinox occurs in D.C., it occurs across the globe.  As the Northern Hemisphere enters the fall season, losing hours of daylight, the Southern Hemisphere, enters the spring season and gains hours of daylight.  Here's a neat view of the equinox and the solstices from space!

NASA

You won't notice anything striking at the time of the equinox, but just think about the sun passing directly over the equator. Then, after Tuesday, the hours of daylight start getting shorter until the winter solstice.

Happy Equinox!

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