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Equal Parts Day & Night, Not on Equinox

Updated: 10/05/2013 8:42 PM
Created: 09/24/2013 8:12 PM KSTP.com

I was asked a question in regards to the almanac, and why the sunrise and sunset still showed more than 12 hours of daylight even though we are passed the Autumnal Equinox.  Here's why.

The Autumnal Equinox, more commonly known as the first day of Fall, started on Sunday (9/22).  This event marks when the sun is located directly above the equator on earth.  That means everyone on earth theoretically sees equal parts of daylight and night time.  However, looking at sunrise/sunset times, this doesn't appear to be the case.

Looking at the almanac for tomorrow's sunrise/sunset times (9/25), it shows that sunrise is at 7:01am with sunset at 7:01pm.  12 hours of sunlight.

So how come it doesn't match up with the Equinox?

It basically comes down to how we measure sunrise/sunset.  Sunrise is the moment the first glimpse of sun eclipses the horizon.  Conversely, sunset is the last glimpse of light as the sun goes below the horizon, not by the center of the sun.

With the equinox, the center of the sun is located over the center (equator) of the earth, not the edges like with sunrise/sunset times leaving there more .

This difference in measuring points subsequently means, the two won't match up perfectly.  At our latitude on earth, in the first couple days of fall, you'll actually see the sun for more than 12 hours.  In the Spring, the last couple of days of winter will see the sun for more than 12 hours.

As you decrease latitude (move towards the equator) this effect is even more pronounced, leaving places as the equator never sees less than 12 hours of daylight in any given year.  This occurs twice a year at both the winter/summer solstices.

So it's all about measurement.  In the grand scheme of things is there a big difference between the two?  Not really.  We're talking about a matter of a couple minutes of difference at maximum anywhere on earth. 

For more information on the subject visit here.

Storm Tracker 6 Chief Meteorologist
Chris Kuball