HAIL TO THE CHIEF: Daylight Longer On The Equinox

James Zahara Weather Blog

With the first day of spring arriving this Thursday, the sun will be directly above the equator. This is what we call the spring equinox. In September, the sun is again above the equator at the autumnal equinox, which takes place on September 22nd.

Now, you have probably learned from way back that day and night are equal length on the spring and fall equinox. However, that is not the case, which may be a surprise to many.   In fact, day and night are about equal length today, March 17th before the time of this upcoming spring equinox.

This is a table of the sunrise and sunset for the city of Moline on the following days leading to the spring equinox, March 20th:

Date              Sunrise        Sunset           Daylight
March 17      6:10 a.m.     6:11 p.m.      12 hours, 1 min.
March 18      6:09 a.m.     6:12 p.m.      12 hours, 3 min.
March 19      6:07 a.m.     6:13 p.m.      12 hours, 6 min.
March 20      6:05 a.m.     6:14 p.m.      12 hours, 9 min.

On the equinox, the sun’s center sets 12 hours after it rose all over the world.

But, sunrise and sunset occur when the top of the sun, not its center, is on the horizon. This alone would make the time of sunrise and sunset a little more than 12 hours apart on these days. Why is that the case? Well, its the way the sun’s light bends or refracts due to the earth’s atmosphere. Check out the graphic below courtesy of the U.S. Naval Observatory.

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As a result, the top of the sun appears to be above the horizon when it is actually below the horizon. We’ll continue to add a smidgen more daylight each day until we arrive to the summer solstice, when an extra 3 additional hours or over 15 hours of daylight takes place!

Thats right, brighter days are coming.    :)

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