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Autumnal Equinox

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It is official! The autumnal equinox occurred on Thursday at 8:03pm CT. That was when the sun was directly over the equator. So fall is here. And of course, the days will shorten until the winter solstice in December. Then the days will begin to lengthen through the spring equinox and on to the summer solstice, the longest day of the year in June.

This is all due, of course, to the fact that the earth does not rotate perpendicular to the direction of its orbit. The earth’s angle of rotation is 23.5 degrees off perpendicular.

It is changes in the directness of sunlight hitting the earth’s surface as the earth revolves around the sun, not the distance to the sun, that causes our seasons to change. For half the year, the north pole is pointed away from the sun while it tilts toward the sun the other half. The winter solstice occurs when the sunlight is least direct in the northern hemisphere. The summer solstice is when it is most direct. And in between are the autumnal and spring equinoxes. It is the opposite in southern hemisphere. It is now early fall in the northern hemisphere, but it is early spring in the southern hemisphere.

You may have noticed that the sun has been noticeably arcing further southward in the sky since the summer solstice back in June. It will continue to arc further southward until the winter solstice in December before beginning its inexorable movement northward again. On the spring and fall equinoxes the daytime and nighttime are of about equal length.

It is surprising how much this angle of rotation influences day length. Sunrise and sunset in Bismarck on September 22 were 7:38am and 7:42pm CT, respectively. That was 12 hours and four minutes between sunrise and sunset. That is not exactly equal, but that is about as close as we can get. What is a bit staggering is that back in June on the summer solstice Bismarck had a whopping 15 hours and 55 minutes between sunrise and sunset which were at 5:47 and 9:42, respectively. And here comes the bad news: When the winter solstice comes around, we will have a little over eight and a half hours between sunrise and sunset, with sunrise coming at 8:23 and sunset at 4:59.

Unless something really bizarre happens, we can expect around three months of shortening days as the earth continues its orbit around the sun. But there is light at the end of the tunnel. On or around December 21, the days will begin to lengthen again!

Chuck Lura has a broad knowledge of “Natural North Dakota” and loves sharing that knowledge with others. Since 2005 he has written a weekly column, “Naturalist at Large,” for North Dakota’s newest newspaper, the Lake Metigoshe Mirror. His columns also appear under “The Naturalist” in several other weekly newspapers across North Dakota.
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