Autumnal Equinox

The autumnal equinox occurs this weekend: On Saturday at precisely 9:49 am CDT the sun will be directly over the equator. That will not happen again until the spring equinox next March. The sun has been 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 it begins its inexorable move northward again.

The term equinox is derived from the Latin terms “aequus” which means equal, and “nox” or night. On the spring and fall equinoxes the daytime and nighttime are of about equal length. As you may know, the longest day of the year is the summer solstice on about June 21. The days then begin to shorten, through the autumnal equinox, until the shortest day of the year, the winter solstice, on around December 21. Days then begin to lengthen through the spring equinox (around March 21) until 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. As a result, the directness of sunlight (not the distance to the sun!) varies, particularly in the high northern and southern latitudes throughout the year. Saturday is our autumnal equinox, but in the southern hemisphere it is the spring equinox.

It is surprising how much this angle of rotation influences day length. Sunrise and sunset in Bismarck on the first of September were 7:03 am and 8:21 pm respectively. Saturday, the autumnal equinox, sunrise and sunset is 7:31 am, and 7:40 pm respectively. That is a whopping loss of an hour and nine minutes of daylight in the last twenty-two days. Most of us will be quite happy when the winter solstice is behind us and days begin to lengthen again.

As you might expect, the equinoxes and solstices were special days for early cultures. Festivities marking them were held in most if not all cultures around the globe. Today, however, we seem to be more removed from celebrating the change of the seasons. But consider finding a way to get outside and celebrate the autumnal equinox.

Chuck Lura

Natural North Dakota is supported by NDSU Central Grasslands Research Extension Center and Minot State University-Bottineau, and by the members of Prairie Public. Thanks to Sunny 101.9 in Bottineau for their recording services.