Autumnal Equinox

What’s an Equinox?

This September 22nd at 1044 AM CDT is designated as the Autumnal Equinox. This is a midway point between the Summer and Winter Solstices of the Northern Hemisphere, where the apparent annual sun motion traverses the equator on its way south.  Compared to the Solstice, the Equinox has more of an astronomical significance than anything else. But for most of us, it can probably mark the end of summer and the growing season. It’s at this time of year that the long days of summer shorten in time to the length of night. In translation, equinox comes from the Latin word aequus meaning equal and nox meaning night.  

From this point on, nights become longer and days become shorter as sunrise appears a little further south on the horizon each day. Its subsequent arc across the sky becomes smaller and smaller. Nighttime frosts become more frequent through the autumn as more heat is radiated to space than we gain during the day. By the time the sun travels 23.5 degrees of latitude below the equator, it is at the Winter Solstice and the sun is working back north. Days start to lengthen from this point but aren’t equal to nighttime again until late March. This point, where the sun arcs across the sky over the equator, and day and night are equal again, is called the Vernal or Spring Equinox.

The graphic below highlights the orientation of earth to sun at the solstices and the equinoxes. The solstices make the most striking differences in earth orientation and subsequent weather. Earth at the equinoxes receives equal amounts of sun at equal orientation, signaling either the beginning or the end of the growing season. The length of the growing season, of course, varies with latitude. Even with an increased length of day, the sun’s incidence angle determines that latitude’s planting and harvesting dates, or even its climate in general.

 



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