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The Equinox: Not Quite Equal

© Keith C. Heidorn

The Autumnal Equinox is just about a week away. On September 22 at 7:05 pm EDT, the sun will be directly over the equator. As the days quickly shorten, soon to be overcome by the hours of darkness, I am reminded of a question a few years back from a friend who taught the lower grades.

She was teaching her students about the Autumnal Equinox, and they were watching the reported times of sunrise and sunset move toward a 12-hour separation, expecting to see it on the date of the Equinox. To her surprise, they were still ten minutes apart on the appointed day. She called and asked: "Why?"

I too had always been taught that on the dates of the equinoxes (Spring as well as Autumn), the day and night lengths were equal. The equations used to determine day length in several computer programs I had written also concurred with my beliefs. So what was wrong?

Before I get into the reason we don't have equal day and night on the equinox, let me define what is meant by sunrise and sunset. The official definitions, according to the US Naval Observatory, are as follows: Sunrise and sunset conventionally refer to the times when the upper edge of the disk of the sun is on the unobstructed horizon, relative to the location of interest.

In general, all celestial bodies, sun, moon, stars and planets included, seem to appear in the sky at the horizon to the East of any particular place (rise), then to cross the sky and again disappear at the horizon to the West (set). The sun and moon complicate the situation because we see them as disks rather than points of light.

The next sentence of the definition states that when determining the time of sun rise or set: "Atmospheric conditions are assumed to be average, and the location is in a level region on the Earth's surface." Now I won't spend time on the latter part of that sentence. I hope you realize that if you ascend a mountain, you can see further before the Earth curves away and thus see the setting sun longer than when at the surface.

The important concept for this discussion stems from "Atmospheric conditions are assumed to be average." Why should atmospheric conditions be important? Here I can get into the science of the sky that links atmospheric conditions with our non-equal day/night lengths.

Last essay, I discussed the inferior mirage and remarked it was one major type of mirage we see. The other main type is the superior mirage, so called because the image is seen above the actual

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