Equivocally equinoctial: The first moment of fall

Cooper Lake (photo by Dion Ogust)

The Autumnal Equinox takes place on Friday afternoon, September 22nd, at 4:02 p.m. At that moment, Earth will angle perfectly sideways to the Sun. Neither pole will tip toward or away from that favorite star of ours. And therefore, as the media never tire of reminding us, days and nights should theoretically be equal.

We’ve often pointed out that this is never true. The air bends the Sun’s image so much that it rises two or three minutes earlier and sets that much later than it would on an airless world, and those extra five minutes of daily undeserved sunshine push the true date of equality to the middle of the following week.

And even then, it’s not strictly accurate to say day and night are equal, because of twilight. The brightest part of dusk is called “civil twilight” – not because people are suddenly more civil to each other, but because most municipal ordinances do not require streetlights to be on until the end of civil twilight, which around here happens a half-hour after sunset.


Next comes nautical twilight, which is still fairly bright. When it ends, a mariner can no longer distinguish the horizon. The line separating the sky from the sea is gone. The final, darkest variety of twilight, called astronomical twilight, brings us to true night.

If useful daylight ends with nautical twilight, and we add in those daily bright-but-non-sunny periods at both dusk and dawn, we find that at our region’s latitude we don’t have equal day and night until November 10. So we get more actual night than daylight for just three months: from then until mid-February.

But never mind the equality business; a more precise equinox event is that this weekend the Sun rises and sets exactly in the east and west, not southeast or northwest or anything else. Sunset will then point the way to places due west of here, like Milwaukee. And the sunrise point will show the precise direction to Boston or, if you have major creditors, Corsica. This directionality is far more accurate than a compass, which around here wrongly indicates north to be 13 degrees left (west) of where it actually lies.

Another equinox phenomenon is that this weekend the Sun moves in a laser-straight line across the sky. A time exposure shows this nicely. By comparison, for the past six months the Sun’s path has displayed an upward curve, concave to the north, like a giant smile. Starting on Saturday its track across the sky bends like a rainbow, with the concave part aimed downward.

As for eggs balancing on Saturday but at no other time, that’s just plain silly. Why should the laws of gravity be repealed just because the Sun illuminates both poles equally that day?

If you take this equinox business so seriously that you have an equinox-obsessive personality (which psychologists call EOP), you’ll pause at 4:02 p.m. Friday and contemplate the equality – as our beloved Sun pauses momentarily, balanced and motionless, before lunging headlong toward the Northern winter.