The equinox - what you "know" is wrong

James Jordan/Flickr

James Jordan/Flickr

Here comes fall, whisking languid thoughts of summer away with a brisk breeze and a bite in the air.

Autumn officially arrives at 10:21 a.m. today when the fall equinox occurs. Casting your mind back to science class, you may recall that “equinox” comes from the Latin words for “equal” and “night.” On the equinoxes, vernal and autumnal, we observe a day when the hours of sunlight and darkness are equal — right?

Wrong, it turns out, and on revisiting the subject I was as startled to learn this as you may be.

For most locations on Planet Earth, there are two days when the length of day and night are very nearly equal, but these are not the equinoxes per se. Astronomically speaking, the equinox marks a singular moment in time when the geometric center of the sun’s disk crosses over the equator. That’s why we have these precise predictions like 10:21 a.m., Eastern Daylight Time.

The day of equal light and dark is known as the equilux. It falls before the spring equinox and after the fall equinox in the northern hemisphere, exactly when being dependent on your latitude, or distance from the equator. Who knew?

The explanation has to do atmospheric refraction, which causes an optical illusion, i.e., the disk of the sun appears higher than it might seem if we had no atmosphere at all (gasp!). Plus, sunrise has to do with appearance of the sun’s leading edge (not its geometric center) and sunset in the same way indicates the sun’s trailing edge has sunk below the horizon.

The times of sunrise and sunset that you see in almanacs are calculated (I learned from the U.S. Naval Observatory Web site) “for the normal atmospheric refraction of 34 minutes of arc and a semidiameter of 16 minutes of arc for the (sun’s) disk ... with the geometric center of the sun actually 50 minutes of arc below a regular and unobstructed horizon for an observer on the surface of the Earth in a level region.” That may be a little too much higher math for me.

Suffice it to say that at the latitude of Newark, N.J., the equilux occurs not on Sept. 22, when sunrise is at 6:44 a.m. and sunset at 6:52 p.m., but on Sept. 25, Sunday, when sunrise and sunset are at 6:47 a.m. and 6:47 p.m., respectively. So there.

Another thing I didn't know was that the September equinox was New Year's Day according to the French Republican calendar, in vogue from 1793 to 1805 — but only en français, I would suppose.

The monarchy was abolished on Sept. 21, 1792, making the day after (the first day of Republican France) the first day of a new era. As it happened, that happy day was the autumn equinox. The start of every year thereafter during this whimsical experiment wasn't fixed but was determined by astronomical calculation, which must have kept the calendar publishers of Paris on their toes.

The other thing you may have heard about the equinoxes is that, the powers of the universe being uniquely poised in equilibrium, these two days are the only dates on the calendar when it is possible to balance an egg on end. Sorry, but this is complete hooey.

Being able to get a common chicken egg to stand at attention has a lot more to so with a large helping of patience and minute irregularities in the shell which act like tiny "legs." (Some say having the eggs at room temperature makes this easier, but that's anecdotal evidence.)

Go ye to your refrigerator and start experimenting with the dozen or so subjects you may have hiding in the cold and dark, just waiting for this moment. Otherwise sane individuals have balanced eggs (On their fat ends! On their pointy ends!) on any old ordinary day of the week. As a matter of fact, the world record was set by a guy with a lot of friends and a lot of time on his hands who engineered the precision balancing of 1,290 eggs on a day not remotely cued to either equinox.

If you are feeling blue about the passing of summer, the arrival of fall and the approach of winter, I have an astronomical consolation for you. Earth's orbit around the sun isn't perfectly circular, but is an ellipse or stretched-out oval thingie. Hence, according to Kepler's law of planetary motion, Starship Earth moves at different speeds through different parts of its orbit.

Before your eyes cross, just know that in the present geological era (this could change in a few million lifetimes) summer in the northern hemisphere is five days longer than winter. This is exactly opposite for our pals in the southern hemisphere, who have five more days of winter — but that's their problem, right?

No worries, mate. We're leaving behind the sultry heat of summer and heading into autumn here in the temperate northern latitudes, where the prevalent deciduous trees put on a final show and go out in a blaze of glory we call fall foliage season. Ain't it grand to live on Earth where we do?