Can you balance an egg on the equinox? Are days and nights truly equal? Bob separates autumnal equinox facts from fiction.
Are Days and Nights Truly Equal?
We have an equinox coming up on September 23, 2018 at 1:54 UTC. That’s Saturday, September 22 (9:54 P.M. ET), for those of us in North American time zones.
At this equinox, the Earth will angle perfectly sideways to the Sun. Neither pole will tip toward or away from it. And therefore, as the media never tire of reminding us, days and nights should theoretically be equal, right?
But this is never true. Our atmosphere bends the Sun’s image upward 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 sunshine push the true date of equality to a few days after the equinox.
And even then it’s not strictly accurate to say day and NIGHT are equal, because of twilight. If useful daylight ends about an hour after sunset, and you add in the dawn twilight too, then most places don’t have equal day and night until around November 10. So we get more actual night than daylight for just three months, from then until mid-February.
Sunrise and Sunset on the Equinox
Never mind the equality business. A more precise equinox event is that the Sun rises and sets exactly in the east and west, not southeast or northwest or anything else. It’s a time of precision, and an opportunity to correctly position your sundial, that task you keep putting off.
The Sun’s Path
Another equinox phenomenon is that the Sun then 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 right after the equinox, the Sun’s track across the sky starts to bend like a rainbow, with the concave part aimed downward.
As for eggs being able to balance on end during the equinox but at no other time, that’s just silly. Why should the laws of gravity be repealed just because the Sun illuminates both poles equally that day? Still, it’s fun to consider, especially if it sparks conversation about the reasons for the seasons.
If you take the equinox so seriously that you have an equinox-obsessive personality, which psychologists call EOP, you’ll contemplate the idea of equality on September 22. That’s when our beloved Sun pauses momentarily, balanced and motionless, before lunging headlong toward the northern winter.