Can you balance an egg on the autumnal equinox? Does the Sun rise due east and set due west for all of us on Earth? Why did the date of the equinox change? How is it related to the Harvest Moon? Bob separates autumnal equinox facts from fiction.
There are a number of questions and misperceptions commonly sent my way. Let’s clear this up, and talk a little about the reasons for the seasons.
A Moment in Time
The equinox happens at the same moment across the Earth (September 22, 2021, at 7:20 UTC) which is pretty cool. It’s not an all-day event such as a birthday or holiday. The equinox is the moment the Sun crosses the celestial equator—that imaginary line in the sky above Earth’s equator. At this instant, Earth’s rotational axis is neither tilted away from nor towards the Sun.
Of course, your “clock time” of this instant depends on your time zone (3:20 P.M.EDT, 2:20 P.M.CDT, 1:20 P.M.MDT and 12:20 P.M.PDT).
Not Always the Same Date
The calendar can affect the date of the equinox as well. The autumnal equinox date is usually on the 22nd or 23rd of September. It varies slightly, since our Gregorian calendar doesn’t perfectly match up with the time it takes the Earth to orbit the Sun (365 days versus approximately 365 and 1/4 days).
If you see a generic calendar stating different dates or times, it’s usually because a universal or different time zone is being used, often unwittingly. (Not true of the Almanac calendars.)
Are Days and Nights Truly Equal on the Equinox?
At the equinox, the Earth will angle perfectly sideways to the Sun. Neither pole will tip toward or away from our bright star. And therefore, as the media never tires of reminding us, days and nights should theoretically be equal, right?
But this is never quite 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.
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.
Never mind the day-night 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. You know, that task you keep putting off.
And this happens no matter where you live on Earth, because we all see the same sky. We all see a due east and due west point on our horizon. That point marks the intersection of your horizon with the celestial equator—the imaginary line above the true equator of Earth.
Another equinox phenomenon is that 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 right after the equinox, the Sun’s track across the sky starts to bend like a rainbow, with the concave part aimed downward.
That Harvest Moon
The full Moon closest to the September equinox is always known as the Harvest Moon. This year, it happens on Monday, September 20—just two days prior to the equinox!
At the Harvest Moon, there are shorter periods between moon rises. This phenomenon occurs due to the low angle that the Moon’s orbit around Earth makes with the horizon during this time of year.
As for the age-old idea that eggs are 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.