The Misunderstood Equinox

Separating Fall Equinox Facts from Fiction!

September 21, 2020
Autumnal Equinox

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.

We have an equinox coming up early in the morning on Tuesday, September 22, 2020See all you need to know about the Autumnal Equinox

There are a number of questions and misperceptions 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 (13:31 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 (9:31 a.m. EDT, 8:31 a.m. CDT, 7:31 a.m. MDT and 6:31 a.m. PDT).

Not Always the Same Date

And the calendar can affect the date itself as well. The Autumnal Equinox date is usually on the 22nd or 23rd. It varies slightly, since our Gregorian calendar doesn’t perfectly match up with the time it takes the Earth to orbin the Sun (365 days versus approximately 365 and ¼ 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?

At the 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 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.

See day length in your location.

Sunrise and Sunset on the Equinox

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 your horizon. That point marks the intersection of your horizon with the celestial equator – the imaginary line above the true equator of the Earth.

See your sunrise and sunset times.

The Sun’s Winding Path

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 the Harvest Moon. This year, due to the timing of the equinox, it falls on Thursday, October 1, and the full Hunter’s Moon on Saturday, October 31. (That’s right—we’ll have a full Moon on Halloween night this year, and it will be a Blue Moon, too!)

At the Harvest Moon, there are shorter period between moon rises.  This phenomenon occurs due to the low angle the Moon’s orbit around Earth makes with the horizon during this time of year.

Eggs on the Equinox

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.

See our Autumnal Equinox page for more equinox facts and folklore!

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe