Autumn Equinox: The Science Behind the First Day of Fall

Smileus/iStock via Getty Images
Smileus/iStock via Getty Images

Today, September 23, the whole world will experience a day and night of equal length when the sun shines directly over the equator—the midpoint of Earth. (For 2019, this moment will happen at 3:50 a.m. ET.) In the Northern Hemisphere, we call this the fall or autumn equinox, and it marks the first day of fall. Around the world, people celebrate the day with ceremonies, some of them ancient, and some less so.

You might be wondering two things. Why on almost every other day of the year (the vernal or spring equinox being the exception) do different parts of the world have days and nights of differing length? And, what do they call the fall equinox in the Southern Hemisphere?

How the Fall Equinox Works

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The answer to each of these questions resides in Earth's axial tilt. The easiest way to imagine that tilt is to think about tanning on the beach. (Stay with me here.) If you lay on your stomach, your back gets blasted by the sun. You don't wait 30 minutes then flop over and call it a day. Rather, as you tan, every once in a while, you shift positions a little. Maybe you lay a bit more on one side. Maybe you lift a shoulder, move a leg. Why? Because you want the sun to shine directly on a different part of you. You want an even tan.

It might seem a little silly when you think about it. The sun is a giant fusion reactor 93 million miles away. Solar radiation is hitting your entire back and arms and legs and so on whether or not you adjust your shoulder just so. But you adjust, and it really does improve your tan, and you know this instinctively.

Earth works a lot like that, except it's operating by physics, not instinct. If there were no tilt, only one line of latitude would ever receive the most direct blast of sunlight: the equator. As Earth revolved around the sun, the planet would be bathed in sunlight, but it would only be the equator that would always get the most direct hit (and the darkest tan). But Earth does have a tilt. Shove a pole through the planet with one end sticking out the North Pole and one end sticking out the South, and angle the whole thing by 23.5°. That's the grade of Earth's tilt.

Now spin our little skewered Earth and place it in orbit around the sun. At various points in the orbit, the sun will shine directly on different latitudes. It will shine directly on the equator twice in a complete orbit—the spring and fall equinoxes—and at various points in the year, the most direct blast of sunlight will slide up or down. The highest latitude receiving direct sunlight is called the Tropic of Cancer. The lowest point is the Tropic of Capricorn. The poles, you will note, are snow white. They have, if you will, a terrible tan—and that's because they never receive solar radiation from a directly overhead sun (even during the long polar summer, when the sun never sinks below the horizon).

When does fall begin?

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The seasons have nothing to do with Earth's distance from the sun. Axial tilt is the reason for the seasons. The sun is directly over the Tropic of Cancer (66.5° latitude in the Northern Hemisphere) on June 21 or 22. When that occurs, the Northern Hemisphere is in the summer solstice. The days grow long and hot. As the year elapses, the days slowly get shorter and cooler as summer gives way to autumn. On September 21 or 22, the sun's direct light has reached the equator. Days and night reach parity, and because the sun is hitting the whole world head-on, every latitude experiences this simultaneously.

On December 21 or 22, the sun is directly over the Tropic of Capricorn in the Southern Hemisphere, meaning the Northern Hemisphere is receiving the least sunlight it will get all year. The Northern Hemisphere is therefore in winter solstice. Our days are short and nights are long. Parity will again be reached on March 21 or 22, the vernal equinox for the Northern Hemisphere, and the whole process will repeat itself.

Now reverse all of this for the Southern Hemisphere. When we're at autumnal equinox, they're at vernal equinox. Happy first day of spring, Southern Hemisphere!

And welcome to fall, Northern Hemisphere! Enjoy this long day of sunlight, because dark days are ahead. You'll get less and less light until the winter solstice, and the days will grow colder. Take solace, though, in knowing that the whole world is experiencing the very same thing. Now it's the Southern Hemisphere's turn to get ready to spend some time at the beach.

This story first ran in 2016.

Mental Floss's Three-Day Sale Includes Deals on Apple AirPods, Sony Wireless Headphones, and More

Apple
Apple

During this weekend's three-day sale on the Mental Floss Shop, you'll find deep discounts on products like AirPods, Martha Stewart’s bestselling pressure cooker, and more. Check out the best deals below.

1. Apple AirPods Pro; $219

Apple

You may not know it by looking at them, but these tiny earbuds by Apple offer HDR sound, 30 hours of noise cancellation, and powerful bass, all through Bluetooth connectivity. These trendy, sleek AirPods will even read your messages and allow you to share your audio with another set of AirPods nearby.

Buy it: The Mental Floss Shop

2. Sony Zx220bt Wireless On-Ear Bluetooth Headphones (Open Box - Like New); $35

Sony

For the listener who likes a traditional over-the-ear headphone, this set by Sony will give you all the same hands-free calling, extended battery power, and Bluetooth connectivity as their tiny earbud counterparts. They have a swivel folding design to make stashing them easy, a built-in microphone for voice commands and calls, and quality 1.18-inch dome drivers for dynamic sound quality.

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3. Sony Xb650bt Wireless On-Ear Bluetooth Headphones; $46

Sony

This Sony headphone model stands out for its extra bass and the 30 hours of battery life you get with each charge. And in between your favorite tracks, you can take hands-free calls and go seamlessly back into the music.

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4. Martha Stewart 8-quart Stainless-Steel Pressure Cooker; $65

Martha Stewart

If you’re thinking of taking the plunge and buying a new pressure cooker, this 8-quart model from Martha Stewart comes with 14 presets, a wire rack, a spoon, and a rice measuring cup to make delicious dinners using just one appliance.

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5. Jashen V18 350w Cordless Vacuum Cleaner; $180

Jashen

If you're obsessive about cleanliness, it's time to lose the vacuum cord and opt for this untethered model from JASHEN. Touting a 4.3-star rating from Amazon, the JASHEN cordless vacuum features a brushless motor with strong suction, noise optimization, and a convenient wall mount for charging and storage.

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6. Evachill Ev-500 Personal Air Conditioner; $65

Evachill

This EvaChill personal air conditioner is an eco-friendly way to cool yourself down in any room of the house. You can set it up at your work desk at home, and in just a few minutes, this portable cooling unit can drop the temperature by 59º. All you need to do is fill the water tank and plug in the USB cord.

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7. Gourmia Gcm7800 Brewdini 5-Cup Cold Brew Coffee Maker; $120

Gourmia

The perfect cup of cold brew can take up to 12 hours to prepare, but this Gourmia Cold Brew Coffee Maker can do the job in just a couple of minutes. It has a strong suction that speeds up brew time while preserving flavor in up to five cups of delicious cold brew at a time.

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8. Townew: The World's First Self-Sealing Trash Can; $90

Townew

Never deal with handling gross garbage again when you have this smart bin helping you in the kitchen. With one touch, the Townew will seal the full bag for easy removal. Once you grab the neatly sealed bag, the Townew will load in a new clean one on its own.

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9. Light Smart Solar Powered Parking Sensor (Two-Pack); $155

FenSens

Parking sensors are amazing, but a lot of cars require a high trim to access them. You can easily upgrade your car—and parking skills—with this solar-powered parking sensor. It will give you audio and visual alerts through your phone for the perfect parking job every time.

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10. Liz: The Smart Self-Cleaning Bottle With UV Sterilization; $46

Noerden

Reusable water bottles are convenient and eco-friendly, but they’re super inconvenient to get inside to clean. This smart water bottle will clean itself with UV sterilization to eliminate 99.9 percent of viruses and bacteria. That’s what makes it clean, but the single-tap lid for temperature, hydration reminders, and an anti-leak functionality are what make it smart.

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Prices subject to change.

This article contains affiliate links to products selected by our editors. Mental Floss may receive a commission for purchases made through these links. If you haven't received your voucher or have a question about your order, contact the Mental Floss shop here.

100 Fascinating Facts About Earth

The best Spaceball.
The best Spaceball.
NASA

Did you know that there’s a place in the South Pacific Ocean called Point Nemo that’s farther from land than any other point on Earth? So far, in fact, that the closest humans are usually astronauts aboard the International Space Station. (And by the way: The map you’re about to look for Point Nemo on might not be entirely accurate; a certain amount of distortion occurs when trying to depict a 3D planet on a 2D surface.)

In this all-new episode of The List Show, Mental Floss editor-in-chief Erin McCarthy is journeying to the center of the Earth, and visiting its oceans, its atmosphere, and even space, in search of 100 facts about our endlessly fascinating planet.

The subjects that fall under the umbrella of “facts about Earth” are nearly as expansive as Earth itself. Geology, biology, astronomy, and cartography, are all fair game—and those are just a few of the many -ologies, -onomies, and -ographies you’ll learn about below. 

Press play to find out more Earth-shattering facts, and subscribe to the Mental Floss YouTube channel for more fact-filled videos here.