Autumn Equinox: The Science Behind the First Day of Fall

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

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?

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.