[static.scientificamerican.com]

Happy September equinox! Or, as we say north of the equator, happy autumnal equinox!

On Saturday, September 23, at 6:50 A.M. UTC (2:50 A.M. EDT or 11:50 P.M. Friday PDT), the sun will be directly over Earth’s equator, which is how astronomers define the equinox.

(Seasons, months and names get a little confusing in this topic because Earth is a tilted ball, and the Southern Hemisphere’s seasons are the opposite of the Northern Hemisphere’s ones. For the duration of this article, I’ll show my own bias by using the northern names and dates. If you live below the equator, first, congratulations; southern skies are far better than northern ones. There are many more interesting things to see. But also, just reverse the seasons and add six months to the dates as you read them, and you’ll be fine.)

In some ways, defining what the equinox isn’t is easier than describing what it is.

For example, it’s not when the day and night have equal lengths. That’s a common misconception and an understandable one. The trouble’s right in the name: “equinox” means “equal night”, implying that day and night are each 12 hours long. But it turns out they’re only mostly equal because of a couple of pernicious physical facts.

First among them is the fact that we measure the length of daytime in a weird way. Daytime starts when the topmost bit of the sun rises above the horizon, but it doesn’t end until the topmost bit sinks below the horizon. If we measured the day starting and ending when the sun’s center breached the horizon, we’d be fine; day and night would be equally parsed. But instead day starts a little earlier and ends a little later, making it longer than night. The difference is the time it takes the sun to move through its own diameter in Earth’s sky, which is about two minutes. So even on the equinox, daytime is fractionally longer than nighttime.

But there’s more! Earth’s air acts as a lens that bends the light from the sun in the same way that a spoon looks bent when it sits in a glass half full of water. This optical effect is called refraction, and it is greatest when the sun’s on the horizon (and thus its light is passing through the greatest amount of air). That sunlight bends in such a way that we still see it even when the sun itself has already physically set below the horizon. It’s essentially a mirage. If Earth had no air, this wouldn’t be an issue (though there would be other uncomfortable consequences). But it does mean that you see the sun rise even when it’s technically still below the horizon, and by the time you see it fully set, it has actually already been below the horizon for some time. This adds several minutes to daytime’s duration, giving day even more of an edge over night at the ostensibly equal equinox.

There are days when, after accounting for these factors, day and night are indeed equal in length. One of these days occurs a few days before the March equinox, and the other is a few days after the September equinox. These two times of year are aptly, if a tad confusingly, called the equilux, or “equal light.”

Incidentally, did you know that a complete cycle of one day and night is called a nychthemeron? That’s just a fun aside because I love odd words like that.

Okay, so that’s what the equinox isn’t. What is it, then?

If you’ve ever seen a globe of Earth in a classroom, you’ll have noticed that it’s tipped over such that the North Pole isn’t pointing straight up but is instead canted at an angle: 23.5 degrees, to be specific. That represents Earth’s axial tilt, which astronomers call its obliquity, relative to the plane of its nearly circular orbit around the sun. (Another fun fact: Mars and Saturn are also inclined by about that same amount.)

newsletter promo
Sign up for Scientific American’s free newsletters.

Sign Up
This is the reason for our seasons! Earth’s axis stays fixed in space and points in the same direction even as it orbits the sun. On June 21 or so every year, Earth’s North Pole reaches its greatest tilt toward the sun. When that happens, the sun’s path across the sky takes it the highest above the horizon all year, making for the longest days of the year and giving the sun maximal time to heat the ground. Many people consider this to be the beginning of summer, and this phenomenon is why (in the Northern Hemisphere, anyway) we associate this time of year with warmth and sunshine.

Six months (half an orbit) later, the North Pole is tipped its farthest away from the sun, and the opposite effects occur. Our star’s path across the sky is low, the days are shorter, and solar heating is minimal. That’s why we have winter.

The equinoxes occur roughly halfway between those two times, when Earth’s axis is pointed 90 degrees away from the sun. At that time, a line drawn between the center of Earth and the sun would go directly through Earth’s equator. During the summer, that line would pass north of the equator, and in winter it would go south.

Another way to visualize this: from Earth, we see the sun rise, follow an arc across the sky and then set. At the summer solstice, that arc is at its highest, and at its apex, the sun is as far north as it gets. During the next six months, the arc lowers, and the apex moves lower toward the southern horizon.

If you measure that movement of the arc’s peak northward from December to June, it moves fastest at the equinoxes and slowest just as it reaches that northward peak. On that day, its movement seems to halt and then reverse, and it moves southward again. Because of this, that day is called the solstice, meaning “sun stands still.”

This movement affects where the sun rises and sets, too. If you were to take a snapshot of the sun as it rises at the December solstice, you’d see that that point on the horizon is to the south of due east. Every day that point moves a little bit further north. At the March equinox, the sun rises due east, and eventually, at the June solstice, it rises as far north of due east as it will all year.

The speed of that movement changes during the year, too! Immediately after the December solstice, the northward progression of the sun’s horizon-cresting point is so slow that it’s hardly noticeable. By the equinox, that point is moving rapidly, and the sun rises at a visibly different spot every morning. Then it begins to slow anew, and by the June solstice, its motion becomes nigh imperceptible once again.

This influences the rate at which daytime ebbs and flows across the seasons. Every day after the December solstice, the length of daylight increases, but it only does so by a small amount at first. That amount increases every day until the equinox, when daytime might be several minutes longer than that of the previous day. Then the trend reverses itself as Earth swoops through the other half of its orbit. The increase in the amount of daytime slows until it reaches a maximum at the June solstice.

This subtle but powerful effect is one reason, I think, that summer and winter seem to last a long time; daylight’s duration doesn’t change much around the solstices. But in the spring and autumn, it changes much more quickly, giving these seasons a more ephemeral feel that isn’t just in your head but rooted in our world’s deepest fundaments.

I can feel autumn coming. The days are getting shorter, the nights are getting cooler and some leaves are already tinged with gold. I don’t mind winter—that’s when Orion is visible, and astronomical delights abound during the long, dark nights—but I also know that there are multiple and subtle cycles at work all the time. Earth still twirls around the sun; the celestial clock keeps ticking; and soon enough, things will change once more.