What Exactly Is the Autumn Equinox?

People attend the spring equinox in front of the Kukulkan Pyramid in Chichen Itza, Mexico, Saturday, March 21, 2009. This Mayan pyramid was built so that the shadows of a corner of the pyramid would fall on a stairway and create the effect of an illuminated serpent. (PHOTO CREDIT: AP Photo/Israel Leal)
People attend the spring equinox in front of the Kukulkan Pyramid in Chichen Itza, Mexico, Saturday, March 21, 2009. This Mayan pyramid was built so that the shadows of a corner of the pyramid would fall on a stairway and create the effect of an illuminated serpent. (PHOTO CREDIT: AP Photo/Israel Leal)

Winter is officially coming. We blew by the longest day of the year with June’s summer solstice, and are coming up on the autumnal equinox on Wednesday—the day when the sun passes directly over Earth’s equator.

In the northern hemisphere, this marks an end to the long days of summer and the beginning of winter’s endless dark nights. For the other half of the planet, Wednesday is the vernal equinox, signaling the beginning of spring.

Even though the equinox happens twice a year, every year, there are a lot of misconceptions about this seasonal transition.

Here Comes the Sun
In a grainy 1980s video, a group of newly fledged Harvard graduates are asked why we have seasons. It seems like a pretty simple question. With varying degrees of confidence, the students explain that the Earth gets warmer or colder based on its distance from the sun. After all, the Earth’s orbit isn’t a perfect circle.

The Harvard grads—captured forever in 1980s hair and their caps and gowns—were wrong.

“The Earth’s orbit is [only] about three percent out of round,” explains Jay Holberg, a senior research scientist at the lunar and planetary lab at the University of Arizona. “So in the northern winter—in December—the sun is actually closest to the Earth by a small amount, and in the summer it’s actually farther away.”

So if it’s not the Earth’s changing proximity to the sun, what gives us seasons?

It’s all in the slant: Earth’s axis isn’t straight up and down relative to the sun, but tilted at a slight angle, about 23.5 degrees.

So as Earth revolves around the sun, it maintains that tilt, and the sun’s light doesn’t hit the entire surface directly. When the planet’s northern hemisphere is tilted toward the sun, sunlight hits it head on, bringing summer’s heat and longer days. At the same time, the southern half of Earth is tilted away from the sun and catches its rays at an angle, causing the cooler, shorter days of winter.

The Earth is bathed evenly in sunlight only twice a year, on the equinoxes.

“What it has to do [with] is the amount of light per square centimeter that’s falling on you,” says Dan Milisavljevic of the Harvard-Smithsonian Center for Astrophysics. “If the light is angled, it’s not going to be as hot.”

So, if the chillier air and crispier leaves have you happily reaching for pumpkin-flavored everything, thank the Earth’s tilt—not its distance from the sun. And if you’re seething about summer’s end, consider moving closer to the equator.

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SOURCE: National Geographic, Rachel A. Becker