Why Are There Seasons? On the First Day of Autumn, Here Is a Reminder

The first brisk morning, the first snowfall, the first budding leaves—we can lose track of them in the bustle of daily life, yet they're all quiet reminders that time is passing. But why do we even have seasons at all?

09_21_autumn_seasons Autumn leaves are a little gift from Earth's orbit to us. Viktor Drachev/AFP/Getty Images

There's a common misconception that seasons are caused by Earth moving closer to or farther from the sun. That's grounded in reality: Earth's distance to the sun certainly does vary over the course of the year. But the difference of about 3.5 million miles doesn't actually make much of a difference in how much heat and light we get here on Earth.

If it helps, remember that whatever season the Northern hemisphere is experiencing, the Southern Hemisphere is seeing the opposite—and the planet is less than 8,000 miles across. If you needed to move Earth 3.5 million miles closer to the sun to change winter to summer, you shouldn't have the opposite seasons happening just 8,000 miles away.

09_21_summer_season Earth's tilt means the sunlight shines more evenly across Earth's surface. Oli Scarff/Getty Images

Instead, the secret to the seasons lies in another quirk of Earth's orbit: The planet is tilted. Imagine a table in front of you, with Earth circling the sun flat on top of it. The Equator wouldn't be parallel to the table's surface, and the north pole wouldn't actually be pointing up, it would point a little to the side—23.5 degrees to the side, to be precise. That angle stays constant throughout the orbit around the sun, so in December the south pole is angled toward the sun and in June the north pole is.

09_21_spring_season April showers don't necessarily bring May flowers, but Earth's tilt definitely does. Oli Scarff/AFP/Getty Images

Earth's tilt is the legacy of the dramatic birth of the moon, likely in a giant collision that knocked Earth sideways. But while the tilt began violently, it now ensures that Earth's surface is more evenly exposed to sunlight. The seasons caused by the tilt are critical to life as we know it.

Many crops need winters and seasonal cycles, and humans would likely be unable to live comfortably in the eternal summer or winter that would reign at the Equator and poles.

09_21_winter_seasons Winter's cold will chill us, and it's going to cost more to warm us up. Drew Angerer/Getty Images

Most of the other planets in our solar system also have seasons, ranging from Venus's at about two-thirds the length of Earth's to Neptune's incredible 40-year-long seasons. Mercury, the planet closest to the sun, is the only exception: it has no tilt, so it has no seasons.

But no matter how much you hate your least favorite season, you're much better off here on Earth than you would be on atmosphere-free Mercury.

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