Earth’s Aphelion Isn’t the Reason for the Seasons

Earth’s Aphelion Isn’t the Reason for the Seasons

Good morning! Top o’ the orbit to you!

At 8:06 P.M. UTC (4:06 P.M. EDT) on July 6, 2023, Earth will reach the point in its orbit when it’s farthest from the sun. In a sense, it’s like our planet will be at the top of a gravitational hill and will then begin to fall toward the sun until January 2, 2024, when it will reach its closest point. Then the cycle will begin anew, with Earth’s momentum taking it farther away from the sun until the next time the planet reaches its maximum distance once again.

This cycle happens because Earth’s orbit isn’t a circle. It’s almost a circle, with a small but significant deviation from Euclidean perfection that actually makes Earth’s orbit a slightly squashed oval—that is, an ellipse. The sun doesn’t sit at the center of Earth’s ellipse but instead at one focus, a point along the long, or “major,” axis.

Our star’s lopsided locale within Earth’s orbital ellipse means that over the course of a year, our planet’s motion brings it alternately a bit closer to the sun and then farther away again. When Earth is closest to the sun, we say it’s at perihelion—from the latinization of the Greek words peri (“near”) and helios (“sun”). The farthest point is called aphelion, from the Greek apo (“away from”).

The exact distances of Earth’s perihelion and aphelion change a bit from year to year because of the gravitational influence of the other planets, as well as that of the moon. But as of 2023’s aphelion, Earth’s center will be 152,093,250 kilometers from the center of the sun.

The average distance between Earth and the sun, what astronomers call an astronomical unit, is defined as 149,597,870.7 km. So this time we’ll be a little bit more than 1.5 percent farther out than average. The last perihelion, if you’re wondering, was on January 4, 2023, and Earth’s center was 147,098,924 km from that of the sun, or about 1.5 percent closer than average. The small values for these orbital offsets explain why our planet’s path around the sun looks so much like a perfect circle. And they correspond with only about a 3 percent change in the sun’s apparent diameter in Earth’s sky—far too small to notice with the naked eye, especially if you’re blinded by inadvisably staring at the sun without proper protection.

The most surprising thing about this cycle, though, is probably when its extremes occur on the calendar: perihelion is in January every year, while aphelion is in July. That means in the Northern Hemisphere we’re closer to the sun in winter and farther in summer—the exact opposite of what you might expect.

The lesson here is that our seasons don’t really depend on Earth’s distance from the sun. The real reason for the seasons is that Earth’s spin axis is tilted by about 23 degrees, compared with the plane of its orbit—an arrangement that tips Earth’s North Pole toward or away from the sun over the course of a year. The North Pole is tipped most toward the sun during the solstice in late June—the one day out of every year in which the sun is highest and spends the most time above the horizon in the Northern Hemisphere. The heat of the Northern Hemisphere’s summer arises from the season’s longer, brighter days, allowing better exposure and more time for the sun to warm the ground. During the other solstice in December, Earth’s northern axis is tipped most away from our home star, the sun is lower in the northern sky, and daytime is shorter in the Northern Hemisphere, resulting in winter’s chill in that part of the globe. Earth’s axial tilt also neatly explains why seasonal temperatures scarcely change around our planet’s equator because the sun’s shifting position overhead is muted at midlatitudes.

Even so, Earth’s changing distance from the sun does affect our planet’s temperature but only by a little bit. Although the physics is a bit involved, in the end, the temperature difference caused by the changing distance is about five degrees Celsius. This is far less than the average seasonal temperature change for midlatitudes. Where I live in Colorado, for example, the temperatures can swing by as much as 60 degrees C over six months, so a five-degree-C change would hardly be noticeable. This effect does somewhat ameliorate the summer highs and winter lows, however.

Conversely—or obversely, really—in the Southern Hemisphere, the situation is reversed: Earth’s South Pole is tipped toward the sun most in December and tipped away from it most in June. The Southern Hemisphere’s seasons are the opposite of those in the Northern Hemisphere. That’s why I’m careful to use the terms “June solstice” and “December solstice” rather than “summer solstice” and “winter solstice,” respectively, when I talk about these events, so as not to confuse anyone on the other half of the planet. Being hemispherist is not a good look.

But this does mean Earth is closer to the sun in austral summer and farther in austral winter, so the corresponding plus-or-minus-five-degree-C shift can amplify seasons to be more extreme in the Southern Hemisphere. Then again, most of the planet’s surface south of the equator is covered by ocean—and water absorbs and retains more heat than land—which tends to dampen such temperature extremes.

If you thought seasons were simple, well, nothing really ever is in science. That’s part of the wonder of it all because there’s always more to observe and more to know. The closer you look, the more details there are, just like examining Earth’s orbit and seeing it’s not a circle but an ellipse. And that’s not hyperbole.

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