To Every Thing There is a Season: But Why?

It’s fall here in the northern hemisphere, but have you ever wondered why the seasons fall when they do?

People sometimes think that the Earth is furthest from the sun in the winter, but this isn’t actually the case, at least north of the equator. We actually make our closest approach to the sun (also known as the perihelion, perigee, or periapsis), in early January and are farthest from the sun (aphelion, apogee, or apoapsis) in early July; the exact dates vary due to the addition of leap days and our orbit actually being 365.2421897 days long. And while most depictions of the Earth’s orbit make it look like a highly elongated ellipse, in reality it’s rather circular. Our closest approach puts us 91,402,640 miles away from the sun, while the farthest we get from the sun is 94,509,460 miles. That’s only a difference of a bit more than 3% but most diagrams, including the one below, make it look like we travel twice as far away on the aphelion.

Diagram of earth's orbit around the sun, in a highly exaggerated oval, showing the equinoxes on March 21 and September 23, solstices on December 21 and June 21, Periapsis (perihelion) on January 3, and Apoapsis (aphelion) on July 3.

Approximate significant dates in our orbit around the sun.
Image via Gothika on Wikipedia.

If our seasons are largely unaffected by our distance to the sun, then what causes the change in seasons? The tilt of the Earth’s axis. The Earth’s axis of rotation is currently offset about 23.4° relative to its orbital plane. We generally talk about the poles tilting toward the sun in summer and away from the sun in winter, but this can give the false impression that the Earth is wobbling back and forth throughout the year. Aside from tiny variations caused by the pull of the moon, changes in ice cover, and powerful earthquakes, the tilt actually remains constant relative to the rest of the galaxy but changes relative to the sun as it travels around it, as seen below.

Animated gif of the earth traveling around the sun with the poles drawn in to show how the north pole sometimes points away from the sun and other times points towards it.

Note the position of the North Pole as it orbits. Science-y gifs make me happy!
Image via Tfr000 on Wikipedia.

Why is it that the relatively small change in distance from the sun due to axial tilt affects temperatures more than the 3 million mile difference due to the elliptical orbit? Changes in the amount of sunlight reaching the surface. On the spring and autumnal equinoxes, the poles are equidistant to the sun, so the days are more or less the same length no matter where on Earth you are. As the Earth continues on its orbit, one pole moves ever so slightly closer each day (leading to increasingly longer days) and the other moves ever so slightly farther way (leading to increasingly shorter days). By the time the solstices arrive about three months later, one pole will be totally in the dark and the other will have sunshine around the clock, though the sun will be very low on the horizon in the nighttime hours. The angle at which the sunlight strikes the earth affects how much heat energy is transferred. In the graphic below, the areas where the sun is directly overhead will be warmer than the areas where the sun strikes at an angle because the energy is less concentrated and travels through a thicker layer of atmosphere to reach the surface. (However, the climate of any particular region is also affected by many other factors: altitude, oceanic currents, proximity to oceans and/or mountains, etc.)

Diagram of how the sun's rays strike the earth during the northern winter/southern summer. Arrows show that the rays strike from directly overhead just below the equator and at steeper angles near the poles

More sunlight strikes the southern hemisphere during its summer.
Image via Rhcastilhos on Wikipedia.

So why don’t the seasons on the calendar really align with the seasons as we experience them? In the northern hemisphere, the first day of summer on the calendar falls around June 21, which is the longest day of the year. Logically, this seems like it should be the hottest day of the year, right? Summer-like weather usually does start before this date, but rather than cooling off as the days get shorter, it actually continues to get hotter through July and August. The reverse holds true in winter; cold weather sets in before the official start of winter around December 21, the shortest day of the year, but the coldest weather comes as days get longer. This seasonal lag mostly occurs because the atmosphere takes a while to fully absorb the extra heat that comes in as the days get longer and to radiate away the heat that isn’t being replaced as the days get shorter.

Have any pressing science questions you’d like to see answered in a future column? Leave them in the comments!

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