Exposing PseudoAstronomy

December 24, 2013

The Reason for the Seasons

Filed under: astronomy,general science — Stuart Robbins @ 6:46 pm
Tags: , , , ,


As the majority of the world’s population celebrated the Winter Solstice a few days ago, a smaller number of persons celebrated a different event, the Summer Solstice. And yet, polls seem to show that somewhere around 20-60%* of people don’t seem to know what causes the seasons, and the statement I made in the first sentence would have them scratching their heads in confusion. Since this is the Exposing PseudoAstronomy blog where the original intent was to cover common misunderstandings and even media mistakes in astronomy, I thought I’d do a post on this.

*Poll 1, Poll 2, Famous 1980s video of Harvard Graduates.

Proximity to Sun

The most common thought is that seasons are caused by Earth’s changing distance from the sun. After all, Earth has an orbital eccentricity of 0.0167, meaning that from its average distance of about 149.6 million km from the sun, it actually varies between 147.1 million at its closest and 152.1 million km at its farthest. “Obviously,” the thinking goes, “this distance change should affect temperature!” It’s a whopping 5 million km.

And it does. Very slightly. Mars, with its higher eccentricity, has significantly warmer southern summers than northern summers because of this.

But on Earth, the day during which we are closest to the sun is around January 2, and it is farthest from the sun around July 5.

If you’re close to a heat source, you should be warm, right? But doesn’t winter happen in January for the majority of people around the world?

Therefore, this can’t be the cause.

What also doesn’t make sense for this explanation is that the northern hemisphere experiences the opposite seasons as the southern hemisphere. Though, many people are not that well traveled and wouldn’t know this from first-hand experience. However, being in Australia right now, and wearing shorts and a t-shirt, I can guarantee you that it is warm down here. Last week, it was around 40 °C, or about 105 °F. I took my friend’s nephew to the beach.

Earth’s Tilt

The only thing that can explain why a spherical object would experience different seasons in one part than another, at the same time, is if it has something not to do with something that applies to the entire object (like distance to the sun), but rather something that applies to one part and then the other, and can’t apply to both parts at the same time in the same way.



The solution is Earth’s tilt. Earth rotates about its axis, and it orbits around the Sun. Earth’s rotational axis is tilted relative to the path around the sun by about 23.5°. The explanation for seasons is given in the diagram above and has to do with the directness of light. Take a flashlight and aim it straight at a wall. You have a certain amount of light, and it’s spread over a small area. Now, aim it down but still at the wall. You have the same amount of light, but it’s spread over a larger area.

The same thing happens with the sun’s light. When the northern hemisphere is tilted more towards the sun, such as in June, the light from the sun strikes it most directly. But, the same quantity of light is spread over a larger area in the southern hemisphere. That given packet of energy cannot heat a wider area the same amount, so it is cooler. Six months later, same thing happens, but in reverse for the hemispheres.


The reason for this post is that I’m here in Australia and it’s warm. A few days ago was the Winter Summer Solstice, the longest day of the year. I didn’t experience the shortest day this year. And I kept having to correct myself a few days ago, just as above, because it really is hemisphere-dependent.

Similarly, right now it’s about noon on December 25, Christmas from TomorrowLand. For someone who grew up and lived his entire life except for 2 weeks (so far) in the northern hemisphere, it does not “feel like” Christmas (despite being an apathetic atheistic agnostic, everyone except perhaps in China and North Korea cannot help but be inundated by Christmas stuff). It’s warm. It’s sunny. I’m <1 km from the beach and last night I walked outside after dark with shorts and a t-shirt on.

It's amazing how many of our experiences are driven by the weather and climate, and how northern-hemisphere-centric many of us are. Even our emotional state and how we feel about certain events are tied with the weather. And yet, the larger, yearly cycles of seasons, are so poorly understood by hundreds of millions of people.

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