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Reasons for the Seasons


Last updated 2/15/2024 at 11:35am

Remember this fact for your next trivia night!

In 1989, filmmaker Matthew H. Schneps released a 20-minute video titled "A Private Universe" in which he interviewed recent Harvard University graduates, faculty members and alumni – some of whom had science backgrounds – and posed to them a simple question: What causes our seasons?

Twenty-one of the 23 people interviewed did not know the answer. Oh, they gave elaborate explanations and tried to be convincing, but it was clear that they had no clue. The most common reason they offered was that the heat of summer occurs because the Earth is closest to the sun at that time of year, and the cold of winter is caused by our greater distance from the sun.

Sounds perfectly reasonable, doesn't it? But it's wrong. Very wrong.

To be fair, most respondents were correct that the Earth orbits the sun not in a circular path, where our distance from our star would remain the same, but rather along an ellipse – a kind of squashed circle. As such, our distance from the sun does change throughout the year.

But this change amounts to only about 3.3%, hardly enough to affect seasonal temperature changes on our planet. And if you want to be really exacting, the Earth reaches its closest point to the sun in January and its farthest in July!

In 2024, the Earth will reach its nearest point to the sun ("perihelion") on Jan. 3 when we will lie 91,404,010 miles away. Of course, this is during the Northern Hemisphere winter. Our farthest point ("aphelion") won't arrive until July 5 (during the Northern Hemisphere summer) when we'll be 94,512,027 miles away from the sun.

So if it's not our changing distance that causes our seasons, what's really going on here?

We experience seasons because our planet's rotational axis is tipped by 23.4 degrees to the plane of our orbit around the sun. This means that, from about mid-September through mid-March, the Earth's Northern Hemisphere is tilted away from the sun, causing the sun's rays to shine on us at a very shallow angle. Just watch the sun move across the daytime sky of winter and you'll see how low it passes and how long your shadow is during midday.

Six months – and half an orbit – later, our planet's tilt aims the Northern Hemisphere toward the sun, and Solar rays shine more directly down upon us. The sun in June, for example, crosses quite high in our sky and sunlight beats directly down upon us. Again, check out how short your shadow appears at midday.

But this simple fact was completely lost on those being interviewed in the video. Oh, did I mention these were graduates and faculty of a prestigious university?

I remember being shocked by this revelation at the time, but if we could do the same interview today, might we get the same results? I suspect we would – perhaps they'd be even worse. Science education in our nation has been woefully inadequate for many decades, and few things demonstrate this more than highly educated people not understanding basic facts of science, like the reason for the seasons.

OK, so maybe they didn't know, but you do!

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