What causes the seasons?

Outside the tropics we experience four seasons – Spring, Summer, Autumn, or Fall, and Winter. These occur because the Earth’s axis about which it rotates once a day is tilted at an angle relative to the Earth’s orbit around the sun. Because the axis always points towards the north star throughout the year, the seasons are cyclical. In the northern hemisphere, when the North Pole points towards the sun, the sun’s light is more directly overhead at mid-day and the sun is in the sky for longer; it is summer. At this time the opposite is true in the Southern hemisphere.

Spring and Autumn are the half-way points when the Earth’s tilt is neither angled towards or away from the sun. These seasons usually have milder temperatures than the extremes of winter and summer. The difference between spring and autumn is essentially one of biology as the organisms experience warming day after day in Spring and respond accordingly and cooling day after day in Autumn.

So, all that in hand, it must have been quite confusing for viewers of the BBC’s latest scientific blockbuster “The Wonders of the Solar System” to watch the earth wobbling like a spinning top as it orbited the sun and learning that this change is the cause of the seasons. Actually, the way they showed it, the Earth was static and the sun was orbiting it.

Now, the producers were actually using the 3D graphics to show the relative position of the tilt of the Earth’s axis to the “fixed” sun. Whereas what they showed did resemble astronomical precession (which does not cause the seasons). Precession is the regular oscillation of the axis of any spinning object whether gyroscope or planet, that occurs as the object rotates.

Precession will be familiar to anyone who has played with a child’s spinning top or a gyroscope (a spinning top for grown ups). The main axial oscillation is slower than the spinning but is quite visible. For the much bigger Earth, spinning on its axis once a day, the oscillation of that axis is much slower. The main large precession of the Earth’s axis takes tens of thousand years to complete a single loop.

Despite precession being a long timescale feature of the Earth, it has a number of observable effects, if you’re willing to wait. First, the positions of the south and north celestial poles appear to move in circles against the space-fixed backdrop of stars, completing one circuit in 25,771.5 years (measured at the year 2000 rate). So, the north star, Polaris, today lies approximately at the north celestial pole, this will change over time, and other stars will become the “north star”, today’s north star was not the north star seen by the earliest human navigators thousands of years ago. It also provides a point of confusion for historians looking at ancient star charts and scientists must take it into account in climate studies, for instance.

I modified this post after Brian Cox commented. I *did* understand what they were showing and what they were trying to show. But, I and others, felt the imagery might have been somewhat confusing for the average viewer.

9 thoughts on “What causes the seasons?”

  1. @Brian Thanks for your comments on the post. Point taken. I’m really enjoying the series, so apologies for any offence. It just seemed that one graphic stood out as likely to confuse. Others seemed to agree that it was an odd representation, although that discussion on John Gribbin’s Facebook page seems to have disappeared.

  2. And one more, while I’m irritated.

    The Northern Lights / spirits issue has also attracted a lot of comment. In the full piece to camera, I recount a Norse myth that our guide told me about the origin of the Aurora. The myth is that if a woman dies as a virgin,her spirit cannot pass from Earth to Heaven, and is trapped in-between. I thought it was a nice bit of mythology, so I told the story. The only bit that made it into the final cut was the last sentence. I do have editorial control over these things, so I could have insisted that either the full story was included or the thing was cut, but I decided that it was harmless enough and let it go. But I assure you I am not about to convert to Odin Worship, or whatever!


  3. David,

    I am more than a little irritated by the tone of this article. Let me explain the graphic:

    The camera position is indeed, as you point out, fixed to the Sun. It follows the Earth around in it’s orbit, and one frame is taken every 24 hours. It’s not too difficult to see that as viewed from the Sun the North Pole won’t be visible in December, and the South Pole won’t be visible in June.

    I agree that one would usually think of point at which the Sun is vertically overhead at noon oscillating about the equator between the tropics during the year, but it is equally valid to fix this position center frame (i.e. camera at the Sun). Something has to oscillate, and if this position is fixed in frame then you get the graphic as shown.

    Now you can argue that you don’t like this reference frame (the idea actually came from a NASA tutorial which myself and the production team thought was an unusual and interesting way of showing the seasons pass) but you should have been able to work out what was happening with a little thought. Indeed, I would say that anyone who vaguely understands axial precession would not have jumped to some of the conclusions you did in this article. It was not badly drawn (because it’s correct – the only approximation is that the model had a circular orbit) and I can assure you that I understand solar system dynamics sufficiently well to understand the seasons.


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