I suggest that you read post 16.4 before this.

You have probably been taught that the sun doesn’t move around the earth. But you can see that it does! What’s happening?

Let’s start with what you can see. This is always a good place to start. It’s much better than blindly accepting what somebody else tells you. In the morning you see the sun rise in the east. It then moves upwards towards the west, reaching its highest point at mid-day. It then moves downwards towards the west. Finally it sets in the west. It certainly seems to be moving around the earth. You could confirm this by phoning someone on the other side of the world. When it is mid-night for you, it is mid-day there. Before the invention of clocks, the shadow cast on a sundial was used to tell the time of day.

If you think about it, you would see the same thing if the earth were rotating, around an axis through the poles, instead of the sun moving around the earth.

Does the earth rotate in this way? The best demonstration that it does was made by the French physicist Foucault in 1851. He hung a huge pendulum inside a very tall building in Paris, called the Panthéon. When it was first hung it was at rest. But as soon as it was displaced, it started to swing. As it swung, backwards and forwards, the pendulum also appeared to rotate around its original rest position. This result can be explained by the earth rotating underneath it, as shown by the video in this web site:

https://en.wikipedia.org/wiki/Foucault_pendulum#/media/File:Foucault-rotz.gif.

By measuring this rotation of his pendulum, and knowing the latitude of Paris, Foucault was able to measure the rotation of the earth. He found that it rotated about its axis once every 24 hours which must be the correct answer because it’s the length of a day.

If we could view the earth from the sun, we would see the earth rotating about its axis once in every 24 hours (1 day). We would also see it in orbit around us, taking 365 ¼ days (1 year) to complete each orbit.

So the sun appears to move around us because we view it from a rotating world. The mathematical way of expressing this is that we are viewing the sun with respect to a rotating frame of reference. It then rotates around us. Is there anything wrong with using a rotating frame of reference? No! We are forced to make our observations in this way because we view the universe from a point on planet earth. If you want to explain how a sundial works, it is much simpler to do it in our rotating frame in which the sun moves around us and the sundial is fixed. For exactly the same reason, physicists often use rotating frames of reference to explain the effects of spinning atomic nuclei which are exploited in medical MRI scans of the human body.

So why does everyone tell you that the earth doesn’t move around the sun. Many of them don’t understand the concept of relative motion (the movement that you observe depends on how you are moving yourself – Post 16.4) and most of them have never heard of a rotating frame of reference.

But there is a more interesting reason for thinking what the universe would look like when viewed from the sun.

Viewed from our rotating frame of reference, the motion of the planets is very complicated and would be difficult to explain. However, if we calculate what things would look like viewed from the sun, a much simpler picture emerges. Now earth and all the other planets we can see move around the sun in elliptical orbits. This provides a much simpler picture of our local part of the universe than that provided by our rotating frame of reference. Following the advice of the monk/philosopher William of Occam, who lived over 600 years ago, we adopt the simple description in preference to the more complicated one. The decision to choose the simplest of a set of possible explanations is called Occam’s razor.

With this view of our local universe, called the solar system, it is possible to explain the movements of the planets. They are then maintained in their orbits by the gravitational pull of the sun. By trying to understand the motion of planets, Newton was able to formulate his theory of gravitation.

As usual, thinking about something is more interesting and useful than simply remembering a “fact” that somebody has told you.

 

Related posts

16.4 Movement

Follow-up posts

16.12 Measuring movement
16.14 Aliasing
17.29 Centrifugal force