Copernicus Journal for Young Scientists

A Copernican Anomaly

Anomaly!

by Flip Schrameijer

Let’s once again take the example of Copernicus (1473-1543; see his bio in this journal). Copernicus found that something must be wrong with the way people explained the motions of the sun, the moon and the planets. He found an anomaly.

Ptolemy

In Copernicus' time the idea that the Earth is the centre of the universe, had existed for at least 2000 years. This theory was written down by the Greek astronomer Ptolemy (90-168 AD, see portrait). Up to Copernicus’ time the thinkers of the Western world still believed this theory. It said that the universe is a closed space – comparable to a glass toy in which it snows when you shake it. Beyond it there is nothing. In the middle is the Earth as a fixed, immovable thing, and all celestial bodies, including the sun and the fixed stars revolve around it.

Sure enough, as has happened often in the history of science, there had been doubts before. Some Greek, Islamic and Christian scientists had even proposed to put the sun in the middle. But they were dismissed. For instance by the Greek philosopher Aristotle, who lived more than three centuries B.C.

Aristotle said that if the Earth actually spun around it’s axis, why then didn’t objects fly off? And if the Earth circled the sun, why didn’t it leave behind the birds flying in the air?

Yes, why indeed? Why don't we fly off the world like in a merry-go-round that runs too fast? Why do we still have an atmosphere that we can breathe? Do you know the answer?

strange movements across the sky

Since even Copernicus thought the stars had fixed positions, as if on the surface of a glass sphere, there was a big problem with the sun, the planets and the moon. With the Earth in the middle, these celestial bodies behaved rather strangely indeed.

Apparent motion of Mars across the sky

Ptolemy’s theory was based on observations of the heavens and contained a great deal of complicated arithmetic. In his system each planet traveled along a path of large circles, while at the same time they moved in smaller circles. Ptolemy used it to explain the irregularities one could see with the bare eye, such as changes in brightness.

Some motions in particular, needed elaborate mathematical explanations. Such as changes in speed. Most puzzling however were certain motions by which planets seemed to stop and move backward and forward. See figure and film. (This is what you see when you look at the sky for 10 months: the apparent movement of Mars across the sky.)

In Ptolemy’s time these movements could still be explained through complicated mathematics. Look here to see how Ptolemy thought the planets were moving around the Earth. Strange isn't it?

In the following centuries however the observations became more accurate.

The two planets circle around the Sun at different speeds!

This made the calculations even more complicated and thereby less and less convincing. At least to Copernicus. These strange back and forth-movements became one of his most important anomalies. He solved this puzzle by putting the sun in the middle of the universe, as one can see in this animation.

Copernicus’ revolution

Looking at the blue line you can see how complicated this movement is, when you believe the Earth is the centre of the universe. And you see too how much simpler it is when you understand the Earth is just one of the planets.

This is Copernicus' model of the Earth and other planets circling the sun.

Although his successors still had a lot of hard work to do, the solution to this problem marks the beginning of Copernicus’ revolution. It is, as it were, the crowbar by which the world as one knew it changed beyond recognition, once and for all.

Links

The Copernican Model , University of Tennessee

Applet Retrograde Motion explains the strange movement of Mars and other planets across the sky

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Last Updated ( Wednesday, 16 August 2006 )

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