# How did Copernicus get the idea of the heliocentric model?

1. Apr 14, 2014

### Yashbhatt

The heliocentric model marked the beginning of modern astronomy. But what were the reasons which led Copernicus to think that the earth orbited the sun. I've read that his model was based on observations made by some other astronomer. Can anyone describe how he got this idea?

2. Apr 14, 2014

### maajdl

Because the mainstream model at that time was becoming utterly complicated.
Heliocentrism was much simpler.

http://en.wikipedia.org/wiki/Deferent_and_epicycle

Also, note that there saying "earth is center of the universe" has simply no meaning.
Similarly saying "sun is the center of the universe" has no meaning.
What really has meaning is saying that any system of coordinates is legitimate but that some are much simpler.

3. Apr 14, 2014

### Mordred

http://www.pitt.edu/~brg/pdfs/brg_iv_3.pdf

"Copernicus’s initial acceptance of the heliocentric
hypothesis that depends primarily on a simple computation of the sidereal periods
of Venus and Mercury (i.e., their heliocentric periods) motivated by his reading of
Aristotle with the commentary of Averroes (twelfth century A.D.), Vitruvius (first century B.C.), and Martianus Capella (early fifth century A.D.), among others.

4. Apr 14, 2014

### Yashbhatt

@maajdi Does being simple mean that the model is correct?

5. Apr 14, 2014

### micromass

Staff Emeritus
First of all, you should realize that Copernicus wasn't the first person to consider the heliocentric model. The Ancient Greek Aristarchus was first: http://en.wikipedia.org/wiki/Aristarchus_of_Samos His model was totally rejected by the ancient Greeks. The criticism was actually quite scientific in nature and not religious. The main criticism was that "if the earth moves, then how come the people and the air aren't left behind?" Of course, this was quite sound criticism and was only really solved much later by Newton's law of gravitation and laws of motion. Still, even Newton's answer would not be accepted by the Greeks because it merely gave a way to calculate things, but it did not tell us what gravity really is.

Another rebuttal against Aristarchus is that if the earth moves, then we should somehow see the stars move. But the Greeks did not see the stars move. Of course, they had no idea how far the stars were from the earth.

So the Greeks rejected heliocentrism, but there were two systems that they accepted. The first was the system invented by Aristoteles and Eudoxos. This consisted out of many (over 50) spheres which move together. The attractive part about this theory was that it explained the movement of the planets and didn't only quantify it. This theory was the most accepted theory by the Greeks and in the middle ages. It is also the theory that the Catholic church accepted.

The second theory was by Ptolemy and said that the planets moved in circles upon circles. These are called epicycles. The theory was quite reliable, but complicated. It was only really accepted in order to calculate stuff.

The only indication of why Copernicus wanted to overthrow the geocentric model is apparently that he was "aroused by divergent thoughts". Maybe he learned the theory from Aristarchus and became interested in it?

Copernicus' model was rejected by most astronomers of the time, and for quite good reasons too. Of course, there was the biblical reason that God was the center of the universe and not the sun. But there were many scientific reasons too. First, his theory was not more accurate than the two Greek theories, it was just less complicated. And of course there were the two arguments above that the Greeks already gave. One astronomer calculated that if the heliocentric system were true, then the stars would move. Since they don't, they had to be very far away and he calculated that they had to be at least four million times the radius of the earth away ($\sim~24\cdot 10^9$ km, which if my calculations are not mistakes is $22$ light hours). This was an inconceivable distance for the astronomers of then. It was only in the nineteenth century that Bessel finally measured the parallax of the stars.

Copernicus' model and the fate of heliocentrism were saved by Kepler who in a work of experimental genius found the correct laws. These laws were even more controversial than heliocentrism. Indeed, he said the planets moved on ellipses and that they did not move with constant speed. This was unacceptable for the time. Circles were the perfect figure so the planets had to move on circles. The notion of nonconstant speed was also as controversial. But fact is that Kepler's laws are very simple, very easy to calculate and supported the observations. This convinced many to throw out their philosophical, religious and physical beliefs and to side with heliocentrism. Of course, it wasn't until Newton that Kepler's laws were actually explained.

6. Apr 14, 2014

### micromass

Staff Emeritus
If a simpler model fits the observations as well or even better, then we should always consider the simpler model.

In the case of heliocentrism, Kepler's laws were simpler and fitted more with observations.

We should keep in mind the goals of science. The goals now are to quantify and make mathematical predictions. If a certain model makes accurate predictions, then it will be accepted. Whether the model is "true" is not a question of science. So the only thing we truly care about is to measure and predict the orbits of the planets. If heliocentrism requires much less computational effort for better observations, then that is the model that will be used.

What happens in the true universe is unkown. Maybe some theory much different than heliocentrism is "really" true. We will never know that. All we care about is mathematical calculations in science. What model is really true is a matter for philosophy.

As a matter of fact, Einstein's theory of relativity implied that heliocentrism too is false. It implied that there is no prefered reference frame to begin with. This theory fits the observations even better than heliocentrism and is so far the best known theory of the universe. Is this the "true" theory? I doubt it.

7. Apr 14, 2014

### maajdl

"Everything Should Be Made as Simple as Possible, But Not Simpler." Said Einstein.

Copernicus was a good example of that.
By removing any useless complication, progress is easier.

Sticking to earth-centrism is always possible.
But the complication it involves blurs the understanding.
Understanding implies simplicity.

8. Apr 14, 2014

### maajdl

Exactly.
A model is true only if it is verified experimentally.
Claiming that "the earth is the center of the universe" cannot be verified experimentally.
This claim has simply no meaning.
Similarly, claiming that the sun is the center of the solar system has no meaning.

What really has meaning is that:
When taking the sun as center of the coordinate system,
the motion of the planet is described by simple trajectories and simple laws.

9. Apr 14, 2014

### Yashbhatt

This was the case when the model was actually correct. But what if someday we find some phenomenon which is against current laws of science but can be explained by some abstract methods, then shall we accept these new ways as "true" and scrap off the other laws? I don't want to sound as if I am against Science or it's aim. I am very much in favor of simple laws. I am asking this just out of curiosity.

P.S: I think such events have already took place with relativity and quantum mechanics.

10. Apr 14, 2014

### micromass

Staff Emeritus
Scientists already are dealing with the case that all the known laws are inconsistent. So yes, there are things that science nowadays cannot explain nor quantify. The famous example here is QM versus GR. So if somebody were to come up with some abstract theory that could actually make accurate predictions that the current theories cannot, then scientists would be forced to accept this abstract theory.

A physical theory does not have to be intuitive or descriptive. Take QM for example, it is very abstract and mathematical. It does not really explain why things happen. But it does offer very accurate predictions, hence it is used.

Now, what if we were to discover some brilliant new theory that would explain more than we know today? Would we then burn all the previous theories? No, those theories remain very good approximations.
For example, Newton's theory of classical mechanics becomes false when the speed is too large or when the size is too small (roughly). But we still teach classical mechanics in high-school, in university, to engineers. The reason is twofold. First, the "more correct" abstract theory would be very complex and is not something you can readily understand without knowledge of the previous 'less correct theories". Second, Classical Mechanics still remains a very good approximation to the real world. The "more correct" theory of relativity is mathematically and conceptually way more complicated. If an engineer now wants to build a house, then classical mechanics is good enough, he will never need to resort to relativity!

So even though there are 'more corect' theories than classical mechanics, that does not mean that classical mechanics is outdated. It is still a very good and simple theory and it still approximates the real world to a very high degree. Relativity just approximates it better, but the better approximations are so insignificant for our daily life that we can just ignore them.

Same things, if a new abstract theory were to be invented, then I'm sure it would be extremely complicated. However, the current theories of QM and GR will usually be good enough approximations to our reality. It is only in very distinct cases that the new abstract theory is needed.

11. Apr 14, 2014

### maajdl

Simplicity without exactness is what a crank produces.
Simplicity is removing anything useless, but keeping things right.
The next step is to see a new things because of the clear view.

12. Apr 14, 2014

### Yashbhatt

@micromass I was just trying to say what you mentioned. Things may get complex. Simple one is not always the right one.

Last edited: Apr 14, 2014
13. Apr 14, 2014

### SteamKing

Staff Emeritus
Yes, scientists have already grappled with this problem of deciding between competing theories. In the absence of other compelling data, the convention is to apply Occam's Razor:

http://en.wikipedia.org/wiki/Occam's_razor

14. Apr 15, 2014

### maajdl

General Relativity is "simpler" than both the Ptolemaic and Copernician system,
in the sense that General Relativity contains less useless concepts.
This is obvious with respect to the Ptolemaic system, where epicycles are useless and easily eliminated by chosing a proper frame of reference.

But it is also true with respect to the Copernician system and the Galilean relativity.
The sun or the inertial frames are not privileged any more and there is no special system of coordinates for this is physically a useless concept.

However, this does not mean that General Relativity is easier to learn!
It doesn't even mean that General Relativity contains less information!
On the contrary, General Relativity cannot only "explain" the older concepts in its own logic, but it can predict or explain many more phenomena than the old theories could.