The Hypothesis of Copernicus and Tycho

15. The same appearances of the planets can be explained by various hypotheses

In the middle of the sea during calm weather, a person on one ship looks at other ships changing their position relative to each other. They can often doubt which of them, even their own ship, is moving (which causes this change in position).

Likewise, when seen from the Earth alone, it is not easy to know which planets are moving.

Their movements are very unequal and complicated. This makes it difficult to explain them unless we choose one particular way among the various ways.

This is why astronomers have devised 3 different hypotheses.

16. ¹ The Ptolemaic hypothesis does not satisfy appearances.

The first of these is Ptolemy’s, which is now generally rejected by all philosophers because it contradicts many phenomena such as the increase and decrease of light observed in Venus as in the Moon.

Therefore, I will not explain it.

17. ^2-3 The hypotheses of Copernicus and Tycho do not differ as hypotheses.

The second is Copernicus’s. The third is Tycho Brahe’s.

These two satisfy the phenomena in the same way. There is not much difference between them, except that Copernicus’s is somewhat simpler and clearer. Tycho did not need to change it, except because he was attempting to explain a hypothesis and the truth itself.

18. Tycho in words denies, but in reality assigns more motion to the Earth than Copernicus.

Copernicus did not hesitate to attribute motion to the Earth. But Tycho considered it very absurd in physics and contrary to common human sense. And so he sought to correct it.

But he did not sufficiently consider the true nature of motion.

And so, he asserted only in words that the Earth is stationary. While in reality, he assigned more motion to it than to the other.

19. I deny the Earth’s motion more accurately than Copernicus and more truly than Tycho.

I differ from both only in this: I will more truly than Tycho and more carefully than Copernicus remove all motion from the Earth.

I will propose here the simplest hypothesis of all, and most suitable both for understanding the phenomena and for investigating their natural causes.

I wish it to be regarded only as a hypothesis, not as the truth.