Hopx asked:

What is the significance of Tycho's model for whether Galileo's model should have been judged as
fitting the orbits of the planets? I don't see any negative evidences in Tycho's model.

What is wrong with the model?

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Tycho was the most accurate observer of the heavens before the invention of the telescope, and his
data as exact as they can be made by looking with the naked eye and measuring instruments.
Therefore his tables are not in question. His measurements of the famous comet's trajectory
demolished the thousands-year-old theory of a division between the sub- and superlunary sphere.
That was a hard nut to swallow, although most people came around in short order to accepting the
implications. The problem lies with the model of the solar system which he devised for the benefit of
those who were scared of Copernicus' system. It was meant to be a compromise between Ptolemy
and Copernicus, but it was a silly idea. Tycho left the Earth in the centre, circled by Mercury, Venus
and the Sun, with all the outer planets revolving around the Sun.I'm sure even you will see at once
how absurd a proposition this is if you draw a graph. (Hint: where is Saturn when it's nearest Earth in
its orbit?). Galileo made mincemeat of it in his book on the World Systems; but the Church had come
to a sort of accommodation with it and the Pope had explicitly asked Galileo not to mention it. In
short, the issue was not scientific alone, but religious and political. Galileo was a hot-tempered,
belligerent man and couldn't stop himself, despite having promised.

But you realise, I hope, that Galileo's model is none other than the Copernican. So I'm not altogether
sure what your first sentence is all about, unless you mean that Kepler had already derived from the
Tychonic tables the world-shattering news that planetary orbits are elliptical, not circular (as
Copernicus still believed). But I don't think this was a problem for Galileo. So I hope, anyway, that the
above tells you what you want to know.

Jürgen Lawrenz

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