Hi, I'm Peter Adamson and you're listening to the History of Philosophy podcast brought to you with the support of the Philosophy Department at King's College London and the LMU in Munich online at Historyofilosvy.net.

Today's episode, best of both worlds, Tiko Brahe.

Let's do an experiment with apologies to visually impaired listeners who may not be able to take part.

Extend your arm and look at your fingertip while closing one eye. Now look at it with the other eye. You should notice that the finger seems to shift its position very slightly.

Now do the same thing holding your finger much closer to your face. You'll see the same effect but with a bigger shift. This is called the phenomenon of parallax.

I mentioned it last time in passing but wanted to start this episode by explaining it more fully because it's going to play such a major role in the 16th century reactions to Copernicus that will be concerning us today.

As Copernicus was well aware, the parallax effect can be used to determine how far away things are.

The calculation is based on the apparent difference in position of the same thing seen from two different places. These could be your left and right eyes, but might also be much further apart, say two different cities, or the same city at two different times as the relative position of the earth and the heavenly bodies changes.

If you're a Copernican, it's the location of the city itself changing as the earth spins and orbits the sun. If you're in the older system of Aristotle and Ptolemy, the city stands still on an unmoving earth while the heavens revolve above.

You might be thinking that the parallax effect should have yielded powerful arguments in favor of the Copernican system. After all, that system was broadly correct. The earth is going around the sun, not vice versa.

So you'd think anything that would provide more accurate data about the situation should have helped the case for heliocentrism.

And as we'll see, parallax did help to undermine the ancient Ptolemaic cosmology, but parallax was also the basis of a powerful objection to Copernicus.

One of those who articulated it was Tico Brahe, the greatest astronomer between Copernicus and Johannes Kepler. He reasoned as follows.

If the earth is going around the sun, then we should have a significant parallax effect as its position changes moving all that way along its annual orbit.

Yet we see no parallax at all for the fixed stars, which were assumed by most astronomers to be embedded in a single sphere at the edge of the visible universe.

It rotates once per day in the old theory and stands still while the earth spins once each day according to Copernicus.

This means that the sphere of fixed stars would have to be further away than supposed by earlier astronomers, much, much further away.

The distance between the last planet Saturn and this outermost sphere of fixed stars would be Brahe calculated, at least 700 times greater than the distance between the Sun and Saturn.

Copernicus simply admitted this consequence of his theory, and he was right. There is parallax for the distant stars.

It was just far too small to see with the instruments available at the time. The tiny effect was first successfully measured in the 19th century.

And the stars are, of course, vastly further away from Saturn than Saturn is from the Sun. Brahe's figure of 700 times isn't even close.

Saturn is 9.5 astronomical units from the Sun, whereas the closest stars in Alpha Centauri are a whopping 268,000 astronomical units away.

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