Explanations of stellar parallax that I have found involve examining the apparent motion of a nearer star relative to a background of more distant stars, as the Earth moves around the sun or as the Earth rotates. We can see the apparent motion because we have telescopes. In Tycho Brahe's time, telescopes were not available, and this kind of motion couldn't be seen. The fact that stellar parallax couldn't be observed is supposed to be evidence that Tycho and others took as evidence that the Earth didn't move.

However, as I understand it, Tycho believed that the stars move together in a fixed sphere, so it seems that he would not even try to look for parallax using the near star/far star method. He would have had to look for motion of the moon or a planet relative to the stars. The moon and planets were in constant motion, however. This doesn't mean that you can't test for parallax, but doing so would be more complicated. How was this done?


Parallax was always measured by measuring the angular distances between stars. If it changes during the year, you see the parallax due to the Earth motion. The accuracy is limited by the accuracy of your angle-measuring device. A typical such device at the time of Brahe could measure to 1' or perhaps 0.5' (1' is the average resolution of the human eye). This is not enough to detect any parallax of "fixed stars". Of course they always attempted to measure it, because this would decide whether the Earth moves or not (with respect to the stars). The negative result was actually one of the strongest arguments in favor of geocentric theories. To deal with this argument against his theory, Copernicus had to postulate really HUGE distance to the stars, the thing which did not look plausible to many.

Parallax was actually detected only in 18th century.

The whole story is told in great detail in this book:

Parallax: The Race to Measure the Cosmos, by Alan W. Hirshfeld

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  • $\begingroup$ Thanks Alexandre. I'll look at that book. I don't understand, however, why anyone would try to measure parallax using two stars if they believed that all stars moved on the surface of a single, fixed sphere. That Tycho believed that the stars were all the same distance is claimed by Richard DeWitt in his book Worldviews, on page 134. This is not a regular scholarly historical study; it's an undergraduate textbook, but so far I've found that DeWitt tries to get the historical research right most of the time. $\endgroup$ – Mars Sep 26 '17 at 21:14
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    $\begingroup$ It is a strange sentence: "why would anyone measure...if they believed...". One reason why "they believed" is exactly that nobody has detected parallax. This argument is given in Ptolemy, together with other arguments. Scientists always try to test such "beliefs". This is how science works. $\endgroup$ – Alexandre Eremenko Sep 27 '17 at 13:29
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    $\begingroup$ But the belief Tycho and Copernican were arguing about, at least as its usually presented, isn't whether the stars were on a fixed sphere, but whether that sphere was near or far. Parallax against a background doesn't resolve this question if all the objects you're observing are on that background. Both models predict the same results. $\endgroup$ – simplicio Sep 27 '17 at 17:26

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