19 000 Earth radii, whereas, as mentioned previously, the lack of annual stellar parallax implied a distance of well over 1 million Earth radii to the stars in the Copernican universe. This enormous implied gap between Saturn and the stars containing no heavenly bodies at all with, as Tycho Brahe would later observe, no conceivable purpose, was yet another reason to be sceptical about the heliocentric theory. The modern values are shown in the third column, and it is clear that Copernicus' theory gave very good results for the relative dimensions of the planetary orbits.

The reception of Copernicus' theory

Since the work of Isaac Newton in the latter part of the seventeenth century, the theoretical calculation of the positions of heavenly bodies and the physical nature of the Universe have been linked inextricably through the theory of gravitation. But in the sixteenth century, most astronomers thought of these as entirely separate subjects, and the two parts of On the Revolutions - the first dealing with the physical reality of the Earth's motion and the second with geometrical methods for astronomical calculations - were received very

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