Routledge History of Philosophy Volume IV

112 SCIENCE AND MATHEMATICS UP TO DESCARTES
in that language rather than the mutually accessible Latin, and the result of
Galileo’s side of the exchange was published in 1613 as Istoria e Dimostrazioni
intorno allé Macchie Solari.
Besides a rather fruitless dispute about priority, the authors disagreed about
the nature of sunspots, with Galileo favouring the view that they were like clouds
around the Sun, thus emphasizing the theme of mutability of the heavens. But
other matters were discussed, the most important of which was undoubtedly the
phases of Venus. It is a notable consequence of Ptolemaic theory that the two
inner planets, Mercury and Venus, never deviate far from the Sun in celestial
longitude—hence the position of Venus as both morning and evening star. Also,
it was almost universally held that they were nearer to us than the Sun. There
accordingly arose the question of how they received the light to make them
visible, for on the common assumption that this came from the Sun they would
be almost entirely illuminated from behind, and at most we should see a small
sliver. One opinion had it that they were possessed of their own light and another
that they were translucent, but basically the problem remained unresolved, or, to
use T.S.Kuhn’s phraseology, was an anomaly within the Ptolemaic paradigm. On
the Copernican hypothesis the predictions were different, for, if these planets
were orbiting the Sun, they should display phases in the manner of the Moon.
And this is what Galileo observed in the case of Venus, and triumphantly
reported in the Letters on Sunspots.
These things leave no room for doubt about the orbit of Venus. With
absolute necessity we shall conclude, in agreement with the theories of the
Pythagoreans and of Copernicus, that Venus revolves about the Sun just as
do all the other planets. 29
So far so good, but what Galileo fails to mention, and what remained a thorn in his
flesh, was that these observations were also perfectly compatible with the
Tychonic system of the world, which, as mentioned above, soon became a
favourite with Jesuit astronomers.
Until the 1610s the Copernican system had aroused little religious discussion,
apart from a few casual references to scriptural passages that seemed to
contradict it, such as the command to the Sun to stand still over Gibeon in order
to lengthen the day, so that Joshua could have time to finish a battle. With the
new popularity that Galileo had brought to the issue, the religious implications
became of major concern, and opposition to the system was probably egged on
by mere conservatism masquerading as high principle. Galileo was himself
drawn into the controversy, and wrote a long letter on the relation of
Copernicanism to the Scriptures. This closely reasoned piece made relatively
liberal use of the principle of accommodation in biblical interpretation. The Bible
was addressed to ignorant people, and appealed to the common understanding of
the time; it was not intended as a textbook in astronomy. In the epigram of one
Cardinal Baronius, which Galileo gleefully quoted, ‘The intention of the Holy