Routledge History of Philosophy Volume IV

RENAISSANCE AND SEVENTEENTH-CENTURY RATIONALISM 103
Moon) would appear from the vantage point of a stationary Earth against the
background of the fixed stars; or, more precisely, in a co-ordinate system
determined by the great circles of the ecliptic (the path of the Sun through the
sky) and the celestial equator. The devices worked by combining circular
motions. Usually we have a deferent circle whose centre is at some distance from
the Earth, and an epicycle, a smaller circle whose centre moves around
the circumference of the deferent, while the planet is imagined to move about the
circumference of the epicycle. Speeds are regulated in terms of an equant point,
situated at the same distance from the centre of the deferent as the Earth but on
the opposite side, so that the centre of the epicycle moves with uniform angular
velocity about this point. The models for Mercury and the Moon were more
complicated than this description may suggest, and that for the Sun simpler, but
all incorporated sufficient flexibility in choice of parameters to make them good
predictive devices. At least, they were good enough for Copernicus, but the
status of the equant point was another matter.
[The theories] were not sufficient unless there were imagined also certain
equant circles, by which it will appear that the star is moved with ever
uniform speed neither in its deferent orb nor about its proper centre, on
which account a theory of this kind seems neither sufficiently absolute nor
sufficiently pleasing to the mind. 16
This famous passage has led some to speak of Copernicus’s Pythagorean
obsession with uniform circular motion, with the implied suggestion that this
was beyond the bounds of rationality. But in fact Aristotle is a better target. In
Copernicus’s time, as it had been from Antiquity onwards, astronomy was
regarded as a branch of mathematics, what Aristotle had referred to as one of its
more physical branches; in the later Middle Ages these were often called middle
sciences, as lying between mathematics and physics. Whatever Ptolemy may
have thought, it was not generally seen as a hallmark of mathematics to look at
causes: that was the province of the physicist. For Aristotle the heavens were
made of the fifth element or aether, to which it was natural to move eternally
with a uniform rotation. In his more detailed astronomical picture, for which he
borrowed from the mathematicians (astronomers) of his time, Aristotle had some
fifty-five spheres, centred on the Earth, of which nine carried planets. Any
deviation from uniformity of motion would need an interfering cause, which
could not seem plausible in the perfection of Aristotle’s celestial realm.
Copernicus did not maintain the Aristotelian distinction between celestial and
elementary regions, but he did demand a cause for deviation from uniformity,
and none seemed available: hence, if for no other reason, farewell the equant.
Here we must emphasize that Copernicus, although more mutedly than
Kepler, was much concerned with causes, and also, in the context of an often
bitter recent controversy, that he almost certainly ascribed a fair degree of reality
to the orbs (be they spheres, circles, orbits, hoops or whatever) to which the