Routledge History of Philosophy Volume IV

RENAISSANCE AND SEVENTEENTH-CENTURY RATIONALISM 109
Nova, whose full title is particularly evocative: New Astronomy, Reasoned from
the Causes, or Celestial Physics, Delivered Up by Considerations of the Motions
of the Star Mars, From the Observations of the Great Tycho Brahe. 25 What we
call the third law linked together the various planets in stating that the square of
the periodic time of a planet was proportional to the cube of its mean distance
from the Sun. It appeared in print in Kepler’s Harmonice Mundi of 1619. As the
title indicates, the principal aim of this work was in a very literal way the search
for musical harmonies in the heavens, and it is illustrated with scales appropriate
to the various planets. This has misleadingly encouraged the anachronistic, and
now thankfully outdated, attitude that Kepler can properly be split into two
distinct halves, the mystical and the scientific.
Another consequence of this attitude was to cause some unnecessary
puzzlement about the subsequent fate of Kepler’s laws. To a superficial modern
eye, it can seem that Kepler had now definitively established the facts about
planetary motion (in a commodious description) which only awaited a Newton in
order to explain them, and indeed the Kepler-Newton motif played a marked role
in later scientific rhetoric, as with Ampère and Maxwell. But this interpretation
raises the question of why Kepler’s ‘laws’ seem to have been so neglected
between their formulation and the time of Newton. Certainly Kepler’s writings
are not easy to penetrate, and the laws themselves are not so clearly sign-posted
as a modern reader may expect. Also Kepler’s second law in particular was not
easy to calculate with, and some astronomers, such as Seth Ward and Ismael
Boulliau, found it easier to combine the ellipse with an equant point at the focus
not occupied by the Sun. In general it seems that knowledgeable astronomers
were well aware of the laws but did not accept them with quite the alacrity that
we might think appropriate, so that even Newton could comment that, ‘Kepler
knew ye Orb to be not circular but oval & guest it to be Elliptical’. 26 The
situation has been much clarified in important articles by Curtis Wilson. The
validity of Kepler’s laws did not rest solely on observational evidence, with the
quasi-animist forces (his ‘mystical’ side) being mere psychological scaffolding
that could be cleared away once the building was erected. These laws depended
on theoretical support as well, for the former was insufficient by itself. But
Kepler’s theory, his system of forces, was very much of his own making, and did
not transfer easily to other workers, especially in an age in which both
conservatives and more mechanistically minded radicals were wary of any
suspicion of unmediated action at a distance, and when the latter were moving
towards a new type of inertial physics in which the continuance of a motion did
not demand a continually acting force to explain it. Thus, before Newton, it was
quite rational to regard Kepler as having provided an ingenious and useful, but
only approximate, account of the planetary motions. When Newton showed how
his inverse square law could be derived from Kepler’s second and third laws, and
then the first law deduced from this, the ‘laws’ were back in business with new
theoretical support, but, despite the use of distance-related forces, this was very
different from that provided by Kepler. It nevertheless fitted well with a new