*Project Gutenberg*. Public domain.

wrote, “What I prophesied two and twenty years
ago, as soon as I discovered the five solids among
the heavenly orbits,—what I firmly believed
long before I had seen Ptolemy’s ’Harmonics’—what
I had promised my friends in the title of this book,
which I named before I was sure of my discovery,—what
sixteen years ago I urged as a thing to be sought,—that
for which I joined Tycho Brahe, for which I settled
in Prague, for which I have devoted the best part
of my life to astronomical computations, at length
I have brought to light, and have recognised its truth
beyond my most sanguine expectations. Great as
is the absolute nature of Harmonics, with all its
details as set forth in my third book, it is all found
among the celestial motions, not indeed in the manner
which I imagined (that is not the least part of my
delight), but in another very different, and yet most
perfect and excellent. It is now eighteen months
since I got the first glimpse of light, three months
since the dawn, very few days since the unveiled sun,
most admirable to gaze on, burst out upon me.
Nothing holds me; I will indulge in my sacred fury;
I will triumph over mankind by the honest confession
that I have stolen the golden vases of the Egyptians
to build up a tabernacle for my God far away from the
confines of Egypt. If you forgive me, I rejoice,
if you are angry, I can bear it; the die is cast,
the book is written; to be read either now or by posterity,
I care not which; it may well wait a century for a
reader, as God has waited six thousand years for an
observer.” He gives the date 15th May,
1618, for the completion of his discovery. In
his “Epitome of the Copernican Astronomy,”
he gives his own idea as to the reason for this Third
Law. “Four causes concur for lengthening
the periodic time. First, the length of the path;
secondly, the weight or quantity of matter to be carried;
thirdly, the degree of strength of the moving virtue;
fourthly, the bulk or space into which is spread out
the matter to be moved. The orbital paths of
the planets are in the simple ratio of the distances;
the weights or quantities of matter in different planets
are in the subduplicate ratio of the same distances,
as has been already proved; so that with every increase
of distance a planet has more matter and therefore
is moved more slowly, and accumulates more time in
its revolution, requiring already, as it did, more
time by reason of the length of the way. The
third and fourth causes compensate each other in a
comparison of different planets; the simple and subduplicate
proportion compound the sesquiplicate proportion, which
therefore is the ratio of the periodic times.”
The only part of this “explanation” that
is true is that the paths are in the simple ratio of
the distances, the “proof” so confidently
claimed being of the circular kind commonly known
as “begging the question”. It was
reserved for Newton to establish the Laws of Motion,
to find the law of force that would constrain a planet
to obey Kepler’s first and second Laws, and to
prove that it must therefore also obey the third.