Kepler: Johannes The
Laws of Planetary Motion
In the interplay between quantitative observation and theoretical construction
that characterizes the development of modern science, we have seen
that Brahe was the master of the first but was deficient in the second. The next
great development in the history of astronomy was the theoretical intuition of
Johannes Kepler (1571-1630), a German who went to Prague to become Brahe's
assistant.
Brahe's Data and Kepler
Kepler and Brahe did not get along well. Brahe apparently mistrusted Kepler,
fearing that his bright young assistant might eclipse him as the premiere
astonomer of his day. He therefore let Kepler see only part of his voluminous
data.
He set Kepler the task of understanding the orbit of the planet
Mars, which was particularly troublesome. It is believed that
part of the motivation for giving the Mars problem to Kepler
was that it was difficult, and Brahe hoped it would occupy
Kepler while Brahe worked on his theory of the Solar System.
In a supreme irony, it was precisely the Martian data that
allowed Kepler to formulate the correct laws of planetary motion, thus
eventually achieving a place in the development of astronomy far surpassing that
of Brahe.
Kepler and the Elliptical Orbits
Unlike Brahe, Kepler believed in firmly the Copernican system. In retrospect,
the reason that the orbit of Mars was particularly difficult was that Copernicus
had correctly placed the Sun at the center of the Solar System, but had erred in
assuming the orbits of the planets to be circles. Thus, in the Copernican theory
epicycles were still required to explain the details of planetary motion.
It fell to Kepler to provide the final piece of the puzzle: after a long struggle, in
which he tried mightily to avoid his eventual conclusion, Kepler was forced
finally to the realization that the orbits of the planets were not the circles
demanded by Aristotle and assumed implicitly by Copernicus, but were instead
the "flattened circles" that geometers call ellipses (See adjacent figure; the
planetary orbits are only slightly elliptical and are
not as flattened as in this example.)
The irony noted above lies in the realization that the
difficulties with the Martian orbit
derive precisely from the fact that the orbit of Mars
was the most elliptical of the planets for which Brahe had extensive data. Thus
Brahe had unwittingly given Kepler the very part of his data that would allow
Kepler to eventually formulate the correct theory of the Solar System and
thereby to banish Brahe's own theory!