NCT
Copernicus revolution
Reception of Copernicus’s theory and the importance of the discovery
Post Copernican cosmological concepts

Kepler’s model

Johannes Kepler (1571–1630) became a supporter of Copernicus’ heliocentric
theory thanks to Michale Maestlin, who introduced him to the arcana of
astronomy at the University of Tübingen. In 1596 Kepler published *Mysterium
Cosmographicum* (*The Secret of the Universe*),* *a spirited
apology of Copernicus’s system, in which he combined the system of six
planetary orbits (of Mercury, Venus, Earth, Mars, Jupiter, and Saturn)
with six Platonic solids. In the centre of this system was located the
Sun, which was to be the source of the force that sets the planets in motion.
At that time it was a revolutionary idea, since astronomy, including that
of Copernicus, was grounded on description of the paths of planet with
the use of uniform circular motions, given to planetary orbs ‘by nature’.
In other words, astronomy was then geometry. From 1600, when Kepler began
to work with Tycho Brahe, he searched for parameters of the orbits of Mars
and Earth, which were presented with the use of traditional circles and,
what is more, were related to the mathematical point called an average
Sun and not the true Sun. But after Brahe’s death, Kepler returned to
the idea of planetary motions influenced by the physical action of the
Sun. He compared the position of planets to the true Sun and determined
that they move not in circles, but in oval curves. Eventually in 1605 he
discovered that the orbit of Mars is an ellipse (Kepler’s first law).
And this shape, according to Kepler, results from the interaction between
a planet and the magnetic emanation radiating from the rotating Sun. The
scientist assumed that this influence is inversely proportional to the
distance of a planet from the Sun. On this basis he proved his second law:
during equal intervals of time, a planet’s radius vector, that is a line
joining a planet with the Sun, sweeps out equal areas. Kepler thus formed
physical astronomy, a matter difficult to accept in his time both by astronomers
and by philosophers, and presented the first description of it in 1609
in his work entitled
*Astronomia Nova*, the full title of which is
the *New Astronomy, Based upon Causes, or Celestial Physics, Treated
by Means of Commentaries on the Motions of the Star Mars, from the Observations
of Tycho Brahe*. His third law, which links the period of circulation
of a planet with the size of its orbit, Kepler discovered later, in 1618,
and published in his work *Harmonice mundi* (*The Harmony of the
World*, 1619).

Jarosław Włodarczyk

Institute for the History of Science

Polish Academy of Sciences

Institute for the History of Science

Polish Academy of Sciences

Further reading:

- B. Stephenson,
*Kepler’s Physical Astronomy*(New York, 1987). - N. M. Swerdlow, Astronomy in the Renaissance, in
*Astronomy before the telescope*, ed. by C. Walker (London, 1996), 187–230.