Mechanics, Planetary Motion and the Modern Science Revolution 47
nothing superfluous or useless but often produces many effects from one
cause.”
It is obvious from this passage that Copernicus was also motivated by
a desire to achieve a simpler picture of heavenly motion. By assuming
that the Earth and the other planets orbit the Sun as well assuming that
the Earth rotates about its axis every 24 hours Copernicus achieved a
simpler description of the heavens in which only 34 circular wheels
instead of 80 were needed. Instead of the entire heavens rotating every
24 hours all of this motion was simply explained in terms of the Earth’s
rotation. Whereas in the Ptolemaic system the stars and the planets
turned in opposite directions and the movement of the Sun and the
planets was completely uncoordinated, in the Copernican system the
stars are fixed; the Earth and the planets orbit in the same direction
with the period of their orbit related to their distance from the Sun. In
addition to its greater symmetry another advantage of the system is that
is also made calculations and predictions easier.
In spite of its distinct advantages the Copernican system did not at
first gain many adherents mainly because it contradicted the existing
view of physics. It was difficult to image that a body as heavy as the
Earth could move. It was much simpler to conceive of the motion of the
heavenly bodies, which were thought to be light and ethereal as had been
proposed by Aristotle. With the new system the tidy explanation of
gravity was also sacrificed. Finally adoption of the Copernican view
required an enormous increase of the distance to the fixed stars. If the
distance to the stars of the Ptolemaic system were retained then one
would be able to detect parallax, the shift in the position of the stars due
to the Earth’s motion. This affect went undetected until the nineteenth
century.
Copernicus realized the difficulties thinkers would have in
adopting his view when he wrote, “If all this is difficult and almost
incomprehensible or against the opinion of many people, we shall, please
God, make it clearer than the Sun, at least to those who also know
mathematics.”
He tried to justify the physics of his scheme by replacing Aristotelian
dynamics with his own principle of circularity and sphericality.
Copernicus argues that it is the natural tendency of all things to
consolidate themselves so as to form a spherical shape. It therefore
follows that not only does the Earth possess gravity but so does
the Sun, the Moon and the other planets. It is from this concept of