Fundamentals of astrodynamics and applications 4th Edition (2013)

8 EQUATIONS OF MOTION 1.1
numbers and data, and details of planetary motion. Pannekoek gives an excellent
account of Copernicus’s arguments, mostly in his own words.
If a motion of the Earth is assumed, this must appear, although in the opposite direction, in
all that is outside, as if things filed past her; and this holds especially for the daily motion.
Since heaven contains all, it is not conceivable that motion should not be attributed to what
is contained therein rather than to what contains all. If then someone should deny that the
Earth occupies the centre of the world but admits to her a distance not large enough to be
measured against the spheres of the fixed stars, but comparable to the orbits of the Sun and
the planets, he could perhaps indicate the cause for the irregularities in the different
motions as due to another centre than the Earth’s. (Pannekoek, 1989:190)
Copernicus’s arguments continue at length to illustrate each of his three main divergences
from Ptolemy’s theories. The mode of argument in the 16 th century was much
more philosophical than modern arguments but just as valid. Copernicus did have difficulty
in removing the equant from Ptolemaic theory: his theory includes small epicycles
and slight modifications to pure circular motion about a central point. He needed them
because the correct orbital theory didn’t exist. The Sun-centered system alleviated some
of the difficulties but didn’t solve the problem.
Copernicus knew his theories were controversial, so he resisted publication of this
work until he was on his deathbed. Were it not for a well-intentioned student of Copernicus
named Georg Rheticus (1514–1576), the world may have missed the great discoveries
for years. Rheticus started the publication process, and after he left to teach in
Leipzig in 1542, Andreas Osiander (1498–1552, a Lutheran theologian) continued it
through the printing in Nürnberg in 1543. A copy from the first printing was delivered to
Copernicus on May 24, 1543, the day he died (Dreyer, 1953:319). Beer and Strand
(1975a:205) suggest that a section discussing elliptical motion was left out of the original
publication. History blames Rheticus; however, Osiander may have had a greater
motive to omit it because he had strong views concerning the religious implications of
Copernicus’s theories. Osiander did change the preface to lessen the work’s impact,
which shows the importance of authors maintaining control over their manuscripts! In
any case, if Copernicus did intend to include a discussion of elliptical motion, it could
have further revolutionized the study of planetary motion. But would the world have
been ready to accept this much change?
In many respects, Galileo Galilei (1564–1642) picked up where Copernicus left off.
In fact, Berry (1961:149) mentions that Galileo (he preferred his first name) adopted the
Copernican ideas a few years before 1597, even though his famous works were published
more than ten years later. The main advantage of his research was his use of the
telescope for regular and dedicated scientific research.
Galileo is perhaps best known for his support of theories which opposed the religious
doctrine of the time. The initial controversy erupted about 1610 over a strictly scientific
discussion to determine whether Sun spots were on the surface of the Sun or were actually
planets passing between the Sun and the Earth. Galileo had observed the spots, as
had Christopher Scheiner (1575–1650). But Scheiner thought the spots were planets
between the Earth and the Sun. They exchanged letters to resolve the problem—a common
practice in the 1600s. Galileo apparently didn’t like the results, and, being a devout