Page 39 - Intro to Space Sciences Spacecraft Applications

CHAPTER 2
Orbital Principles

Around 350 B.C., the renowned Greek philosopher Aristotle made the
deduction that was to influence man’s perception and understanding of the
motions of the planets for over two thousand years! Aristotle was of the
opinion that all the heavenly bodies, including the sun, the wanderers
(planets), and the stars, were in circular motion about a fixed and unmov-
ing Earth. So authoritative was Aristotle that the Catholic church later
accepted this view as its official doctrine on the matter and used its reli-
gious influence to enforce this position. As a direct result, Galileo was
arrested for espousing views to the contrary in 1600 A.D.! However, a
century before Galileo, the first serious questioning of Aristotle’s views
had already begun.
Astronomy (or rather astrology) was very popular at the time of Coper-
nicus, around 1500 A.D., and the inability to accurately describe the
motions of the heavenly bodies increased efforts to find a better explana-
tion than Aristotle’s. Copernicus was both an astronomer and mathemati-
cian, and he used the observed angular positions of the planets and
trigonometry to correctly place the solar system in its proper order, with
the sun at the center and the earth as just another wanderer in motion
about the sun like the other planets. However, the idea of a moving earth
and similarity with the other planets was quite radical at the time and
made his heliocentric (sun-centered) hypothesis hard to accept.
For instance, Tycho Brahe, the late sixteenth-century astronomer, com-
pletely rejected the notion. Tycho, in the years prior to his death in 1601,
conducted the most exhaustive and accurate recording of the movements
of the planets to date. He was sure that his data held the secret to the mys-
tery of planetary movements, but his mathematical ability was too poor to
check out his theories. Therefore, Tycho solicited the assistance of math-
ematicians like Johannes Kepler.
After Tycho’s death, Kepler came into possession of most of the obser-
vational records kept over the many years. Kepler believed in the Coperni-

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