On this system the
apparent motion of each planet in the epicycle was represented by
a motion of the earth around the sun, and the problem of
correcting the position of the planet on account of the epicycle
was reduced to finding its geocentric from its heliocentric
position. This was the greatest step ever taken in theoretical
astronomy, yet it was but a single step. So far as the materials
were concerned and the mode of representing the planetary motions,
no other radical advance was made by Copernicus. Indeed, it is
remarkable that he introduced an epicycle which was not considered
necessary by Ptolemy in order to represent the inequalities in the
motions of the planets around the sun.
The next great advance made in the theory of the planetary motion
was the discovery by Kepler of the celebrated laws which bear his
name. When it was established that each planet moved in an ellipse
having the sun in one focus it became possible to form tables of
the motions of the heavenly bodies much more accurate than had
before been known. Such tables were published by Kepler in 1632,
under the name of Rudolphine Tables, in memory of his patron, the
Emperor Rudolph. But the laws of Kepler took no account of the
action of the planets on one another.
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