If it did not appear that a planet moved in a fixed circle,
then the ready answer was provided by Ptolemy's theory that the
circle in which the planet did move was itself in motion, so that its
centre described another circle.
When Kepler had before him that wonderful series of observations of
the planet, Mars, which had been accumulated by the extraordinary
skill of Tycho, he proved, after much labour, that the movements of
the planet refused to be represented in a circular form. Nor would
it do to suppose that Mars revolved in one circle, the centre of
which revolved in another circle. On no such supposition could the
movements of the planets be made to tally with those which Tycho had
actually observed. This led to the astonishing discovery of the true
form of a planet's orbit. For the first time in the history of
astronomy the principle was laid down that the movement of a planet
could not be represented by a circle, nor even by combinations of
circles, but that it could be represented by an elliptic path. In
this path the sun is situated at one of those two points in the
ellipse which are known as its foci.
[PLATE: KEPLER.]
Very simple apparatus is needed for the drawing of one of those
ellipses which Kepler has shown to possess such astonishing
astronomical significance.
Pages:
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121