Another error
of a somewhat similar kind seemed to Ptolemy to be demonstrated. He
had shown that the earth was an isolated object in space, and being
such was, of course, capable of movement. It could either be turned
round, or it could be moved from one place to another. We know that
Ptolemy deliberately adopted the view that the earth did not turn
round; he had then to investigate the other question, as to whether
the earth was animated by any movement of translation. He came to
the conclusion that to attribute any motion to the earth would be
incompatible with the truths at which he had already arrived. The
earth, argued Ptolemy, lies at the centre of the celestial sphere.
If the earth were to be endowed with movement, it would not lie
always at this point, it must, therefore, shift to some other part of
the sphere. The movements of the stars, however, preclude the
possibility of this; and, therefore, the earth must be as devoid of
any movement of translation as it is devoid of rotation. Thus it was
that Ptolemy convinced himself that the stability of the earth, as it
appeared to the ordinary senses, had a rational philosophical
foundation.
Not unfrequently it is the lot of the philosophers to contend against
the doctrines of the vulgar, but when it happens, as in the case of
Ptolemy's researches, that the doctrines of the vulgar are
corroborated by philosophical investigation which bear the stamp of
the highest authority, it is not to be wondered at that such
doctrines should be deemed well-nigh impregnable.
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