No doubt,
other philosophers, in groping after knowledge, may have set forth
certain assertions that are more or less equivalent to this
fundamental truth. It is to Ptolemy we must give credit, however,
not only for announcing this doctrine, but for demonstrating it by
clear and logical argument. We cannot easily project our minds back
to the conception of an intellectual state in which this truth was
unfamiliar. It may, however, be well imagined that, to one who
thought the earth was a flat plain of indefinite extent, it would be
nothing less than an intellectual convulsion for him to be forced to
believe that he stood upon a spherical earth, forming merely a
particle relatively to the immense sphere of the heavens.
What Ptolemy saw in the movements of the stars led him to the
conclusion that they were bright points attached to the inside of a
tremendous globe. The movements of this globe which carried the
stars were only compatible with the supposition that the earth
occupied its centre. The imperceptible effect produced by a change
in the locality of the observer on the apparent brightness of the
stars made it plain that the dimensions of the terrestrial globe must
be quite insignificant in comparison with those of the celestial
sphere.
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