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Great Astronomers by Sir Robert S. (Robert Stawell) Ball
page 26 of 309 (08%)
or a hundred times, or a thousand times, as far as other stars.
Nevertheless, to his eye the stars appear on the surface of the
sphere, it is on that surface that his measurements of the relative
places of the stars are made; indeed, it may be said that almost all
the accurate observations in the observatory relate to the places of
the stars, not as they really are, but as they appear to be projected
on that celestial sphere whose conception we owe to the genius of
Ptolemy.

This great philosopher shows very ingeniously that the earth must be
at the centre of the sphere. He proves that, unless this were the
case, each star would not appear to move with the absolute uniformity
which does, as a matter of fact, characterise it. In all these
reasonings we cannot but have the most profound admiration for the
genius of Ptolemy, even though he had made an error so enormous in
the fundamental point of the stability of the earth. 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
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