Great Astronomers | Page 9

this supposition
must have appeared monstrous, even to Ptolemy. He knew that the
earth was a gigantic object, but, large as it may have been, he knew that
it was only a particle in comparison with the celestial sphere, yet he
apparently believed, and certainly succeeded in persuading other men
to believe, that the celestial sphere did actually perform these
movements.
Ptolemy was an excellent geometer. He knew that the rising and the
setting of the sun, the moon, and the myriad stars, could have been
accounted for in a different way. If the earth turned round uniformly
once a day while poised at the centre of the sphere of the heavens, all
the phenomena of rising and setting could be completely explained.
This is, indeed, obvious after a moment's reflection. Consider yourself
to be standing on the earth at the centre of the heavens. There are stars
over your head, and half the contents of the heavens are visible, while
the other half are below your horizon. As the earth turns round, the
stars over your head will change, and unless it should happen that you
have taken up your position at either of the poles, new stars will pass
into your view, and others will disappear, for at no time can you have
more than half of the whole sphere visible. The observer on the earth
would, therefore, say that some stars were rising, and that some stars
were setting. We have, therefore, two totally distinct methods, each of
which would completely explain all the observed facts of the diurnal
movement. One of these suppositions requires that the celestial sphere,
bearing with it the stars and other celestial bodies, turns uniformly
around an invisible axis, while the earth remains stationary at the centre.
The other supposition would be, that it is the stupendous celestial
sphere which remains stationary, while the earth at the centre rotates
about the same axis as the celestial sphere did before, but in an opposite
direction, and with a uniform velocity which would enable it to
complete one turn in twenty-four hours. Ptolemy was mathematician
enough to know that either of these suppositions would suffice for the
explanation of the observed facts. Indeed, the phenomena of the
movements of the stars, so far as he could observe them, could not be
called upon to pronounce which of these views was true, and which

was false.
Ptolemy had, therefore, to resort for guidance to indirect lines of
reasoning. One of these suppositions must be true, and yet it appeared
that the adoption of either was accompanied by a great difficulty. It is
one of his chief merits to have demonstrated that the celestial sphere
was so stupendous that the earth itself was absolutely insignificant in
comparison therewith. If, then, this stupendous sphere rotated once in
twenty-four hours, the speed with which the movement of some of the
stars must be executed would be so portentous as to seem well-nigh
impossible. It would, therefore, seem much simpler on this ground to
adopt the other alternative, and to suppose the diurnal movements were
due to the rotation of the earth. Here Ptolemy saw, or at all events
fancied he saw, objections of the weightiest description. The evidence
of the senses appeared directly to controvert the supposition that this
earth is anything but stationary. Ptolemy might, perhaps, have
dismissed this objection on the ground that the testimony of the senses
on such a matter should be entirely subordinated to the interpretation
which our intelligence would place upon the facts to which the senses
deposed. Another objection, however, appeared to him to possess the
gravest moment. It was argued that if the earth were rotating, there is
nothing to make the air participate in this motion, mankind would
therefore be swept from the earth by the furious blasts which would
arise from the movement of the earth through an atmosphere at rest.
Even if we could imagine that the air were carried round with the earth,
the same would not apply, so thought Ptolemy, to any object suspended
in the air. So long as a bird was perched on a tree, he might very well
be carried onward by the moving earth, but the moment he took wing,
the ground would slip from under him at a frightful pace, so that when
he dropped down again he would find himself at a distance perhaps ten
times as great as that which a carrier-pigeon or a swallow could have
traversed in the same time. Some vague delusion of this description
seems even still to crop up occasionally. I remember hearing of a
proposition for balloon travelling of a very remarkable kind. The
voyager who wanted to reach any other place in the same latitude was
simply to ascend in a balloon,