Transmission of Light
TRANSMISSION OF LIGHT.
"The progressive transmission of light being established, let us deduce from it our demonstration of the earth's rotation. If the earth is immoveable we ought not to see the stars the moment they arrive at the horizon or at the meridian, but only after the time acquired for the rays they emit to reach us. If, on the contrary, the earth turns, we ought to see the stars the moment they arrive, either at the horizon or at the meridian; for in consequence of the rotary motion, the eyes will fall into the line of the rays which had set out some time before from the stars, and which now arrive at the points of space traversed by our horizon. Now we do see the stars the instant of their arrival. The proof of this is, that the culminations of Mars,
for instance, would be more or less advanced or retarded according as that planet approached or receded from us, if we did not see it the moment it arrived at the meridian, but no appearance of the kind is noticed; the earth, therefore, must turn." 1
It is difficult to understand in what way the language of the above paragraph can be applied to prove the motion of the earth that does not equally apply to the proof of sidereal motion. The Newtonian astronomers, however, felt the necessity for practical proof of their leading assumptions; and hence have always been anxious to seize upon whatever could, by any kind of treatment, be made to appear like an argument in their favour.
In the above case they have been as premature and unfortunate as they have notoriously been in connection with other phenomena.