Mr. Youmans informs us, also, that “Copernicus did not abolish, but rather revised, the old astronomy.” If the words “old astronomy” are taken to express merely the knowledge of celestial phenomena, we have nothing to reply; but if those words be understood to mean the Ptolemaic theory, the assertion is ridiculous. Indeed, Copernicus, as Mr. Youmans says, “simply recentred the solar system”; that is, he simply put the sun, instead of the earth, in the centre of the planetary orbits. Nothing but that. But who does not see that to give a new centre to the solar system was to suppress the old centre, and therefore to _abolish_ the geocentric theory? Why Mr. Youmans should labor to insinuate the contrary we cannot really understand. Dr. Taylor, against whom he writes, had said nothing concerning either the personal views of Copernicus or the old system of astronomy, but had simply maintained that the so-called Copernican theory, as mentioned by Prof. Huxley, and as understood by all—that is, as perfected by Kepler, Newton, and others—stands to-day on such a basis of undeniable facts that we can no longer hesitate about its truth. This statement might have been contradicted two centuries ago; but we fancy that it ought not to give rise to the least controversy on the part of a modern cultivator of science, however much determined to find fault with his opponent.
Dr. Taylor had said, as we have noticed, that the Copernican theory “rests on facts presently occurring before our eyes.” Mr. Youmans answers: “So does the Ptolemaic theory; and not only that, but, if the test is what occurs before our eyes, then the Ptolemaic theory is a thousand times stronger than the Copernican.” If this answer expresses the real opinion of Mr. Youmans, we must conclude that he alone, among physicists, is ignorant of the fact that terrestrial gravitation is modified by the centrifugal force due to the rotation of the earth, and that this fact is established by experiments which “occur before our eyes” when we make use of the pendulum in different latitudes. What shall we say of the aberration of light? Is not this phenomenon a proof of the movement of the earth? Or does it not “occur before our eyes”? Mr. Youmans may say that these facts do not occur before all eyes, but only before the eyes of scientific men. But Dr. Taylor had not maintained that all the facts connected with the Copernican theory occur before all eyes; and, on the other hand, Foucault’s pendulum, even though oscillating before unscientific eyes, makes visible to the dullest observer the shifting of the horizontal plane from its position at a rate proportional to the sine of the latitude of the place, thus showing to the eye the actual movement of our planet. It is true, therefore, that the Copernican theory “rests on facts presently occurring before our eyes.”