[20] Tycho Brahé discovered two out of the principal inequalities in the Moon’s motion—known to astronomers as the Variation and the Annual Equation; the third, which is the most obvious of all and is called the Evection, was discovered by Ptolemy.
[21] The figurative interpretation, however, in this instance is as old as St. Augustine, though his speculations lead him to a different conclusion from that of modern scientific men; namely, that of supposing the actual creation to be the work of one moment.
[22] It is, I think, Mr. Proctor who uses this argument in one of his works, to prove how very doubtful a thing is the existence of highly organised and rational beings on the other planets.
[23] It is quite possible, as Mr. Lockyer has recently argued, that many objects that appear to us as stars, are in reality nebulæ in a more or less advanced stage of condensation.
[24] The relative distances could be computed geometrically, even before the absolute distances were known, and in fact were so; Kepler’s third law affords a simple rule for calculating them, but they were known even previously.
[25] I may, perhaps, be permitted to recall to the reader’s mind, in a note, one or two of the main objections urged by the anti-Copernicans. One of these was that it would leave the atmosphere behind, the true answer to which is that the atmosphere itself is attracted by the force of gravity to the earth, and is carried round by the rotation, as everything else is; this Galileo did not perfectly understand, as may be seen by his remarks, both in the second and the fourth day’s dialogue. Another was this—and it was put forward by no less a man than Tycho Brahé—a stone dropped from a high tower ought to fall to the westward of the tower, because the tower would be carried on to the east by the earth’s rotation, and the stone would not; this, however, being contrary to experience. The real fact is that the stone partakes of the rotatory movement as much as the tower does, the two forces of rotation and gravity being combined according to the second law of motion, while the stone is falling; this Galileo did know. Supposing a very high tower, the stone ought to fall slightly to the east, on account of the superior velocity of rotation at the top of the tower to that at the bottom. It is said this experiment has been successfully tried, as stated in note, page 55.
[26] There are other laws, besides that of the inverse square of the distance, which would cause a body to move in an ellipse, at least if the force acting on it were placed, not in the focus, but in the centre of the orbit. The question has been discussed with reference to some of the binary stars which appear to move round one another in ellipses. No doubt is thereby raised as to the prevalence of the law of the inverse square in our own solar system, where it has been verified by long and careful observation; the doubt (I think we may say a comparatively slight one) is whether the same law extends to the whole stellar universe, where, of course, accurate observation is impracticable.
[27] I do not think the truth of this is affected by any of the great modern discoveries; though that of the Conservation of Energy approaches more nearly than others to Universal Gravitation in its importance.
Transcriber’s Notes
Punctuation, hyphenation, and spelling were made consistent when a predominant preference was found in the original book; otherwise they were not changed.