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I do not think that it has at all interfered with the facts of physical 
astronomy ; but at the same time one cannot help feeling that there is a 
great deal to be said on the other side. I think considerable light is thrown 
upon it by the well-written and cautious paper published in the name of a 
Cambridge “ Wrangler.” The question is well put, not whether the law of 
gravitation is true or false, but whether we are to adopt the Copernican or 
the Ptolemaic theory. It is generally assumed that the theory of gravitation 
can only be supported on the Copernican theory. What do physical astro- 
nomers do in the celebrated problem of the three bodies ? As to the lunar 
motion, they assume that the earth is in the centre, with the moon moving 
round the earth nearly in a circle, and the sun also moving round the 
earth at a certain distance nearly in a circle (Mr. Reddie. — Hear, 
hear) ; and then they apply the differential calculus to get a differential 
equation, which assumes the three laws of motion and the law of attraction 
— of gravity. Thus they get a differential equation, which they cannot solve 
(hear, hear), and then, by various extremely clever devices, and a suc- 
cessful series of mathematical dodges, they get at — not the real motion 
of the moon, because they take the earth as the centre — but the 
apparent motions of the moon, as seen from the earth. (Hear, hear.) But, 
supposing the earth to be perfectly still, and the moon moving round it, 
the theoretical path of the moon is not an ellipsis, and not any known curve ; 
and, moreover, it is not in any one plane, but in a plane which is constantly 
in a state of oscillation. Thus you get for the motion of the moon one of the 
most complicated curves that the mind can conceive. But why did astro- 
nomers reject the Ptolemaic theory and accept the Copernican ? Because 
the latter was supposed to give the simplest possible motions. But modern 
physical astronomy gives us motions of such an exceedingly complicated 
character, that the argument of simplicity does not apply to the present 
system any more than to the Ptolemaic. Then Mr. Brooke very pertinently 
said that one of the greatest proofs of current physical astronomy was its 
power of predicting eclipses and the moon’s motions. To my mind, one of 
the most astounding things is that little nautical almanac, in which you have 
the moon’s position calculated years before. Now, does that agree with the 
theory, or does it not ? You will say that if it does agree with the theory it 
will prove the modern theory to be true, and not the Ptolemaic. The Cam- 
bridge “ Wrangler ” says it does not do anything of the kind. Your mathe- 
matical analysis has been based upon the assumption that the earth is 
standing still. (Mr. Reddie. — Hear, hear : that is just my argument.) You 
have calculated all these motions upon an hypothesis which is as likely to be 
true as the converse ; so that anything that you prove with regard to the 
motions of the moon, or with regard to eclipses, can be held to be equally 
true, whether you take the current system of belief of the sun being fixed in 
the centre, with the moon rotating round the earth, and the earth round the 
sun ; or, suppose the earth to be fixed, and the moon and sun rotating round 
it. (Hear, hear.) Therefore you get no direct proof from the lunar theory 
of the present system of current physical astronomy. But the Cambridge 
