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watchword of Copernicus and Galileo was virtually that appearances are 
deceptive. It seems to have been forgotten that if appearances are deceptive 
as regards the motion of the earth, they may be equally deceptive as regards 
the motions of the moon. The moon’s variation, in fact, was first discovered 
by Tycho Brahe, who held a geocentric hypothesis, and who would naturally 
therefore speak of the moon’s apparent increase and decrease of velocity as 
not only apparent but real, for so he believed them to be ; and so they would 
have been, if his geocentric hypothesis were true. 
26. It is for us, however, now, whatever others may have done, to get rid 
of all unrealities and deceptive appearances in science. We are bound, as 
rational beings, if we accept a heliocentric system, to look at all its conse- 
quences. In some respects the puzzling motions of the moon may probably 
be better understood if we regard her actual path on that system. The 
moon’s variation, the alteration of the place of her nodes, and the progression 
of the apse — probably, also, her annual equation, would, perhaps, all be more 
simply explained and better understood, by dealing with her actual motions 
and velocities, instead of fictions.* I do not say there will not be found 
other difficulties of another kind. But that is nothing to the purpose. We 
may not get over difficulties in science by having recourse to mere false 
appearances. For instance, there may be a difficulty from the non-coincidence 
of the moon’s path with the plane of the ecliptic, as this will make her path 
not a simple undulatory wave-line crossing and recrossing the earth’s orbit in 
the same plane, but a kind of drawn-out spiral path round the orbit of the 
earth. On the other hand, even this may be found a simplification that may 
serve to explain the apparent librations and some of the other various per- 
turbations of the moon, so far as they may not be mere optical effects of 
changed position and varying refractions. 
27. At all events we must not flinch from the consequences that flow from 
our adopted hypothesis. The opposite course has been far from satisfactory. 
With a heterogeneous mixture of effects which are only apparent in the 
moon’s motions, explained by a physical cause believed to be real ; with a 
fictitious orbit never described by the moon if the earth revolves, in which, 
also, the very direction of her real motions is sometimes reversed, and, as a 
natural consequence, is accounted for by an influence which must really 
repel, mistaken for a force that attracts, we need not be astonished that the 
result has been perplexities and complications. “ Of these applications of 
the theory of gravitation to explain the different perturbations of the moon” 
(says Professor Airy), “ a great deal might be said. It is a subject involved 
in mathematical perplexity beyond anything else that I know” (p. 183). One 
of the latest of these perplexities is the famous dispute as to the acceleration 
of the moon’s mean motion, described in Lord Wrottesley’s Address to the 
British Association at Oxford in 1860. I allude to it now, because it cer- 
tainly is one of those difficulties from which all that has been in dispute 
between several eminent analysts of England and the Continent, is en- 
tirely cleared away, when we have regard to the moon’s real path round 
the sun instead of to a fictitious path round the earth. His Lordship 
said : “ Professor Adams asserts that his predecessors have improperly 
omitted the consideration of the effect produced by the action of that part 
of the sun’s disturbing force which acts in the direction of a tangent to 
* So also, the various phenomena of the tides may be more simply 
explained by the hypothesis of a repulsive influence than they are by the 
theory of the attractions of the sun and moon ; especially considering that 
there are no tides at the Equator, where the theory of attraction requires 
them (and Newton and his followers actually represent them) to be greatest! 
