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It is a familiar principle in elementary mechanics that a uniform motion of the whole system under consideration makes no difference to the phenomena, and may be ignored. For example, in calculating the motions of the planets round the sun, we do not need to pay any attention to the fact that the whole solar system is travelling towards the constellation Lyra. It does not seem to us that there is anything surprising or needing explanation in this principle; in fact, when we try to examine the idea of motion through an empty space without fixed landmarks, we are conscious of something illusory in the conception.
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- Copyright © Mathematical Association 1971
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* Note that we do not reject the aether, but only velocity with respect to the aether; that is to say, the nature of the aether is such that it does not provide a standard with respect to which a velocity can be measured.
* Some complication is introduced by the fact that the word “Now” may-refer not to an exact instant of time, but to a period of time which, though short, is not infinitesimal. The instant in New York which is simultaneous with the instant here is indeterminate (except by arbitrary mathematical convention); but there are limits to the ambiguity, and we can fix on a period of about sV sec. in New York, which certainly overlaps the instant “Now” here. As we go further away the limits widen. Thus the lover who says to himself “She is thinking of me now” need not be troubled by the ambiguity, for her thought would certainly endure 
of a second. But if the absent one were on the planet Neptune (where the ambiguity owing to the increased distance amounts to about four hours), he would be deprived of this consolation.