THE MOON’S MOTIONS.

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by the above reasoning has received this name. As amatter of fact, the moon’s mean motion is subject toan acceleration nearly twice as great as the change inthe ellipticity of the terrestrial orbit will account for;and astronomers have been led to suspect that a por-tion of this acceleration may be only apparent anddue to a real retardation of the earth’s rotation,—thatis, a slight increase in the sidereal day, the unit bywhich we measure astronomical time. With this cir-cumstance, however, we are not at present concerned,save in so far as it relates to the history of thatinteresting cause of acceleration which has been de-scribed above. Halley had been led to suspect thatthe moon had advanced somewhat farther in herorbit than was consistent with the accounts of certainancient eclipses.* Further inquiries confirmed the

* I quote here some remarks on Halley’s researches by Mr. J. M.Wilson, of Rugby, from a valuable paper contributed to The Eagle,a magazine supported by members of St. John’s College, Cambridge (No. xxvi. vol. v.). “Halley,” he says, “seems to have been thefirst who considered this question. With astonishing clearness heseized the conditions of the question, saw that the knowledge ofthe elements, on which the solution was to be founded, was as yetincomplete, and saw also the probability that when the accurateknowledge was obtained, it would appear that there was a pecu-liarity in the moon’s motion entirely unforeseen by others, that itwas now moving faster and performing its revolution in a shortertime than it did in past time. If the longitudes of Bagdad,Antioch, and other places, were accurately known, ‘ I could then,’fie says, 1 pronounce in what proportion the moon’s motion doesaccelerate; which that it does, I think I can demonstrate, andshall (God willing) one day make it appear to the public.’ Newtonadds to his second edition of the Principia the words,—‘ Halleius