THE MOON’S MOTIONS.

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is the general law of matter, and prevails, as far asmatter prevails,—throughout the universe.

But Newton was sensible that a law of this naturecould not be established unless some special evi-dence, suited to attract the attention of scientificmen to the subject, were adduced and insisted upon.The discovery must throw light on some facts hithertounexplained,—must in effect achieve some strikingsuccess, — before men could be expected to lookfavourably upon it.

What Newton determined to do, then, was this.The law had been shown to accord with the generalfeatures of the lunar motions. But the moon’s motionis characterized by many peculiarities. At one timeshe takes a longer, at another a shorter time incircling around the earth, than that average periodcalled the sidereal lunar month. At one time she is inadvance of her mean place, calculated on the supposi-tion of a simple elliptic orbit; at another time she isbehind her mean place. The inclination of her path isvariable, as is the position of its plane; so also theeccentricity of her path and the position of her perigeeare variable. Newton saw that if the law of gravita-tion be true, the moon’s motion around the earthmust necessarily be disturbed by the sun’s attraction.If he could show that the peculiarities of the moon’smotion vary in accordance with the varying effectsof the sun’s perturbing influence, and, still more, if hecould show that the extent of the lunar perturbationscorresponds with the actual amount of the sun’s