. Astronomy for students and general readers . ame motion, or the same accelerating force. SUNIS ATTRACTION ON MOON. 157 the sun—that is, nearly in an ellipse, having the earth inits focns. But there is always a small difference betweenthe attractive forces of the sun upon the moon and upon theearth, and this difference constitutes a disturbing forcewhich makes the moon deviate from the elliptic orbitwhich it would otherwise describe, and, in fact, keeps theellipse which it approximately describes in a state of con-stant change. A more precise idea of the manner in which the sun disturbs themo
. Astronomy for students and general readers . ame motion, or the same accelerating force. SUNIS ATTRACTION ON MOON. 157 the sun—that is, nearly in an ellipse, having the earth inits focns. But there is always a small difference betweenthe attractive forces of the sun upon the moon and upon theearth, and this difference constitutes a disturbing forcewhich makes the moon deviate from the elliptic orbitwhich it would otherwise describe, and, in fact, keeps theellipse which it approximately describes in a state of con-stant change. A more precise idea of the manner in which the sun disturbs themotion of the moon around the earth ma,y be gathered fromFig. 57. Here 8 represents the sun, aiid the circle F Q M Nrepre-sents the orbit of the moon. First suppose the moon at N, the posi-tion corresponding to new moon. Then the moon, being nearer tothe sun than the earth is, will be attracted more powerfully by itthan the earth is. It will therefore be drawn away from the earth,or the a,ction of the sun will tend to separate the two Fig. 57. Next suppose the moon at F the position corresponding to fullmoon. Here the action of the sun upon the earth will be morepowerful than upon the moon, and the earth will in conscijuence I)edrawn away from the moon. In this position also the effect of thedisturbing force is to separate the two bodies. If, on the otherhand, the moon is near the first quarter or near Q, the sun will exerta nearly equal attraction on both bodies ; and ince the lines of at-traction B S and Q S then converge toward S, it follows that therewill be a tendency to bring the two bodies together. The samewill evidently be true at the third quarter. Hence the influence ofthe disturbing force changes back and forth twice in the course ofeach lunar month. The disturbing force in question may be constructed for any po-sition of the moon in its orbit in the following way, which is be-lieved to be due to Mr. R. A. Pkoctok : Let Mhe the position ofthe moon ; let us
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