THE MOON’S MOTIONS.

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are reduced slightly below their mean values ; for thesun’s releasing effect is then at a minimum. Thus themoon lags somewhat during the winter months, andregains her place by slightly hastening during thesummer months. She is farthest behind her meanplace, so far as this circumstance is concerned, inspring and autumn (at those epochs when she is ather mean distance), for it is at these times that theloss begins to change into gain, or vice versa. Thegreatest possible amount of lagging accruing in springis such that the moon is behind her mean place byabout a third of her own diameter. In autumn shegets in advance of her mean place by about the sameamount.

This peculiarity of the moon’s motion is called theannual equation, and was discovered by Tycho Brahe .

Associated with this variation is another of muchgreater delicacy, and having a period of much greaterlength. We have seen that the eccentricity of theearth’s orbit affects the amount of the sun’s perturbinginfluence, insomuch that this influence is sometimesgreater and sometimes less than when the earth is ather mean distance. It might appear that as there isthus an excess at one season and a defect at another,the general result for the year would be the same asthough the earth travelled in a circular orbit at herpresent mean distance from the sun. This, however, isnot the case. If we consider that, supposing the earthto revolve always at her mean distance she woulddescribe a circle having a diameter as great as the