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at e g it is at a maximum as at first. The range of theeccentricity is so considerable as to exceed |ths ofthe mean value of the eccentricity. So that repre-senting this mean value as 5, the maximum is aboutone-fifth greater, or 6; and the minimum about one-fifth less, or 4. Thus the greatest eccentricity exceedsthe least in the proportion of 3 to 2. Since the meaneccentricity of the lunar orbit is 0054908, thegreatest and least values of the eccentricity are re-spectively about 0’066 and about 0044.
The irregularity of the perigeal motion and thevariation of the eccentricity are oscillatory dis-turbances; and their combined influence on theactual position of the moon in her orbit is thereforealso oscillatory in its effects. It will be easily inferredthat the moon’s position is importantly modified attimes by these causes, especially by the variation inthe eccentricity, since the eccentricity causes themoon’s motion in longitude to be unequal, and it isso much the more unequal as the eccentricity isgreater. The moon, owing to these causes, may bein advance of, or behind, the place she would have ifthese perturbations had no existence, by no less than1° 18'. This perturbation is called the evection, andis the only lunar perturbation which the ancientastronomers discovered. The discovery is commonlyattributed to Ptolemy , though there are reasons forbelieving that it was actually made by Hipparchus .
Hitherto we have supposed the lunar orbit to he wthe plane of the ecliptic, since we have regarded the