DISTANCE, SIZE, AND MASS.

41

Stone detected, Leverrier adopted the value . Pro-fessor Newcomb adopted the valueThese values were deduced by an independentmethod, the last remaining to be described, and onthe whole perhaps the most satisfactory. Owing tothe attraction of the sun and moon on the bulgingequatorial parts of the earth, the axis of the earthundergoes the disturbance called precession. Nowthis disturbance, whose period is about 25,868 years,depends on the inclination of the earth’s equator-planeto lines drawn from the sun and moon. The portiondue to the moon’s action depends on the inclina-tion of the equator-plane to a line from the moon.Now of course this inclination varies during themoon’s circuit of the earth, because she twice crossesthe celestial equator in such a circuit, and at thesetimes the moon’s action vanishes. But these changesare comparatively unimportant so far as the progressof the displacement of the earth’s axis is concerned,simply because the displacement during a month isexceedingly small. There is, however, a change which,having a much longer period, is clearly recognizable.The moon’s orbit is inclined to the ecliptic by rathermore than five degrees. If the orbit thus inclinedhad a constant position, its inclination to the earth’sequator (assumed also to have a constant position,which is approximately the case), would also be constant.But we shall see in the next chapter that the directionof the line in which the moon’s plane intersects theecliptic, makes a complete revolution once in about