Page:Popular Astronomy - Airy - 1881.djvu/192

178 attracts the protuberance E, which, amounts to the same thing as pushing the protuberance E away from the sun, there is a tendency to bring E towards b. It is very important that you should see this clearly: that if I were standing in the place S where the sun is, and if I had a line fastened by a hook to the place D, and if I pulled it, I should tend to bring that part towards a. The immediate tendency of this pull, therefore is, so to change the position of the earth that its axis will become more nearly perpendicular to the plane of the ecliptic. You might suppose then, that the effect of that pulling will be to change the inclination of the earth's axis to the line which connects the earth and sun. No such thing; the effect is entirely modified by the rotation of the earth. Undoubtedly, if the earth were not revolving, and if the earth were of a spheroidal shape, the attraction of the sun would tend to pull it into such a position that the axis of the earth would become perpendicular to the line SC; or (if in the position of the winter solstice) it would become perpendicular to the plane of the ecliptic; but, in consequence of the rotation of the earth, the attraction produces a perfectly different effect. Let us consider the motion of a mountain in the earth's protuberance, which, passing through the point c on the distant side of the earth, would, in the semi-revolution of half-a-day, describe the arc c D e, if the sun did not act on it, (c and e being the points at which this circle c D e intersects the plane of the ecliptic, or the plane of the circle a b that passes through the sun). While the protuberant mountain is describing the path c D e it is constantly nearer to the sun than the earth's centre is; the difference of the sun's actions therefore tends to pull that mountain towards S, and, therefore, (as it cannot be separated