Page:Popular Astronomy - Airy - 1881.djvu/214

200 swelling part of the earth which is nearest to it more powerfully than it attracts the central part of the earth; and that it attracts the centre of the earth more powerfully than the parts further off. The sun, therefore, is endeavouring to pull, as it were, the nearest part of the earth from the centre towards the sun; and it is endeavouring to pull the centre from the distant parts towards the sun, which is the same as saying, that it is pushing this distant part from the sun. Now, if the earth were a complete sphere, the pulling at any part above the ecliptic would not disturb its motion; because there would be a corresponding pull on the corresponding part below the ecliptic; but, inasmuch as the earth is not a complete sphere, but has this flattened, turnip-like shape, protuberant at the middle; at the time of the solstice this protuberant part in the direction of the sun being above the ecliptic, then the extraordinary pull which the sun makes on that place is not balanced by a corresponding pull at the part below the ecliptic, because there is no protuberant part there on which the sun can act; and the action of the sun on that part which is above the plane of the ecliptic tends to pull it down towards that plane.

I remarked that the first effect of this would seem to be, to change the inclination of the axis on which the earth revolves, and to bring that axis more nearly perpendicular to the ecliptic. And that would be the effect, if the earth were not revolving on its axis; but in consequence of the revolution of the earth round its axis, a totally different effect is produced. I illustrated that by calling your attention to the motion of a single point, as for instance, a mountain at the earth's equator. While that mountain is near the sun and above the ecliptic, the force of which I have