Page:Popular Astronomy - Airy - 1881.djvu/191

Rh towards S. In like manner, if the sun were to pull the centre C of the earth and the part B with equal force, it would not tend either to push B towards the centre or to draw it away from the centre; but, as it pulls the centre more powerfully than it pulls B, it does tend to separate them, not by pulling the opposite side B from the centre, but by pulling the centre from the opposite side B. The general effect of the sun's attraction, therefore, as tending to affect the different parts of the earth, is this: that it tends to pull the nearest parts towards the sun, and to push the most distant parts from the sun.

If the earth were a perfect sphere, this would be a matter of no consequence—it would produce tides of the sea, but it would not affect the motion of the solid parts. But the earth is not a sphere; it is flattened like a turnip, or has the form of which I have spoken to you under the description of a spheroid. Moreover, the axis of the earth is not perpendicular to the ecliptic; the earth's equator is inclined to the line joining the earth's centre with the sun at all times, excepting at the equinoxes.

Let us now consider the position of the earth at the winter solstice, represented in Figure 46. The North Pole is distant from the sun, the South Pole is turned towards the sun. This spheroidal earth, at this time, has its protuberance, not turned exactly towards the sun, but elevated above it. As I said before, the attraction of the sun is pulling the part D of the earth more strongly than it pulls the centre. What is the tendency of that action? The immediate tendency of that action is to bring the part D towards a, supposing a to be in the horizontal circle passing through S. In like manner, in consequence of the sun attracting the centre of the earth more than it