Page:Popular Astronomy - Airy - 1881.djvu/148

134 where the force is so great, it has been accelerated so much that its velocity also is very great. If the body had not been moving quicker, its path would have been much curved; but in consequence of its great velocity, its path may be very little curved; the power of the sun may be unable to bridle it any longer, and it may go on from that point increasing its distance from the sun. When it has thus reached a point as M', if we resolve the sun's force into two forces, one perpendicular to the orbit, and one in the direction of the orbit, then that which is perpendicular to the orbit bends it, but that which is in the direction of the orbit retards it; and when it has got to a certain distance, its velocity is small indeed; and though it is so far off that the sun's force is very small, nevertheless, in consequence of the planet's diminished speed, the attractive power of the sun may be able to pull it in and make it describe the same orbit again; and thus the planet need not either fall into the sun when nearest, or go quite away when farthest.

There is another thing which I think it very proper to mention, because many persons have a very erroneous notion upon it. Some persons have a notion that there is some remarkable adjustment, so that if anything however small was to disturb the motion of a planet, it would either fall to the sun or go quite away from the sun. That is not the case; the effect of the disturbance of a planet would be to change its orbit, but nothing else. I will endeavour to point out to you what I mean. Suppose that a planet has been going on describing the orbit LPRM, Figure 39, for ages, continually describing the same curve in an ellipse round the sun. Now, I will suppose that when it is nearest to the sun at L, something comes