It then occurred to Newton, that though the moon is at a distance of two hundred and forty thousand miles from the earth, yet the attractive power of the earth must extend to the moon.  He was particularly led to think of the moon in this connection, not only because the moon is so much closer to the earth than are any other celestial bodies, but also because the moon is an appendage to the earth, always revolving around it.  The moon is certainly attracted to the earth, and yet the moon does not fall down; how is this to be accounted for?  The explanation was to be found in the character of the moon’s present motion.  If the moon were left for a moment at rest, there can be no doubt that the attraction of the earth would begin to draw the lunar globe in towards our globe.  In the course of a few days our satellite would come down on the earth with a most fearful crash.  This catastrophe is averted by the circumstance that the moon has a movement of revolution around the earth.  Newton was able to calculate from the known laws of mechanics, which he had himself been mainly instrumental in discovering, what the attractive power of the earth must be, so that the moon shall move precisely as we find it to move.  It then appeared that the very power which makes an apple fall at the earth’s surface is the power which guides the moon in its orbit.

[Plate:  Sir Isaac Newton’s telescope.]

Once this step had been taken, the whole scheme of the universe might almost be said to have become unrolled before the eye of the philosopher.  It was natural to suppose that just as the moon was guided and controlled by the attraction of the earth, so the earth itself, in the course of its great annual progress, should be guided and controlled by the supreme attractive power of the sun.  If this were so with regard to the earth, then it would be impossible to doubt that in the same way the movements of the planets could be explained to be consequences of solar attraction.

It was at this point that the great laws of Kepler became especially significant.  Kepler had shown how each of the planets revolves in an ellipse around the sun, which is situated on one of the foci.  This discovery had been arrived at from the interpretation of observations.  Kepler had himself assigned no reason why the orbit of a planet should be an ellipse rather than any other of the infinite number of closed curves which might be traced around the sun.  Kepler had also shown, and here again he was merely deducing the results from observation, that when the movements of two planets were compared together, the squares of the periodic times in which each planet revolved were proportional to the cubes of their mean distances from the sun.  This also Kepler merely knew to be true as a fact, he gave no demonstration of the reason why nature should have adopted this particular relation between the distance and the periodic