It is based on a comparison of the moon's distance with the length of the earth's radius.

On making this calculation, Newton found that the pull of gravitation--if that were really the force that controls the moon--gives that body a fall of slightly over fifteen feet in the first minute, instead of thirteen feet.

Here was surely a suggestive approximation, yet, on the other band, the discrepancy seemed to be too great to warrant him in the supposition that he had found the true solution.

He therefore dismissed the matter from his mind for the time being, nor did he return to it definitely for some years.
{illustration caption = DIAGRAM TO ILLUSTRATE NEWTON'S LAW OF GRAVITATION (E represents the earth and A the moon.

Were the earth's pull on the moon to cease, the moon's inertia would cause it to take the tangential course, AB.