But perhaps the most signal of Newton’s applications of the law of gravitation was connected with certain irregularities in the movements of the moon.  In its orbit round the earth our satellite is, of course, mainly guided by the great attraction of our globe.  If there were no other body in the universe, then the centre of the moon must necessarily perform an ellipse, and the centre of the earth would lie in the focus of that ellipse.  Nature, however, does not allow the movements to possess the simplicity which this arrangement would imply, for the sun is present as a source of disturbance.  The sun attracts the moon, and the sun attracts the earth, but in different degrees, and the consequence is that the moon’s movement with regard to the earth is seriously affected by the influence of the sun.  It is not allowed to move exactly in an ellipse, nor is the earth exactly in the focus.  How great was Newton’s achievement in the solution of this problem will be appreciated if we realise that he not only had to determine from the law of gravitation the nature of the disturbance of the moon, but he had actually to construct the mathematical tools by which alone such calculations could be effected.

The resources of Newton’s genius seemed, however, to prove equal to almost any demand that could be made upon it.  He saw that each planet must disturb the other, and in that way he was able to render a satisfactory account of certain phenomena which had perplexed all preceding investigators.  That mysterious movement by which the pole of the earth sways about among the stars had been long an unsolved enigma, but Newton showed that the moon grasped with its attraction the protuberant mass at the equatorial regions of the earth, and thus tilted the earth’s axis in a way that accounted for the phenomenon which had been known but had never been explained for two thousand years.  All these discoveries were brought together in that immortal work, Newton’s “Principia.”

Down to the year 1687, when the “Principia” was published, Newton had lived the life of a recluse at Cambridge, being entirely occupied with those transcendent researches to which we have referred.  But in that year he issued from his seclusion under circumstances of considerable historical interest.  King James the Second attempted an invasion of the rights and privileges of the University of Cambridge by issuing a command that Father Francis, a Benedictine monk, should be received as a Master of Arts in the University, without having taken the oaths of allegiance and supremacy.  With this arbitrary command the University sternly refused to comply.  The Vice-Chancellor was accordingly summoned to answer for an act of contempt to the authority of the Crown.  Newton was one of nine delegates who were chosen to defend the independence of the University before the High Court.  They were able to show that Charles the Second, who had issued a mandamus under somewhat similar circumstances, had been induced after due consideration to withdraw it.  This argument appeared satisfactory, and the University gained their case.  Newton’s next step in public life was his election, by a narrow majority, as member for the University, and during the years 1688 and 1689, he seems to have attended to his parliamentary duties with considerable regularity.