The determining of the places of the fixed stars which Flamsteed carried out so efficiently in his _British Catalogue of Stars_--the first 'Census of the Sky' made with the aid of a telescope--was but half of the work imposed upon him. The other half, equally necessary for the solution of the problem of the longitude at sea, was the 'Rectifying the Tables of the Motions of the Heavens.' This second duty was not less necessary than the other, for, if we may again use the old simile of the clock-face, the fixed stars may be taken to represent the figures on the vast dial of the sky, whilst the moon, as it moves amongst them, corresponds to the moving hand of the timepiece. To know the places of the stars, then, without being able to predict the place of the moon, would be much like having a clock without its hands. But if not less necessary, it was certainly more difficult. The secret of the movements of the moon and planets had not then been grasped, and the only tables which had been calculated were based upon observations made before the days of telescopes. It is one of the most fortunate and remarkable coincidences in the whole history of science, that at the very time that Greenwich Observatory was being called into existence, the greatest of all astronomers was working out his demonstration of the great fundamental law of the material universe--the law that every particle of matter attracts every other particle with a force which varies directly with the mass and inversely with the square of the distance. Several other of the great minds of that time, in particular Dr. Hooke, the Gresham Professor of Astronomy, had seen that it was possible that some such law might supply the secret of planetary motion; but it is one thing to make a suggestion, and a very different matter indeed to be able to demonstrate it; and the latter was in Newton's power alone. He did much more than demonstrate it--he brought out a whole series of most important and far-reaching consequences. He showed that the ebb and flow of the tides was due to the attraction of both sun and moon, especially the latter, upon the waters of our oceans. He pointed out certain irregularities which must take place in the motion of our moon, due to the influence of the sun upon it. He showed, too, what was the cause of that swinging of the axis of the earth which gives rise to precession. He deduced the relative weights of the earth, the sun, and of Jupiter and Saturn, the planets with satellites. He proved also that comets, which had seemed hitherto to men as perfectly lawless wanderers, obeyed in their orbits the self-same law which governed the moon and planets. The whole vast system of celestial movements, which had long seemed to men irregular and uncontrolled, now fell, every one of them, into its place, as but the necessary manifestations of one grand, simple order.