GRAVITATION (Fr. gravitation, from Fr. graviter, to gravitate, from Lat. gravitas, weight, from gravis, heavy). The idea that there is an action between the earth and its moon and the sun and its planets perfectly analogous to that between the earth and a falling body occurred to many astronomers and students of science pre vious to the announcement by Newton in 1687 of his famous law of universal gravitation; but it was reserved for Newton to give this law exact mathematical expression. All bodies, when raised into the air and left unsupported, fall to the earth. The force which causes them to do so is termed gravity, and, universal experience shows, acts toward the earth's centre; more strictly, it acts in a direction perpendicular to the surface of still water. But if a body, such as a stone, be projected obliquely into the air, it describes a curved path, and when it meets the earth in its descent, its direction is not toward the centre, but inclined to it at the angle of projection. (See PROJECTILES.) Observing this, and that the body, if not stopped by the earth's surface, would continue to move in a curve, it is easy to imagine that it might circulate round the earth's centre as the moon does round the earth. (See CENTRAL FORCES.) Observing now the time of revolution of the moon, we can calculate the force with which it tends to leave the path (centrifu gal force). This must be balanced by an equal attractive force, or we should lose the moon. But then the question arises, is this attractive force the same as the force acting on bodies near the surface of the earth ? The answer is that it is a force 3600 times less energetic. If, then, gravity be the force which really holds the moon to its path, we must explain why it acts upon it so much more feebly than it would were it a body on the earth's surface. The explanation is given at once if we suppose gravity to be a force whose magnitude diminishes with increase of distance, and inversely as the squares of the distances at which it is exerted; for the distance of the moon from the earth's centre .is about 60 times that of the earth's surface from its centre, and 3600: 1:: 1. We infer that this explanation is correct from the fact that there is nothing inadmissible in such a diminution of force with increase of distance, and in the argument drawn from the necessity of otherwise supposing some other force than gravity to be employed in de flecting the moon, and the force of gravity to cease at some unknown level. On these views
Newton is understood to have at first rested his law of universal gravitation. Every particle of matter in the universe attracts every other particle with a force directly proportional to the mass of the attracting particle, and inversely to the square of the distance between them. New ton, before conceiving the law, had explained the three great Keplerian laws of motion obtaining in the solar system by reference to an attractive force residing in the sun. These laws are: (1) That the planets revolve round the sun in ellipses, having the sun for a common focus. (2) That every planet moves in such a way that the line drawn from it to the sun sweeps over equal areas in equal times. (3) That the squares of the times occupied by the several planets in their revolutions in their elliptic orbits are propor tional to the cubes of their mean distances from their common focus, the sun. From the law of equal areas Newton inferred that every planet is retained in its orbit by a force of attraction directed toward the centre of the sun; from the orbits being elliptical, he inferred that in each case this force varies in intensity according to the inverse square of the body's distance from the sun; while from the third law he inferred the homogeneity of the central force throughout the solar system.
It was then, after being familiar with the no tion of terrestrial gravity and its action, through the researches of Galileo, Huyghens, and Hooke, and with the notion of a central force acting in versely as the square of the distance through his explanations of the laws of Kepler, that he put to himself the question: Is not the force with which the moon is pulled to the earth the same with gravity? A question answered affirmatively on the supposition of gravity, like the sun's at traction, being a force diminishing with increase of distance and according to the same law. The result was to bring the whole solar system within the range of the law of gravitation. The phe nomena of double stars justify the extension and the statement of the law as we have given it to universal terms. It may be observed, in con clusion, that the Keplerian laws, which may be said to have been the basis of Newton's re searches, are, owing to perturbations (q.v.) caused by the mutual action of the planets, etc., only approximately correct, and that these per turbations afford, when examined, a further proof of the truth and universality of the law of gravi tation.