MOTION, LAWS OF. Before the time of Galileo (1564 1642) very little attention had been paid to scientific study of the motion of terrestrial bodies. With regard to celestial bodies the case was somewhat different. The regularity of their diurnal revolutions could not escape notice; and before the end of the znd century A.D. the Greek astronomers had observed and re corded a great deal about the more complicated motions of the sun and moon and planets. They adopted uniform motion in a circle as a fundamental type of motion, and expressed the mo tion of each body by a combination of motions of this type. This scheme did not suggest any relation between celestial and terres trial motions. Copernicus (1473-1543) simplified it by taking the sun as the centre of the solar system. The first astronomical discovery which was a direct contribution to a general theory of motion was made by Kepler (1571-163o) a contemporary of Galileo. A time had come, at the end of the 16th century, when the theory of planetary orbits, from which tables were calcu lated, had been shown to be seriously faulty. So it was necessary to correct it, either by a rearrangement of the old scheme, or in some other way. Kepler undertook the difficult work of recon struction, stimulated by a conviction that there was something of an orderly kind to be discovered; and in 1609 and 1619 he published his new laws of planetary motion. (See ASTRONOMY.) The establishment of the approximate agreement of these laws with a theory of motion founded on terrestrial experiments was one of the first steps in Newton's subsequent work on the subject.
Dealing then with projectiles, he treated their motion as the result of compounding horizontal motion with constant velocity and vertical motion with his constant acceleration, so that the path of a projectile, in a vacuum, would be a parabola. This composition of motions is illustrated by the path of a stone dropped from the mast of a ship, sailing at uniform speed, so that it alights at the foot of the mast. These results, and Galileo's careful teaching, some of which is recorded in his dialogues, effec tively introduced a new view of the whole subject, namely, the idea that the acceleration of a moving body is the feature of its motion which the surrounding conditions determine, and that, if it were freed from all influence due to other matter, it would move with uniform velocity in a straight line. This doctrine, and Galileo's composition of motions, are directly applicable only to a body whose motion can be treated geometrically as that of a point. But any material system can be regarded as built up of particles of this character, conceived to be as small as may be necessary for the purpose of calculations.
The most important contributor to the subject, in the period between Galileo and Newton, was Huygens (1629-95). He in vestigated the acceleration of a point moving in any curve, and understood the nature of centrifugal force ; and when it was discovered, by means of clocks, that the acceleration of falling bodies varies with the latitude, he suggested the earth's rotation as a cause of this. He also took the considerable step of com paring the motions of pendulums consisting of rigid bodies of various shapes, finding their centres of oscillation. He based this calculation on an assumption which amounted to a denial of what is popularly called perpetual motion. It would be classed now as an application of the theory of energy. At the same time various experiments were made on the collision of hard bodies, establishing a comparison of their masses by inertia, which agreed with the comparison by weighing. Inertia is the general term applied to the resistance of bodies to a change of motion.