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GRAVITATION (from Lat. gravis, heavy)

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Originally appearing in Volume V12, Page 385 of the 1911 Encyclopedia Britannica.
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GRAVITATION (from See also:Lat. gravis, heavy) , in See also:physical See also:science, that mutual See also:action between masses of See also:matter by virtue of which every such See also:mass tends toward every other with a force varying directly as the product of the masses and inversely as the square of their distances apart. Although the See also:law was first clearly and rigorously formulated by See also:Sir See also:Isaac See also:Newton, the fact of the action indicated by it was more or less clearly seen by others. Even See also:Ptolemy had a vague conception of a force tending toward the centre of the See also:earth which not only kept bodies upon its See also:surface, but in some way upheld the See also:order of the universe. See also:John See also:Kepler inferred that the See also:planets move in their orbits under some See also:influence or force exerted by the See also:sun; but the See also:laws of See also:motion were not then sufficiently See also:developed, nor were Kepler's ideas of force sufficiently clear, to admit of a precise statement of the nature of the force. C. See also:Huygens and R. See also:Hooke, contemporaries of Newton, saw that Kepler's third law implied a force tending toward the sun which, acting on the several planets, varied inversely as the square of the distance. But two requirements necessary to generalize the theory were still wanting. One was to show that the law of the inverse square not only represented Kepler's third law, but his first two laws also. The other was to show that the gravitation of the earth, following one and the same law with that of the sun, extended to the See also:moon. Newton's researches showed that the attraction of the earth on the moon was the same as that for bodies at the earth's surface, only reduced in the inverse square of the moon's distance from the earth's centre. He also showed that the See also:total gravitation of the earth, assumed as spherical, on See also:external bodies, would be the same as if the earth's mass were concentrated in the centre.

This led at once to the statement of the law in its most See also:

general See also:form. The law of gravitation is unique among the laws of nature, not only in its wide generality, taking the whole universe in its See also:scope, but in the fact that,-so far as yet known, it is absolutely unmodified by any See also:condition or cause whatever. All other forms of action between masses of matter, vary with circumstances. The mutual action of electrified bodies, for example, is affected by their relative or See also:absolute motion. But no conditions to which matter has ever been subjected, or under which it has ever been observed, have been found to influence its gravitation in the slightest degree. We might conceive the rapid motions of the heavenly bodies to result in some See also:change either in the direction or amount of their gravitation towards each other at each moment; but such is not the See also:case, even in the most rapidly moving bodies of the See also:solar See also:system. The question has also been raised whether the action of gravitation is absolutely instantaneous. If not, the action would not be exactly in the lineadjoining the two bodies at the instant, but would be affected by the motion of the See also:line joining them during the See also:time required by the force to pass from one See also:body to the other. The result of this would be seen in the motions of the planets around the sun; but the most refined observations show no such effect. It is also conceivable that bodies might gravitate differently at different temperatures. But the most careful researches have failed to show any apparent modification produced in this way except what might be attributed to the surrounding conditions. The most See also:recent and exhaustive experiment was that of J.

H. Poynting and P. See also:

Phillips (Prot. See also:Roy. See also:Soc., 76A, p. 445). The result was that the change, if any, was less than dliy of the force for one degree change of temperature, a result too See also:minute to be established by any See also:measures. Another cause which might be supposed to modify the action of gravitation between two bodies would be the interposition of masses of matter between them, a cause which materially modifies the action of electrified bodies. The question whether this cause modifies gravitation admits of an easy test from observation. If it did, then a portion of the earth's mass or of that of any other See also:planet turned away from the sun would not be subjected to the same action of the sun as if directly exposed to that action. See also:Great masses, as those of the great planets, would not be attracted with a force proportional to the mass because of the hindrance or other effect of the interposed portions. But not the slightest modification due to this cause is shown.

The general conclusion from everything we see is that a mass of matter in See also:

Australia attracts a mass in See also:London precisely as it would if the earth were not interposed between the two masses. We must therefore regard the law in question as the broadest and most fundamental one which nature makes known to us. It is not yet experimentally proved that variation as the inverse square is absolutely true at all distances. Astronomical observations extend over too brief a See also:period of time to show any attraction between different stars except those in each other's neighbourhood. But this proves nothing because, in the case of distances so great, centuries or even thousands of years of accurate observation will be required to show any action. On the other See also:hand the enigmatical motion of the See also:perihelion of See also:Mercury has not yet found any plausible explanation except on the See also:hypothesis that the gravitation of the sun diminishes at a See also:rate slightly greater than that of the inverse square—the most See also:simple modification being to suppose that instead of the exponent of the distance being exactly – 2, it is -2.000 000 161 2. The See also:argument is extremely simple in form. It is certain that, in the general See also:average, See also:year after year, the force with which Mercury is See also:drawn toward the sun does vary from the exact inverse square of its distance from the sun. The most plausible explanation of this is that one or more masses of matter move around the sun, whose action, whether they are inside or outside the See also:orbit of Mercury, would produce the required modification in the force. From an investigation of all the observations upon Mercury and the other three interior planets, See also:Simon See also:Newcomb found it almost out of the question that any such mass of matter could exist without changing either the figure of the sun itself or the motion of the planes of the orbits of either Mercury or See also:Venus. The qualification " almost " is necessary because so complex a system of actions comes into See also:play, and accurate observations have extended through so See also:short a period, that the See also:proof cannot be regarded as absolute. But the fact that careful and repeated See also:search for a mass of matter sufficient to produce the desired effect has been in vain, affords additional See also:evidence of its non-existence.

The most obvious test of the reality of the required modifications would be afforded by two other bodies, the motions of whose pericentres should be similarly affected. These are See also:

Mars and the moon. Newcomb found an excess of motions in the perihelion of Mars amounting to about 5" per See also:century. But the See also:combination of observations and theory on which this is based is not sufficient fully to establish so slight a motion. In the case of the motion of the moon around the earth, assuming the gravitation of the latter to be subject to the modification in question, the See also:annual motion of the moon's See also:perigee should be greater by 1.5" than the theoretical motion. E. W. See also:Brown is the first investigator to determine the theoretical motions with this degree of precision; and he finds that there is no such divergence between the actual and the computed motion. There is therefore as yet no ground for regarding any deviation from the law of inverse square as more than a possibility. (S.

End of Article: GRAVITATION (from Lat. gravis, heavy)

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