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# Constants

Constants, in Mathematics and Physics, are those quantities which remain of the same value throughout the case studied. In discussing, for example, the motion of bodies on the earth's surface, the attraction due to gravity is very nearly constant, and will therefore produce a constant acceleration. But in discussing the effect of the earth's gravitation on other heavenly bodies the intensity of this acceleration varies; it can no longer be treated as a constant. The period of a sidereal day is almost exactly a constant, and it is by referring to such quantities, whose unchangeableness may be relied upon, that we are able to express the magnitudes of similar quantities in terms of a definite number of absolute units. If this day varied in duration, it would be a matter of difficulty to obtain a convenient standard unit of time. This illustrates the importance of such physical constants.