Perturbations

Perturbations, in Physical Astronomy, are the disturbances produced in the simple elliptic motion of one heavenly body about another by the action of a third body, or by the non-sphericity of the principal body. Thus, for instance, were there no bodies in space except the earth and moon, the moon would describe accurately an ellipse about the earth's centre as focus, and its radius-vector would pass over equal areas in equal times; but only if both bodies were homogeneous and truly spherical, or had their constituent matter otherwise so arranged that they might attract each other as if each were collected at some definite point of its mass. The oblateness of the earth's figure, therefore, produces perturbations in what would otherwise be the fixed elliptic orbit of the moon. Again, when we consider the sun's action it is obvious that in no position of the moon can the sun act equally upon both earth and moon; for at new moon the moon is nearer to the sun than the earth is, and is therefore more attracted (in proportion to its mass) than the earth—i.e. the difference of the sun's actions on the earth and moon is equivalent to a force tending to draw the moon away from the earth. At full moon, on the other hand, the earth (in proportion to its mass) is more attracted than the moon is by the sun; and the perturbing influence of the sun is again of the nature of a force tending to separate the earth and moon. About the quarters, again, the sun's attraction (mass for mass) is nearly the same in amount on the earth and moon, but the direction of its action is not the same on the two bodies, and it is easy to see that in this case the perturbing force tends to bring the earth and moon nearer to each other. For any given position of the moon, with reference to the earth and sun, the difference of the accelerating effects of the sun on the earth and moon is a disturbing force; and it is to this that the perturbations of the moon's orbit, which are the most important, and amongst the most considerable, in the solar system, are due. See MOON, PLANETS, &c.