a planet has not. The reason is this: the mass of a body is known from the pull it exerts, inasmuch as this pull depends, by the law of gravitation, upon its mass and the square of its distance. If then we know the pull and the distance from which it is exerted, we can find the mass. Now we gauge the pull from its effects in causing some other body to move. By measuring, therefore, the motion of this other body, we learn the mass of the first one. To get this accurately the motion must be large enough to admit of satisfactory measurement in the first place, and be as uncomplicated with motions due to pulls of other bodies as possible, in the second. As each body pulls every other, and it is only their relative displacement we can measure, as we have no foothold in space, even the case of only two bodies presents difficulties of apportionment. We can learn the aggregate mass of the two, but not the separate mass of either alone unless it so happen that the mass of one is so insignificant compared with the other that the mass of that other may be taken as the mass of both. Now this is substantially realized in the case of the solar system. Owing to the greatly disproportionate size of primary and secondary bodies in it, the great size of the Sun as compared with that of any of the planets, and the great size of the planets as compared with their satellites
