Since weight is nothing else than the pull which the earth exerts on a body, it follows that, big and massive as our planet is, it must, nevertheless, be weightless; for it cannot with any degree of propriety be said to pull itself. It is incapable of producing even an infinitesimal change in the position of its mass-center, or center of gravity, as this centroid is sometimes called. The earth attracts _itself_ with no force whatever; but is attracted and governed in its annual movement by the sun, the central controlling body of our system, while the moon and planets play only the part of petty disturbers. It would, however, be right to speak of the _weight_ of the earth _relatively_ to the sun; for the sun attracts the mass of our planet with a certain definite force, readily calculable from the familiar formula for central force, viz., mv^2/r., in which _m_ is the mass of the earth, _v_ its orbital velocity and _r_ its distance from the sun. Supplying the numbers, the weight of the earth relatively to the sun, comes out to be 3,000000,000000,000000 or 3×10^{18} tons weight, or, in words, three million million million tons weight. It may here be noted that the velocity _v_ of the earth in its orbit is a varying quantity, depending on distance from the sun. As this distance is least in December and greatest in June, it follows that the earth is heavier relatively to the sun in winter than it is in summer. The _mass_ of the earth, on the other hand, is not a relative and variable quantity, but a constant and independent one, which would not be affected either by the sudden annihilation of all the other members of the solar system or by the instantaneous or successive addition of a thousand orbs. Mass being the product of volume by density, that of the earth is 6000,000000,000000,000000 or 6×10^{21} tons mass, which reads six thousand million million million tons mass. The number which expresses the mass of the earth is thus very different from that which represents its weight relatively to the sun. It is obvious that the latter would be a much greater quantity if our planet were transferred to the orbit of Venus and very much less if transferred to that of far-off Jupiter, but the number which expresses its _mass_ would remain precisely the same in both cases, viz., the value given above.