The range of planetary masses, in fact, is very curious, and is doubtless of much significance in the cosmogony, with which we deal later. If we consider the sun and his eight planets, the mass or weight of each of the nine bodies far exceeds the combined mass of all the others which are lighter than itself. To illustrate: suppose we take as our unit of weight the one-billionth part of the sun's weight; then the planets in the order of their masses will be Mercury, Mars, Venus, Earth, Uranus, Neptune, Saturn, and Jupiter. According to their relative masses, then, Mercury being a five-millionth part the weight of the sun will be represented by 200; similarly Venus, a four hundred and twenty-five thousandth part by 2,350, and so on. Then we have Mercury 200 Mars 340 ------ Sum of weights of Mercury and Mars 540 Venus 2,350 ------ Sum of weights of Mercury, Mars, and Venus 2,890 The Earth 3,060 ------ Sum of weights of four inner planets 5,950 Uranus 44,250 ------ Sum of weights of five planets 50,200 Neptune 51,600 ------- Sum of weights of six planets 101,800 Saturn 285,580 --------- Sum of weights of seven planets 387,380 Jupiter 954,300 --------- Sum of weights of all the planets 1,341,680 Mass or weight of the sun 1,000,000,000 Curious and interesting it is that Saturn is nearly three times as heavy as the six lighter planets taken together, Jupiter between two and three times heavier than all the other planets combined, while the sun's mass is 750 times that of all the great planets of his system rolled into one. All the foregoing masses, except those of Mercury and Venus, are pretty accurately known because they were found by the satellite method just indicated. Mercury's mass is found by its disturbing effects on Encke's comet whenever it approaches very near. The mass of Venus is ascertained by the perturbations in the orbital motion of the earth. In such cases the Newtonian law of gravitation forms the basis of the intricate and tedious calculations necessary to find out the mass by this indirect method.