"Neptune, invisible to the naked eye, but seen as a pale blue star in the telescope, is 2,780 million miles from the sun, and makes a revolution in 165 years. His diameter is about 35,000 miles, and his density rather less than that of water.
"Neptune has one satellite, at a distance of 202,000 miles, which, like those of Uranus, revolves about its primary in an orbit at a considerable angle to his own in five days twenty-one hours. Both Neptune and Uranus are probably dying suns.
"Comets of unknown number travel in long elliptical or parabolic orbits round the sun at great velocities. They seem to consist partly of glowing vapours, especially hydrogen, and partly of meteoric stones. 'Shooting stars,' that is to say, stones which fall to the earth, are known to swarm in their wake, and are believed to be as plentiful in space as fishes in the sea."
"The trash or leavings of creation," said I reflectively.
"And the raw material, for nothing is lost," rejoined Gazen. "Now, in spite of all its diversity, there is a remarkable symmetry in the solar system. The planets are all moving round the sun in one direction along circular paths. As a rule each is nearly as far again from the sun as the next within it. Thus, if we take Mercury as ¾ inch from the sun, Venus is about 1¼ inches, the Earth 2¼, Mars 2, the planetoids 5¼, Jupiter 9¾, Saturn 14, Uranus 36, and Neptune 60 inches. On the same scale, by the way, Enckes' comet at Aphelion, its farthest distance from the sun, would be about 12 feet; Donatis almost a mile; and Alpha Centauri, a near star in the Milky Way, some ten miles.
"The stately march of the planets in their orbits becomes slower the farther they are from the sun. The velocity of Mercury in its orbit is thirty, that of Jupiter is eight, and that of Neptune is only three miles a second. On the other hand, the inner planets, as a rule, take some twenty-four hours, and the outer only ten hours to spin round their axis. The inner planets are small in comparison with the outer. If we represent the sun by a gourd, 20 inches in diameter, Mercury will seem a bilberry (⅟₁₆ inch) Venus, a white currant, the Earth a black currant (¼ inch), Mars a red currant (⅛ inch), the planetoids as fine seed, Jupiter an orange or peach (2 inches), Saturn a nectarine or greengage (1 inch), Uranus a red cherry (¾ inch), and Neptune a white cherry (barely 1 inch in diameter). By putting the sun and planets in a row, and drawing a contour of the whole, we obtain the figure of a dirk, a bodkin, or an Indian club, in which the sun stands for the knob (disproportionately big), the inner planets for the handle, and the outer for the blade or body. Again, the average density of the inner planets exceeds that of the outer by nearly five to one, but the mass of any planet is greater than the combined masses of all which are smaller than it. The inner planets derive all their light and heat from the sun, and have few or no satellites; whereas the outer, to all appearance, are secondary suns, and have their own retinue of worlds. On the similitude of a clan or house we may regard the inner planets as the immediate retainers of the chief, and the outer as the chieftains of their own septs or families."