The table and the following caption are from “A popular handbook and atlas of astronomy”, by Sir William Peck (1862-1925), published in New York in 1891.
The sun occupies the centre of the planetary system, producing and regulating the movements of all orbs lying within the sphere of his mighty attraction, and bountifully supplying them with his light and heat. These orbs, revolving round him, constitute the solar family, which consists of eight large planets and numerous smaller ones, divided into distinct classes, revolving in orbits widely separated from each other. The eight principal orbs are separated into two groups, which may be designated inner and outer planets; while revolving between these are the tiny orbs of the system — the asteroids, or small planets. The planet lying immediately outside the orbit of our earth is the most distant member of the inner group from the sun — the planet MARS; while the nearest member of the outer planets — JUPITER — is situated at a distance from the central orb no less than five times that of the earth. In the order of their distances from the sun, the inner planets are MERCURY, VENUS, THE EARTH, and MARS; and the outer planets, JUPITER, SATURN, URANUS, and NEPTUNE. There are, therefore, in the planetary system, three different groups of orbs. First, there is the inner, or terrestrial group, from MERCURY to MARS, of which our earth is the largest member; second, the small planets, or asteroids, revolving between the orbits of MARS and JUPITER; and third, the outer, or giant planets, from JUPITER to NEPTUNE, of which JUPITER is by far the largest.
Amongst the various members of these classes of planets there exists the most striking difference, in dimensions, in weight, and in condition. JUPITER, for instance, exceeds our earth, the largest of the inner planets, no less than 1300 times in volume, and is actually larger and heavier than all the other planets put together. In a greater proportion even, our globe exceeds the largest of the asteroids. In density, or comparative weight of material, there is likewise a marked contrast. The average density of the inner planets (about five times greater than that of water) is much greater than that of the outer orbs. The outer planets, on the other hand, are, on the average, about the same density as that of water. With regard to the velocity of rotation, there is also a considerable difference. Notwithstanding their enormous size, the outer planets rotate very rapidly; while the inner orbs perform their rotations in comparatively slow periods. But even in their present physical constitution there exists a marked contrast. The members of the inner group are chiefly solid globes, which, from their comparatively insignificant dimensions, have long since cooled from their former hot and molten condition, and are rapidly approaching that stage in each orb’s existence that may be called planetary old age. The outer planets, on the other hand, are mostly red-hot masses, surrounded by dense atmospheres, and fitly represent planetary youth, or the fiery condition of an orb before radiation has accomplished the process of cooling.
