These notes are equally applicable to the other terrestrial spheres of our - and no doubt any other - solar system. Each planet is the result of the condensation and cooling of a massive ejection of the Sun. We see similar eruptions, abet on a far smaller scale, regularly, along with sunspots. These lesser ejections are likely the source of meteor swarms and other interplanetary - perhaps inter stellar - debris. Only the largest ejections have had sufficient mass to pull themselves together and form proto-planetary systems.
As the ejecta cools it condenses into separate masses, orbiting a common center. These are the primitive gasses that will condense into planets and moons. It is noteworthy that all such masses are approximately equal; the greatest planet of the Solar System has not much more than twice the mass of the smallest true moon. (There are, of course, smaller "false" moons, being captures asteroids. Earth herself has three such, so small they are little more than steadily orbiting meteors, so small they are obscured when back-lit by Luna and scarcely cause a twinkling when they pass between a terrestrial observer and a star.)
Just as these spheres of superheated gaseous mater orbit a common center, so too does the matter within each sphere orbit the center of its sphere. Slowly, the surface of the spheres gives up their heat to the infinite chill of space. This cooling is very uneven, for the interior - a superheated and thus most tenuous of gaseous mixtures - is slow to cool, surrounded as it is by like heated gaseous matter. When the exterior has already cooled to form a solid crust, the interior is still hot enough to be exerting pressure upon the rapidly - by geologic time-scales - congealing surface.
When the interior does, in its turn, cool, the heavier material forms first droplets and then particles. These are pushed by centrifugal force, which it increases by its condensation, to adhere to the inner side of the crust. Thus are laminated layers within the shell, layers ordered, at least initially, in inverse order by their rate of cooling and condensation. Naturally, a great deal of gas condenses to form but a relatively little amount of solid matter. Finally, the remaining gas falls in upon itself. Its remaining heat is primarily expelled in the form of light; thus is formed the inner sun.
Due to the centrifugal forces of the spinning sphere, the matter becomes thickest and densest about the equatorial region while thinning at the poles. Eventually, one or, as in the case of Earth, pole may spin out into an aperture, Polar or Arctic and Antarctic Openings. Atmospheric irregularities caused by the edge and the extreme cold of the outer polar surface contrasted with the tropical inner polar surface make observation surprisingly difficult.
The planets clearly differ greatly in size. This is, however, not due as much to their mass (and therefore gravity) as it is their ages. True, their masses do vary, but not substantially. As a planet ages, centrifugal force causes its hollow shell to expand. Thankfully, this is a slow, gradual effect, which, although it may result in the occasional spasm of earthquakes or volcanic activity, has only rarely resulted in a planet or moon actually spinning itself apart and shattering into an asteroid belt or ring. Astronomical observances which effected calculations yielding estimates of greatly differing gravities, while patently absurd today, were due to a lack of understanding of the effect of the omnipenitrative and transubstantial liquid aether, that very same nonphysical substance against which the propellers of the interplanetary steamships push. The larger a planet spins out to be, the greater its volume. The particular compliment of minerals which individualize a given heavenly body also affects how it interacts with the aether through which all things travel.
Moons are smaller, not because they have less matter, but because they have smaller interior hollows. A moon's tendency to expand is transferred to the planet it orbits, just as it absorbs a portion of its mother-planet's spin. (Obviously, Luna is actually more properly considered a twin of Earth, and is not a true moon.) Further difference is often found in the particular nature of the matter an individual heavenly body is composed of, some being more elastic than others.
INNER MOONS and FLOATING ISLANDS: One peculiarity of the condensation of such quantities of matter into solid state with in such an ocean of molten gas is that some clumps form as froth with gas bobbles within. These become as floating solid clouds, suspended between the surface of the inner world and the interior sun. They may float about, their movements determined orbitally if larger and by atmospheric current if smaller, indefinitely, islands in the sky. Larger masses are likely to clump up, forming as last the equivalent of inner moons. By virtue of their size, these inner moons are likely, if of different sizes and thus speeds, to catch up with each other and congeal together; only those which "balance" each other will remain apart, and their influence upon each other will tend to stabilize their equidistant positions upon their common equatorial orbit.
These inner moons are of significance as they may not only reflect the light of the inner sun, like a plaque reflecting the light of a candle in front of it, but more significantly cast a shadow below, where the lighting is but by reflection from the further surface of the inner sphere. Such an inner moon will create a cycle of short nights in its shadow alternating with long days when it is not directly above. Of course, this will only pertain to an equatorial zone or belt of a width determined by the size of the inner moon and its distance from the inner sun. Inside worlds lacking such an inner moon, and, indeed, away from the equatorial regions of those which to have such a moon-shadow night, the inner sun will stand perpetually at noon, warmer on an infinite day.
Lesser clumps of such "solid clouds" generally circulate at lower altitudes than do the inner moons. Notably, some do not circulate at all, but, whether due to size, magnetism, or other reason, remain in a stationary orbit, seeming fixed in place. Such set sky-islands cast shadows of "perpetual night" below them, with climactic effects reaching far beyond.
The weather inside the world would be utterly even, were it not for the differential, temporary cooling caused by the moon-shadow. Thus, in inverse to the outer world, it is the poles that are tropical and the equator that is temperate. True cold it to be found primarily upon the upper summits of the greatest mountains, as is the case with the glaciated peaks of the highest mountains of even the torrid regions of the external world. Cooling air moves down and out from the traveling spot of night, and, following the abrupt dawn as the crescent edge of the inner moon moves on, the ground, warming, causes the air above it to rise, and thus be drawn from the region around it. Seas, which stabilize temperatures or carry them by liquid currents, to far regions, affect the local climate. Mountains rising high create cool places by virtue of giving off heat in the thinner atmosphere of the heights. Moisture is squeezed out of the air as it rises to pass about the peaks. Perpetual cloudbanks are often dependent from their downwind side, which creates a moist clime there.
The hollow insides of other, apparently arid, worlds may retain all their youthful lushness and moisture even when the exteriors have turned dry and barren. Some speculate that they are dry, not because of evaporation - for surely evaporating water rising too high crystallized and fall back down to perpetuate the atmospheric vapor - but because a portion of that water which seeps downwards into the soil and between the rocks fails to ever rise again. It is speculated that such water must needs dissolve away a space for itself, creating an interlocking series of sea-vast chambers, each lower than the last, until the waters at last reach the plane which defines that place mid-way between the Inner and Outer worlds of each of the Heavenly Spheres of our - and, no doubt, any other - solar system.
Preventing the easy fulfillment of such a settling out of the waters of a world are the subterranean fires, which drive waters surfaceward again. The pressure created by the immense weight of the crust's inner and outer sides, and the incredible energies resulting from the slippage, however geologically slow in its proceeding, of the inner and outer surfaces, results in the heating of the masses of rock under the places of greatest weight or friction. That is to say, the earth is heated to magma, either in pockets or in belts like subterranean rivers of lava, under the highest mountains or along the edges of ranges. We often see the result as volcanic activity along mountain ranges. Where we do not see this, as, for example, Mount Kenya, we can be assured that there is an area of volcanic activity all the greater at the corresponding location on the other surface. This is the reason we see some volcanic activity in places where there is no mountain of sufficient size to explain the local volcanism - or, at least there is no such massif on our side of the crustal sphere! Naturally, such volcanic forces diminish at the sphere ages, just as its inner sun fades by infinitesimal degrees and the sphere's orbit slowly unwinds, running further and further from the Sun, making way for the next, younger planetary system in the succession.
As the planets closer to the Sun reflect younger geologic ages, so do they reflect younger stages of biologic evolution. (It should also be noted that, as the interiors take longer to cool, the Inner Worlds are younger than the Outer Worlds.) The biology of the various planets reflects this, but, as each has evolved in its own way, they vary greatly. In contrast are the many species, both animal and vegetable, which are not merely similar but exactly identical, upon the various worlds. Obviously, in antediluvian times some ancient civilization - no doubt originating upon one of the outer planets - carried on interplanetary - some suggest interstellar - traffic.