Mercury and Venus are the only planets as yet that turn a constant face to their overruling lord. The reason for this appears when one goes into the matter analytically. The tidal force is not the direct pull of the Sun on a particle of the body, but the difference in the pulls upon a particle at the centre and one at the circumference. Being differential, it depends directly upon the radius of the distorted body and inversely upon the third power of its distance away. As the space through which the force acts is proportional to the force itself, the effect is as the squares of the quantities mentioned, or, inversely, as the sixth power of the distance and as the square of the body’s radius. The result thus proves greatest on the planets nearest to the Sun, and diminishes rapidly as we pass outward from him. If, then, the solar force had had time enough to produce its effects, it would be first in Mercury and then in Venus that it should be seen. And this is precisely where we observe it.
The Moon presents us a well-known case of such filial regard, resulting in permanent incompetency of action on its own account. It turns always the same face to us, following us about with the mute attention of a dog to its master. Here again the libration may be detected, for no dog but makes excursions on the road. This case differs from those of Mercury and Venus in that the body to which the regard is paid is not also the dispenser of light and warmth. In consequence, though the side of the Moon with which we are presented remains always the same, we do not always see it; the light creeping over it with the progress of the lunation, from new to full. On this account the worst that happens to our Moon in its old age is that its day becomes its month.
Moon—full and half, photographed at the Lowell Observatory.
Our Moon is not peculiar in having its day and its month the same. On the contrary, it is now the rule with satellites thus to protract their days. So far as we can observe, all the large satellites of Jupiter turn the same face to him; those of Saturn pay him a like regard; while about those of Uranus and Neptune we are too far off to tell. Their direct respect for their primary, with only secondary recognition of the Sun, keeps them from the full consequences of their fatal yielding to attraction. It is bad enough to have the day half a month long, but worse to have one that never ends, or, still worse, perpetual night.
In our diagnosis of the cause of death in planets, we now pass from paralysis to heart failure. For so we may speak of the next affection which ends in their taking off, since it is due to want of circulation and lack of breath. It comes of a planet’s losing first its oceans and then its air.
To understand how this distressing condition comes about, we must consider one of the interesting scientific legacies of the nineteenth century to the twentieth: the kinetic theory of gases.
