Galileo himself wrote: “I do not believe that the body of the Moon is composed of earth and water.” The name of maria was given to the great grey plains of the Moon by Hevelius, but this was simply for convenience of nomenclature, not because he actually believed them to be seas. One observation is, in itself, sufficient to prove that the maria are not water surfaces. The Moon’s “terminator,” that is to say, the line dividing the part in sunlight from that in darkness, is clearly irregular when it passes over the great plains; were they actually sea it would be a bright line and perfectly smooth. The grey plains are therefore not expanses of water now, nor were they in time past. It is obvious that in some remote antiquity their surface was in a fluid condition, but it was the fluidity of molten rock. This is seen by the way in which the maria have invaded, breached, broken down, and submerged many of the circular formations on their margins. Thus the Mare Humorum has swept away half the wall of the rings, Hippalus and Doppelmayer, and far out in the open plain of the Mare Nubium, great circles like Kies, and that immediately north of Flamsteed, stand up in faint relief as of half-submerged rings. Clearly there was a period after the age in which the great ring mountains and walled plains came into existence, when an invasive flood attacked and partially destroyed a large proportion of them. And the flood itself evidently became more viscous and less fluid the further it spread from its original centre of action, for the ridges and crumpling of the surface indicate that the material found more and more difficulty in its flow.
We have evidence just as direct that there is no atmosphere. This is very strikingly shown when the Moon, in its monthly progress among the stars, passes before one of them and occults it. Such an occultation is instantaneous, and is particularly impressive when either a disappearance or a reappearance occurs at the defective limb; that is to say, at the limb which is not illuminated by the Sun, and is therefore invisible. The observer may have a bright star in the field of view, showing steadily in a cloudless sky; there is not a hint of a weakening in its light; suddenly it is gone. The first experience of such an observation is most disconcerting; it is hardly less disconcerting to observe the reappearance at the dark limb. One moment the field of view of the telescope is empty; the next, without any sort of dawning, a bright star is shining steadily in the void, and it almost seems to the observer as if an explosion had taken place. If the Moon had an atmosphere extending upwards from its surface in all directions and of any appreciable density, an occultation would not be so exceedingly abrupt; and, in particular, if the occultation were watched through a spectroscope, then, at the disappearance, the spectrum of the star would not vanish as a whole, but the red end would go first, and the rest of the spectrum would be swept out of sight successively, from orange to the violet. This does not happen; the whole spectrum goes out together, and it is clear that no appreciable atmosphere can exist on the Moon. In actual observation so inappreciable is it that its density at the Moon’s surface is variously estimated as 1⁄300th of that of the Earth by Neison, and as 1⁄10000th by W. H. Pickering. If the Moon possessed an atmosphere bearing the same proportion to her total mass as we find in the case of the Earth, she would have a density of one-fortieth of our atmosphere at the sea level.
The Moon is at the same mean distance from the Sun as the Earth, and therefore, surface for surface, receives from it on the average the same amount of light and heat. But it makes a very different use of these supplies. Bright as the Moon appears when seen at the full on some winter night, it has really but a very low power of reflection, and is only bright by contrast with the darkness of the midnight sky. If the full Moon is seen in broad daylight, it is pale and ghost-like. Sir John Herschel has put it on record that when in South Africa he often had the opportunity of comparing the Moon with the face of Table Mountain, the Sun shining full upon both, and the Moon appeared no brighter than the weathered rock. The best determinations of the albedo of the Moon, that is to say, of its reflective power, give it as 0·17, so that only one-sixth of the incident light is reflected, the other five-sixths being absorbed. It is difficult to obtain a good determination of the Earth’s albedo, but the most probable estimate puts it as about 0·50, or three times as great as that of the Moon. This high reflective power is partly to be accounted for by the great extent of the terrestrial polar caps, but chiefly by the clouds and dust layer always present in its atmosphere.
A larger proportion, therefore, of the solar rays are employed in heating the soil of the Moon than in heating that of the Earth, and in this connection the effect of an important difference between the two worlds must be noted. The Earth rotates on its axis in 23 hours 56 minutes 4 seconds, the mean length of its rotation as referred to the Sun being 24 hours. The rotation of the Moon, on the other hand, takes 27 days 7 hours 43 minutes to accomplish, giving a mean rotation, as referred to the Sun, of 29 days 12 hours 44 minutes. The lunar surface is therefore exposed uninterruptedly to the solar scorching for very nearly fifteen of our days at a time, and it is, in turn, exposed to the intense cold of outer space for an equal period. As the surface absorbs heat so readily, it must radiate it as quickly; hence radiation must go on with great rapidity during the long lunar night. Lord Rosse and Prof. Very have both obtained measures of the change in the lunar heat radiation during the progress of a total eclipse of the Moon, with the result that the heat disappeared almost completely, though not quite at the same time as the light. Prof. Langley succeeded in obtaining from the Moon, far down in the long wave lengths of the infra-red, a heat spectrum which was only partly due to reflection from the Sun; part coming from the lunar soil itself, which, having absorbed heat from the Sun, radiated it out again almost immediately. In 1898, Prof. Very, following up Langley’s line of work, concluded that the temperature of the lunar soil must range through about 350° Centigrade, considerably exceeding 100° at the height of the lunar day, and falling to about the temperature of liquid air during the lunar night. So wide a range of temperature must be fatal to living organisms, particularly when the range is repeated at short, regular intervals of time. But this range of temperature comes directly from the length of the Moon’s rotation period; for the longer the day of the Moon, the higher the temperature which may be attained in it; the longer the night, the greater the cold which will in turn be experienced. We learn, therefore, that the time of rotation of a planet is an important factor in its habitability.
CHAPTER VI
THE CANALS OF MARS
Both of the two worlds best placed for our study are thus, for different reasons, ruled out of court as worlds for habitation. The Sun by its vastness, its intolerable heat and the violence of its changes, has to be rejected on the one hand, while the Moon, so small, and therefore so rigid, unchanging and bare, is rejected on the other.
Of the other heavenly bodies, the planet Mars is the one that we see to best advantage. Two other planets, Eros and Venus, at times come nearer to us, but neither offers us on such occasions equal facilities for their examination. But of Mars it has been asserted not only that it is inhabited, but that we know it to be the case, since the evidence of the handiwork of intelligent beings is manifest to us, even across the tremendous gulf of forty or more million miles of space.