The Moon: considered as a planet, a world, and a satellite.

by James Nasmyth; James Carpenter

Speaking generally, the details of the lunar surface seem to us to be devoid of colour. To the naked eye of ordinary sensitiveness the moon appears to possess a silvery whiteness: more critical judges of colour would describe it as presenting a yellowish tinge. Sir John Herschel, during his sojourn at the Cape of Good Hope, had frequent opportunities of comparing the moon’s lustre with that of the weathered sandstone surface of Table Mountain, when the moon was setting behind it, and both were illuminated under the same direction of sunlight; and he remarked that the moon was at such times “scarcely distinguishable from the rock in apparent contact with it.” Although his observations had reference chiefly to brightness, it can hardly be doubted that similarity of colour is also implied; for any difference in the tint of the two objects would have precluded the use of the words “scarcely distinguishable;” a difference of colour interfering with a comparison of lustre in such an observation, though it must be remembered that he observed through a dense stratum of atmosphere. Viewed in the telescope, the same general yellowish-white colour prevails over all the moon, with a few exceptions offered by the so-called seas. The Mare Crisium, Mare Serenetatis, and Mare Humorum have somewhat of a greenish tint; the Palus Somnii and the circular area of Lichtenberg incline to ruddiness. These tints are, however, extremely faint, and it has been suggested by Arago that they may be mere effects of contrast rather than actual colouration of the surface material. This, however, can hardly be the case, since all the “seas” are not alike affected; those that are slightly coloured are, as we have said, some green and some red, and contrast could scarcely produce such variations. The supposition of vegetation covering these great flats and giving them a local colour is in our view still more untenable, in the face of the arguments that we shall presently adduce against the possibility of vegetable life existing upon the moon.

It appears to us more rational to consider the tints due to actual colour of the material (presumably lava or some once fluid mineral substance) that has covered these areas; and it may well be conceived that the variety of tint is due to different characters of material, or even various conditions of the same material coming from different depths below the lunar surface; and we may reasonably suppose that the same variously-coloured substances occur in the rougher regions of the lunar surface, but that they exist there in patches too small to be recognized by us, or are “put out” by the brightness to which polyhedral reflexion gives rise.

Seeing that volcanic action has had so large a share in giving to the moon’s surface its structural character, analogy of the most legitimate order justifies us in concluding not only that the materials of that surface are of kindred nature to those of the unquestionably volcanic portions of the earth, but also that the tints and colours that characterize terrestrial volcanic and Plutonian products have their counterparts on the moon. Those who have seen the interior and surroundings of a terrestrial volcano after a recent eruption, and before atmospheric agents have exercised their dimming influences, must have been struck with the colours of the erupted materials themselves and the varied brilliant tints conferred on these materials by the sublimated vapours of metals and mineral substances which have been deposited upon them. If, then, analogy is any guide in enabling us to infer the appearance of the invisible from that which we know to be of kindred nature and which we have seen, we may justly conclude that were the moon brought sufficiently near to us to exhibit the minute characteristics of its surface, we should behold the same bright and varied colours in and around its craters that we behold in and about those of the earth; and in all probability the coloured materials of lunar volcanoes would be more fresh and vivid than those of the earth by reason of the absence of those atmospheric elements which tend so rapidly to impair the brightness of coloured surfaces exposed to their influence.

Situated as we are, however, as regards distance from the moon, we have no chance of perceiving these local colours in their smaller masses; but it is by no means improbable, as we have suggested, that the faint tints exhibited by the great plains are due to broad expanses of coloured volcanic material.

But if we fail to perceive diversity of colour upon the lunar surface, we are in a very different position in regard to diversity of brightness or variable light-reflective power of different districts and details. This will be tolerably obvious to those casual observers who have remarked nothing more of the moon’s physiography than the resemblance to a somewhat lugubrious human countenance which the full moon exhibits, and which is due to the accidental disposition of certain large and small areas of surface material which have less of the light-reflecting property than other portions; for since all parts seen by a terrestrial observer may be said to be equally shone upon by the sun, it is clear that apparently bright and shaded parts must be produced by differences in the nature of the surface as regards power of reflecting the light received.

When we turn to the telescope and survey the full disc of the moon with even a very moderate amount of optical aid, the meagre impression as to variety of degree of brightness which the unassisted eye conveys is vastly extended and enhanced, for the surface is seen to be diversified by shades of brilliancy and dullness from almost glittering white to sombre grey: and this variety of shading is rendered much more striking by shielding the eye with a dusky glass from the excessive glare, which drowns the details in a flood of light. Under these circumstances the varieties of light and shade become almost bewildering, and defy the power of brush or pencil to reproduce them.

We may, however, realize an imperfect idea of this characteristic of the lunar surface by reference to the self-drawn portrait of the full moon upon [Plate III]. This is, in fact, a photograph taken from the full moon itself, and enlarged sufficiently to render conspicuous the spots and large and small regions that are strikingly bright in comparison with what may in this place be described as the “ground” of the disc. As an example of a wide and irregularly extensive district of highly reflective material, the region of which Tycho is the central object, is very remarkable. We may refer also to the bright “splashes” of which Copernicus and Kepler are the centres. So brilliant are these spots that they can easily be detected by the unassisted eye about the time of full moon. Still brighter but less conspicuous by its size is the crater Aristarchus, which shines with specular brightness, and almost induces the belief that its interior is composed of some vitreous-surfaced matter: the highly reflective nature of this object has often caused it to become conspicuous when in the dark hemisphere of the moon, unilluminated by the sun, and lighted only by the light reflected from the earth. At these times it appears so bright that it has been taken for a volcano in actual eruption, and no small amount of popular misconception at one time arose therefrom concerning the conditions of the moon as respects existing volcanic activity—a misconception that still clings to the minds of many.

The parts of the surface distinguished by deficiency of reflecting power are conspicuous enough. We may cite, however, as an example of a detail portion especially remarkable for its dingy aspect, the interior of the crater Plato, which is one of the darkest spots (the darkest well defined one) upon the hemisphere of the moon visible to us. For facilitating reference to shades of luminosity, Schroeter and Lohrman assorted the variously reflective parts into 10 grades, commencing with the darkest. Grades 1 to 3 comprised the various deep greys; 4 and 5 the light greys; 6 and 7 white; and 8 to 10 brilliant white. The spots Grimaldi and Riccioli came under class 1 of this notation; Plato between 1 and 2. The “seas” generally ranged from 2 to 3; the brightest mountainous portions mostly between degrees 4 and 6; the crater walls and the bright streaks came between these and the bright peaks, which fell under the 9th grade. The maximum brightness, the 10th grade, is instanced only in the ease of Aristarchus and a point in Werner, though Proclus nearly approaches it, as do many bright spots, chiefly the sites of minute craters, which make their appearance at the time of full moon.

In photographic pictures produced by the moon of itself, there is always an apparent exaggeration in the relation of light to dark portions of the disc. The dusky parts look, upon the photograph, much darker than to the eye directed to the moon itself, whether assisted or not by optical appliances. It may be that the real cause of this discrepancy is that the eye fails to discover the actual difference upon the moon itself, being insensible to the higher degrees of brightness or not estimating them at their proper brilliance with respect to parts less bright. On the other hand, it is probable that the enhanced contrast in the photograph is due to some peculiar condition of the darker surface matter affecting its power of reflecting the actinic constituent of the rays that fall upon it.

The study of the varying brightness or reflective power of different regions and spots of the lunar disc leads us to the consideration of the relative antiquity of the surface features; for it is hardly possible to regard these variations attentively without being impressed with the conviction that they have relation to some chronological order of formation. We cannot, in the first place, resist the conviction that the brightest features were the latest formed; this strikes us as evident on primâ facie grounds; but it becomes more clearly so when we remark that the bright formations, as a rule, overlie the duller features. The elevated parts of the crust are brighter than the “seas” and other areas; and it is pretty clear that the former are newer than the latter, upon which they appear to be super-imposed, or through which they seem to have extruded.[13] The vast dusky plains are in every instance more or less sprinkled with spots and minute craters, and these last were obviously formed after the area that contains them. One is almost disposed to place the order of formations in the order of relative brightness, and so consider the dingiest parts the oldest and the brightest spots and craters the newest features, though, in the absence of an atmosphere competent to impair the reflective power of the surface materials, we are unable to justify this classification by suggesting a cause for such a deterioration by time as the hypothesis pre-supposes.