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

by James Nasmyth; James Carpenter

We have now reached that stage of our subject at which it behoves us to repair to the telescope for the purpose of examining and familiarising ourselves with the various classes of detail that the lunar surface presents to our view.

That the moon is not a smooth sphere of matter is a fact that manifested itself to the earliest observers. The naked eye perceives on her face spots exhibiting marked differences of illumination. These variations of light and shade, long before the invention of the telescope, induced the belief that she possessed surface irregularities like those that diversify the face of the earth, and from analogy it was inferred that seas and continents alternated upon the lunar globe. It was evident, from the persistence and invariability of the dusky markings, that they were not due to atmospheric peculiarities, but were veritable variations in the character or disposition of the surface material. Fancy made pictures of these unchangeable spots: untutored gazers detected in them the indications of a human countenance, and perhaps the earliest map of the moon was a rough reproduction of a man’s face, the eyes, nose and mouth representing the more salient spots discernible upon the lunar disc. Others recognised in these spots the configuration of a human form, head, arms and legs complete, which a French superstition that lingers to the present day held to be the image of Judas Iscariot transported to the moon in punishment for his treason. Again, an Indian notion connects the lunar spots with a representation of a roebuck or a hare, and hence the Sanskrit names for the moon, mrigadhara, a roebuck-bearer, and ’sa’sabhrit, a hare-bearer. Of these similitudes the one which has the best pretensions to a rude accuracy is that first mentioned; for the resemblance of the full moon to a human countenance, wearing a painful or lugubrious expression, is very striking. Our illustration of the full moon ([Plate III].) is derived from an actual photograph;[4] the relative intensities of light and shade are hence somewhat exaggerated; otherwise it represents the full moon very nearly as the naked eye sees it, and by gazing at the plate from a short distance,[5] the well-known features will manifest themselves, while they who choose may amuse themselves by arranging the markings in their imagination till they conform to the other appearances alluded to.

We may remark in passing that by one sect of ancient writers the moon was supposed to be a kind of mirror, receiving the image of the earth and reflecting it back to terrestrial spectators. Humboldt affirmed that this opinion had been preserved to his day as a popular belief among the people of Asia Minor. He says, “I was once very much astonished to hear a very well educated Persian from Ispahan, who certainly had never read a Greek book, mention when I showed him the moon’s spots in a large telescope in Paris, this hypothesis as a widely diffused belief in his country: ‘What we see in the moon,’ said the Persian, ‘is ourselves; it is the map of our earth.’” Quite as extravagant an idea, though perhaps a more excusable one, was that held by some ancient philosophers, to the effect that the spots on the moon were the shadows of opaque bodies floating in space between it and the sun.

PLATE III.
FULL MOON.

An observer watching the forms and positions of the lunar face-marks, from night to night and from lunation to lunation, cannot fail to notice the circumstance that they undergo no easily perceptible change of position with respect to the circular outline of the disc; that in fact the face of the moon presented to our view is always the same, or very nearly so. If the moon had no orbital motion we should be led from the above phenomenon to conclude that she had no axial motion, no movement of rotation; but when we consider the orbital motion in connection with the permanence of aspect, we are driven to the conclusion—one, however, which superficial observers have some difficulty in recognising—that the moon has an axial rotation equal in period to her orbital revolution. Since the moon makes the circuit of her orbit in twenty-seven days and one-third (more exactly 27d. 7h. 43m. 11s.) it follows that this is the time of her axial rotation, as referred to the stars, or as it would be made out by an observer located at a fixed position in space outside the lunar orbit. But if referred to the sun this period appears different; because the moon while revolving round the earth is, with the earth, circulating around the sun. Suppose the three bodies, moon, earth, and sun, to be in a line at a certain period of a lunation, as they are at full moon: by the time the moon has completed her twenty-seven days’ journey around the earth, the latter will have moved along twenty-seven days’ march of its orbit, which is about twenty-seven degrees of celestial longitude: the sun will apparently be that much distant from a straight line passing through earth and moon, and the moon must therefore move forward to overtake the sun before she can assume the full phase again. She will take something over two days to do this; hence the solar period of her revolution becomes more than twenty-nine days (to be exact, 29d. 12h. 44m. 2s. ·87). This is the length of a solar day upon the moon—the interval from one sunrise to another at any spot upon the equator of our satellite, and the interval between successive reappearances of the same phase to observers on the earth. The physical cause of the coincidence of times of rotation and revolution was touched upon in a previous chapter.

We have said that the moon continuously presents to us the same hemisphere. This is generally true, but not entirely so. Galileo, by long scrutiny, familiarised himself with every detail of the lunar-disc that came within the limited grasp of his telescopes, and he recognised the fact that according as the position of the moon varied in the sky, so the aspect of her face altered to a slight degree; that certain regions at the edge of her disc, alternately came in sight and receded from his view. He perceived, in fact, an apparent rocking to and fro of the globe of the moon; a sort of balancing or libratory motion. When the moon was near the horizon he could see spots upon her uppermost edge, which disappeared as she approached the zenith, or highest point of her nightly path; and as she neared this point, other spots, before invisible, came into view, near to what had been her lower edge. Galileo was not long in referring this phenomenon to its true cause. The centre of motion of the moon being the centre of the earth, it is clear that an observer on the surface of the latter, looks down upon the rising moon as from an eminence, and thus he is enabled to see more or less over or around her. As the moon increases in altitude, the line of sight gradually becomes parallel to the line joining the observer and the centre of the earth, and at length he looks her full in the face: he loses the full view and catches another side face view as she nears the horizon in setting. This phenomenon, occurring as it does, with a daily period, is known as the diurnal libration.

But a kindred phenomenon presents itself in another period, and from another cause. The moon rotates upon her axis at a speed that is rigorously uniform. But her orbital motion is not uniform, sometimes it is faster, and at other times slower than its average rate. Hence, the angle through which she moves along her orbit in a given time, now exceeds, and now falls short of the angle through which she turns upon her axis. Her visible hemisphere thus changes to an extent depending upon the difference between these orbital and axial angles, and the apparent balancing thus produced is called the libration in longitude. Then there is a libration in latitude due to the circumstance that the axis of the moon is not exactly perpendicular to the plane of her orbit; the effect of this inclination being, that we sometimes see a little more of the north than of the south polar regions of our satellite, and vice versâ.[6]

The extent of the moon’s librations, taking them all and in combination into account, amounts to about seven degrees of arc of latitude or longitude upon the moon, both in the north-south and east-west directions. And taking into account the whole effect of them, we may conclude that our view of the moon’s surface, instead of being confined to one half, is extended really to about four-sevenths of the whole area of the lunar globe. The remaining three-sevenths must for ever remain a terra incognita to the habitants of this earth, unless, indeed, from some catastrophe which it would be wild fancy to anticipate, a period of rotation should be given to the moon different from that which it at present possesses. Some highly fanciful theorists have speculated upon the possible condition of the invisible hemisphere, and have propounded the absurd notion that the opposite side of the moon is hollow, or that the moon is a mere shell; others again have urged that the hidden half is more or less covered with water, and others again that it is peopled with inhabitants. There is, however, no good reason for supposing that what we may call the back of the moon has a physical structure essentially different from the face presented towards us. So far as can be judged from the peeps that libration enables us to obtain, the same characteristic features (though of course with different details) prevail over the whole lunar surface.

The speculative ideas held by the philosophers of the pre-telescopic age, touching the causes which produced the inequalities of light and shade upon the moon, received their coup de grâce from the revelations of Galileo’s glasses. Our satellite was one of the earliest objects, if not actually the first, upon which the Florentine turned his telescope; and he found that the inequalities upon her surface were due to differences in its configuration analogous to the continents and islands, and (as might then have been thought) the seas of our globe. He could trace, even with his moderate means, the semblance of mountain-tops upon which the sun shone while their lower parts were in shadow, of hills that were brightly illuminated upon their sides towards the sun, of brightly shining elevations, and deeply shadowed depressions, of smooth plains, and regions of mountainous ruggedness. He saw that the boundary of sunlight upon the moon was not a clearly defined line, as it would be if the lunar globe were a smooth sphere, as the Aristotelians had asserted, but that the terminator was uneven and broken into an irregular outline. From these observations the Florentine astronomer concluded that the lunar world was covered not only with mountains like our globe, but with mountains whose heights far surpassed those existing upon the earth, and whose forms were strangely limited to circularity.