The Story of the Solar System :: CHAPTER VI. THE MOON.

CHAPTER VI. THE MOON.

The Moon being merely the satellite of a planet, to wit, the Earth, it should, according to the plan of this book, be included in the chapter which deals with its primary; but for us inhabitants of the Earth the Moon has so many special features of interest that it will be better to give it a special chapter to itself.

We may regard the Moon in a twofold aspect, and consider what it is as a mere object to look at, and what it does for us; probably my present readers will prefer that most prominence shall be given to the former aspect. The Moon as seen with the naked eye exhibits a silvery mass of light, which at the epoch of what is called “full Moon” has a seemingly even circular outline. Full or not full, its surface appears to be irregularly shaded or mottled. The immediate cause of this shading is the fact that the surface of the Moon, not being really smooth, reflects irregularly the Sun’s light which falls upon it. The causa causans of this is the existence of numerous mountains and valleys on its surface, and which were first discovered to be such by Galileo. That there are mountains is proved by the shadows cast by their peaks on the surrounding plains, when the Sun illuminates the Moon obliquely—that is, when the Moon is shining either as a crescent or gibbous. Such shadows, however, disappear at the phase of “Full-Moon,” because the Sun’s rays then fall perpendicularly on the Moon’s surface. When the Moon presents either a crescent or a gibbous form (in point of fact when it presents any form except that of “Full-Moon”), the boundary line which separates the illuminated from the unilluminated portion (and which boundary line is generally spoken of as the “terminator”) has a rough, jagged appearance; this is due to the fact that the Sun’s light falls first on the summits of the peaks, and that the adjacent valleys and declivities are in shade. These remain so till by reason of the Moon’s progress in its orbit a sufficient time has elapsed for the Sun to penetrate to the bottom of the valleys. With this explanation the reader will have no difficulty in realising why the terminator always exhibits an irregular or jagged edge.

Fig. 11.—Mare Crisium. (Lick Observatory photographs.)

Various mountains on the Moon to the number of more than a thousand have been mapped, and their elevations calculated. Of these fully half have received names, being those of men of various dates and nationalities, who have figured conspicuously in the annals of science, including some, however, who have not done so. Whilst many of these mountains are isolated elevations, not a few form definite chains of mountains, and to certain of these chains definite names, borrowed from the Earth, have been given. Thus we find on maps of the Moon the “Apennines,” the “Alps,” the “Altai Mountains,” the “DÖrfel Mountains,” the “Caucasus Mountains,” and so on.

Besides the mountains there exist on the Moon a number of plains analogous in some sense to the “steppes” of Asia and the “prairies” of North America. These were termed “seas” in the early days of the telescope, because it was assumed that as they were so large and so smooth they were vast tracts of water. This supposition has long ago been overthrown, but the names have been retained as a matter of convenience. Hence it comes about that in descriptions of the Moon one meets with such names as Mare Imbrium, the “Sea of Showers”; Mare Serenitatis, the “Sea of Serenity”; Mare Tranquillitatis, the “Sea of Tranquillity”; and so on. It seems probable that the so-called seas represent in nearly its original form what was once the original surface of the Moon before the mountains were formed. A confirmation of this idea is to be found in the fact that though these plains are fairly level surfaces compared with the masses of mountains which hedge them in on all sides, yet the plains themselves are dotted over with inequalities (small elevations and pits), which seem to suggest that some of them might eventually have developed into mountains if the further formation of mountains had not been arrested by the fiat of the Creator.

Though hitherto we have been speaking of the mountains of the Moon under that generic title, it is necessary for the reader to understand that the Moon’s surface exhibits everywhere remarkable illustrations of those geological processes which we on the earth associate with the word “volcano.” There cannot be the least doubt that the existing surface of the Moon, as we see it, owes all its striking features to volcanic action, differing little from the volcanic action to which we are accustomed on the earth. That this theory is well founded may be very easily inferred by comparing the structural details of certain terrestrial volcanoes and their surroundings with a typical lunar mountain, or indeed, I might say, with any lunar mountain. This point was very well worked out some 40 years ago by Professor Piazzi Smyth, who placed on pictorial record his results of an examination and survey of the Peak of Teneriffe. Any person seeing side by side one of Smyth’s pictures of Teneriffe and a picture of any average lunar crater would find great difficulty if the pictures were not labelled in determining which was which.

The one special feature of the Moon, which never fails to attract the attention of everybody who looks at our satellite for the first time through a telescope, are the crater mountains, which indeed constitute an immense majority of all the lunar mountains. Their outline almost always conforms, more or less, to that of the circle, but when seen near either limb of the Moon they often appear considerably oval simply because they are then seen considerably foreshortened. In their normal form they exhibit a basin bounded by a ridge, with a conical elevation in the centre of the basin, the basin and the cone together being evidently the result of an uprush of gases breaking through the outer crust of the Moon and carrying with them masses of molten lava. This lava, with perhaps the materials in fragments, projected in the first instance up into the air, fell back on to the Moon forming first of all the outer edge of the basin, and subsequently, as the eruptive force became weakened, the small central accumulation, which took, as it naturally would do, a conical shape. An experimental imitation of the process thus inferred was carried out some years ago by a French physicist, Bergeron, who acted upon a very fusible mixture of metals known as Wood’s alloy by forcing through it a current of hot air. The success of this experiment was complete, and Bergeron considered that his experiments, taken as a whole, were calculated to throw much light on the past history of the Moon.

Several observers at various times have fancied they have seen signs that the lunar mountain Aristarchus was an active volcano even up to the present century; but it admits of no doubt that this idea is altogether a misconception, and that what they saw as a faint illumination of the summit of Aristarchus was no more than an effect of earth-shine. On the general question of volcanic action on the Moon, Sir John Herschel summed up as follows:—“Decisive marks of volcanic stratification arising from successive deposits of ejected matter, and evident indications of lava currents, streaming outwards in all directions, may be clearly traced with powerful telescopes. In Lord Rosse’s magnificent Reflector the flat bottom of the crater called Albategnius is seen to be strewed with blocks not visible in inferior telescopes, while the exterior ridge of another (Aristillus) is all hatched over with deep gulleys radiating towards its centre.”

The valleys and clefts or rills visible on the Moon’s surface constitute another remarkable feature in the topography of our satellite. The valleys, properly so-called, require no particular comment, because they are just what their name implies—hollows often many miles long and several miles wide. The clefts or rills, however, are more mysterious, by reason of their great length and remarkable narrowness. One is almost led to infer that they are naught else but cracks in the lunar crust, the result of sudden cooling, how caused is of course not known.

There is another lunar feature to be mentioned somewhat akin to the foregoing in appearance but apparently, however, owing its origin to a different cause. I refer to the systems of bright streaks which, especially at or near the time of full Moon, are seen to radiate from several of the largest craters, and in particular from Tycho, Copernicus, Kepler and Aristarchus. These bright streaks extend in many cases far beyond what may fairly be considered as the neighbourhood of the craters from which they start, traversing distant mountains, valleys and other craters in a way which renders it very difficult to assign an explanation of their origin.

There are 13 areas on the Moon, which used to be regarded as “seas,” one of them, however, bearing the name of “Oceanus Procellarum,” the “Ocean of Storms”; but besides these there are several bays, termed in Latin Sinus, of which the most important is the Sinus Iridum or the “Bay of Rainbows,” a beautiful spot on the northern border of the Mare Imbrium, and best seen when the Moon is between 9 and 10 days old. The summits of the semi-circular range of rocks which enclose the bay are then strongly illuminated and a greenish shadow marks the valley at its base. By the way, it is worth mentioning that not a few of the lunar seas, so-called, seem to be pervaded by a greenish hue, though no particular explanation of this fact is forthcoming.

Much controversy has ranged round the question whether or not the Moon has an atmosphere. Without doubt the preponderance of opinion is on the negative side, though it must be admitted that some observers of eminence have suggested that there are indeed traces of an atmosphere to be had, but that it is extremely attenuated and of no great extent, otherwise it must render its presence discoverable by optical phenomena which it is certain cannot be detected.

A brief reference may here be made to a curious phenomenon sometimes seen in connection with occultations of stars by the Moon. Premising that an “occultation” is the disappearance of a star behind the solid body of the Moon by reason of the forward movement of the Moon in her orbit, it must be stated that though generally the Moon extinguishes the star’s light instantaneously, yet this does not invariably happen, for sometimes the star seems to hang upon the Moon’s limb as if reluctant to disappear. No very clear or satisfactory explanation of this phenomenon has yet been given; the existence of a lunar atmosphere would be an explanation, and accordingly this anomalous appearance, seen on occasions, has been advanced in support of the theory that a lunar atmosphere does exist; but, nevertheless, astronomers do not accept that idea.

Any one desirous of carrying out a careful study of the Moon’s surface must be provided with a good map, and for general purposes none is so convenient or accessible as Webb’s, reduced from Beer and MÄdler’s Mappa Selenographica published in 1837, of which another reproduction is given in Lardner’s Astronomy. Those, however, who would desire to study the Moon with the utmost attention to detail must provide themselves with Schmidt’s map published in 1878 at the expense of the German Government. When it is stated that this map represents the Moon on a circle 7½ feet in diameter, the size and amount of detail in it will be readily understood. Special books on the Moon furnishing numerous engravings and detailed descriptions have been written by Carpenter and Nasmyth (jointly) and by Neison.

Various attempts have been made to determine the amount of light reflected by the Moon, and also the question whether it yields any measurable amount of heat. As regards the light of the full Moon compared with that of the Sun, the estimates range from ¹/₃₀₀₀₀₀ to ¹/₈₀₀₀₀₀, a discrepancy not perhaps greater than might be expected under the circumstances of the case.

With respect to the heat possessed by, or radiated from the Moon’s surface, the conclusions of those who have attempted to deal with the matter are less consistent. As regards the surface of the Moon itself Sir John Herschel was of opinion that it is heated at least to the temperature of boiling water, but that owing to the radiant heat having to pass through our atmosphere, which acts as an obstacle, it is no wonder that it should be difficult for us to become conscious of its existence. In 1846 Melloni, by concentrating the rays of the Moon with a lens 3 feet in diameter, thought he detected a sensible elevation of temperature; and in 1856 C. P. Smyth at Teneriffe, but with inferior instrumental appliances, arrived at the same conclusion. Though Professor Tyndall in 1861 obtained a contrary result, yet the most recent experiments by the younger Earl of Rosse, Professor Langley, and others, all tend to show that the Moon does really radiate a certain infinitesimally small amount of heat. Perhaps, however, it will be best to give Langley’s ideas as to this in his own words:—“While we have found abundant evidence of heat from the Moon, every method we have tried, or that has been tried by others, for determining the character of this heat appears to us inconclusive; and without questioning that the Moon radiates heat earthward from its soil, we have not yet found any experimental means of discriminating with such certainty between this and reflected heat that it is not open to misinterpretation.” It is obvious from the foregoing that we on the Earth need not concern ourselves very much about lunar heat; and I will only add that F. W. Very, by an ingenious endeavour to localise the Moon’s radiant heat, has been able, he thinks, to establish the fact that on the part of the Moon to which the Sun is setting, what he calls the heat-gradient (using a phrase suggested by terrestrial meteorology) appears to be steeper than on that part to which the Sun is rising. Generally, Very’s observations accord fairly with Lord Rosse’s.

The Moon revolves round the Earth in 27 d. 7 h. 43 m. 11 s. at a mean distance of 237,300 miles, in an orbit which is somewhat, but not very, eccentric. Its angular diameter at mean distance is 31' 5, or, say, just over ½°. The real diameter may be called 2160 miles.

A few words will probably be expected by the reader on the subject of lunar influences on the weather, and generally; this being a matter highly attractive to the popular mind. The truth appears to lie, as usual, between two extremes of thought. The Moon, of course, is the main cause of the tides of the Ocean, and it is not entirely inconceivable that tidal changes imparted to vast masses of water may be either synchronous with, or may in some way engender, analogous movements in the Earth’s atmosphere; though no distinct proofs of this, as a determinate fact, can be brought forward.

There is no doubt whatever that at or near the time of full Moon, evening clouds tend to disperse as the Moon comes up to the meridian, and that by the time the Moon has reached the meridian a sky previously overcast will have become almost or quite clear. Sir John Herschel has alluded to this by speaking of a “tendency to disappearance of clouds under a full Moon”; and he considers this “fully entitled to rank as a meteorological fact.” He goes on, not unnaturally, to suggest the obvious thought that such dissipation of terrestrial clouds is due to the circumstance that, assuming heat really comes by radiation from the Moon (and we have seen on a previous page the probability of this) such radiant heat will be more potential if it falls on the Earth perpendicularly, as from a Meridian Moon, than if it comes to us at any one locality from a Moon low down in the observer’s horizon, and therefore has to pass through the denser strata of the Earth’s atmosphere and suffer material enfeeblement accordingly. I am aware that Mr. Ellis, late of the Royal Observatory, Greenwich, has sought to show by a seemingly powerful array of statistics that the idea now under consideration is unfounded, but I consider that we have here only one more illustration of the familiar statement that you can prove anything you like by statistics. I am firmly convinced, as the result of more than 30 years’ observation, that terrestrial clouds do disperse under the circumstances stated. Sir J. Herschel added that his statement proceeded from his own observation “made quite independently of any knowledge of such a tendency having been observed by others. Humboldt, however, in his Personal Narrative, speaks of it as well known to the pilots and seamen of Spanish America.” Sir John Herschel further remarked:—“Arago has shown from a comparison of rain, registered as having fallen during a long period, that a slight preponderance in respect of quantity falls near the ‘new’ Moon over that which falls near the ‘full.’ This would be a natural and necessary consequence of a preponderance of a cloudless sky about the ‘full,’ and forms, therefore, part and parcel of the same meteorological fact.”

Bernadin has asserted it to be a fact that many thunderstorms occur about the period of “new” or “full” Moon. But what I want most to warn the reader against is that popular idea (wonderfully wide-spread it must be admitted) that at the epochs of what are called, most illogically, the Moon’s “changes,” changes of weather may certainly be expected. There is absolutely no foundation whatever for this, and still more void of authority (if such a phrase is admissible) is a table of imaginary weather to be expected at changes of the Moon, often met with in books published half a century ago, and still occasionally reprinted in third-rate almanacs, and designated “Dr. Herschel’s Weather Table.” This precious production is not only devoid of authenticity as regards its name, but may easily be seen to be fraudulent in its reputed facts any month in the year.

It would be beyond both my present available space and the legitimate objects of this work to attempt even an outline of the influences over things terrestrial ascribed to, or associated, rightly or wrongly, with the Moon, and of which the word “lunatic” perhaps affords the most familiar exponent.

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