CHAPTER V.

ON THE EXISTENCE OR NON-EXISTENCE OF A LUNAR ATMOSPHERE.

At the close of the preceding chapter we stated that any force acting in opposition to that of gravity would be six times more effective on the moon than on the earth. But, in fact, it would in many cases be still more so; at all events, so far as projectile forces are concerned; for the reason that “the powerful coercer of projectile range,” as the earth’s atmosphere has been termed, has no counterpart, or at most a very disproportionate one, upon the moon.

The existence of an atmosphere surrounding the moon has been the subject of considerable controversy, and a great deal of evidence on both sides of the question has been offered from time to time, and is to be found scattered through the records of various classes of observations. Some of the more important items of this evidence it is our purpose to set forth in the course of the present chapter.

With the phenomena of the terrestrial atmosphere, with the effects that are attributable to it, we are all well familiar, and our best course therefore is to examine, as far as we are able, whether counterparts of any of these effects are manifested upon the moon. For instance, the clouds that are generated in and float through our air would, to an observer on the moon, appear as ever changing bright or dusky spots, obliterating certain of the permanent details of the earth’s surface, and probably skirting the terrestrial disc, like the changing belts we perceive on the planet Jupiter, or diversifying its features with less regularity, after the manner exhibited by the planet Mars. If such clouds existed on the moon it is evident that the details of its surface must be, from time to time, similarly obscured; but no trace of such obscuration has ever been detected. When the moon is observed with high telescopic powers, all its details come out sharp and clear, without the least appearance of change or the slightest symptoms of cloudiness other than the occasional want of general definition, which may be proved to be the result of unsteadiness or want of homogeneity in our own atmosphere; for we must tell the uninitiated that nights of pure, good definition, such as give the astronomer opportunity of examining with high powers the minute details of planetary features, are very few and far between. Out of the three hundred and sixty-five nights of a year there are probably not a dozen that an astronomer can call really fine: usually, even on nights that are to all common appearance superbly brilliant, some strata of air of different densities or temperatures, or in rapid motion, intervene between the observer and the object of his observation, and through these, owing to the ever-changing refractions which the rays of light coming from the object suffer in their course, observation of the delicate markings of a planet is impossible: all is blurred and confused, and nothing but bolder features can be recognized. It has in consequence sometimes happened that a slight indistinctness of some minute detail of the moon has been attributed to clouds or mists at the lunar surface, whereas the real cause has been only a bad condition of our own atmosphere. It may be confidently asserted that when all indistinctness due to terrestrial causes is taken account of or eliminated, there remain no traces whatever of any clouds or mists upon the surface of the moon.

This is but one proof against the existence of a lunar atmosphere, and, it may be argued, not a very conclusive one; because there may still be an atmosphere, though it be not sufficiently aqueous to condense into clouds and not sufficiently dense to obscure the lunar details. The probable existence of an atmosphere of such a character used to be inferred from a phenomenon seen during total eclipses of the sun. On these occasions the black body of the moon is invariably surrounded by a luminous halo, or glory, to which the name “corona” has been applied; and, further, besides this corona, apparently floating in it and sometimes seemingly attached to the black edge of the moon, are seen masses of cloud-like matter of a bright red colour, which, from the form in which they were first seen and from their flame-like tinge, have become universally known as the “red-flames.” It used to be said that this corona could only be the consequence of a lunar atmosphere lit up as it were by the sun’s rays shining through it, after the manner of a sunbeam lighting up the atmosphere of a dusty chamber; and the red flames were held by those who first observed them to be clouds of denser matter floating in the said atmosphere, and refracting the red rays of solar light as our own clouds are seen to do at sunrise and sunset. But the evidence obtained, both by simple telescopic observation and by the spectroscope, from recent extensively observed eclipses of the sun has set this question quite at rest; for it has been settled finally and indisputably that both the above appearances pertain to the sun, and have nothing whatever to do with the moon.

The occurrence of a solar eclipse offers other means in addition to the foregoing whereby a lunar atmosphere would be detected. We know that all gases and vapours absorb some portion of any light which may shine through them. If then our satellite had an atmosphere, its black nucleus when seen projected against the bright sun in an eclipse would be surrounded by a sort of penumbra, or zone of shadow, in contact with its edge, somewhat like that we have shown in an exaggerated degree in the annexed cut (Fig. 11), and the passage of this penumbra over solar spots and other features of the solar photosphere would to some extent obscure the more minute details of such features. No such dusky band has however been at any time observed. On the contrary, a band somewhat brighter than the general surface of the sun has frequently been seen in contact with the black edge of the moon: this in its turn was held to indicate an atmosphere about the moon; but Sir George Airy has shown that a lunar atmosphere, if it really did exist, could not produce such an appearance, and that the cause of it must be sought in other directions. If this effect were really due to the passage of the solar rays through a lunar atmosphere a similar effect ought to be produced by the passage of the sun’s rays through the terrestrial atmosphere: and we might hence expect to see the shadow of the earth projected on the moon during a lunar eclipse surrounded by a sort of bright zone or halo: we need hardly say such an appearance has never manifested itself. Similarly as we stated that the delicate details of solar spots would be obscured by a lunar atmosphere, small stars passing behind the moon would suffer some diminution in brightness as they approached apparent contact with the moon’s edge: this fading has been watched for on many occasions, and in a few cases such an appearance has been suspected, but in by far the majority of instances nothing like a diminution of brightness or change of colour of the stars has been seen; stars of the smallest magnitude visible under such circumstances retain their feeble lustre unimpaired up to the moment of their disappearance behind the moon’s limb.

Again, in a solar eclipse, even if there were an atmosphere about the moon not sufficiently dense to form a hazy outline or impair the distinctness of the details of a solar spot, it would still manifest its existence in another way. As the moon advances upon the sun’s disc the latter assumes, of course, a crescent form. Now if air or vapour enveloped the moon, the exceedingly delicate cusps of this crescent would be distorted or turned out of shape. Instead of remaining symmetrical, like the lower one in the annexed drawing (Fig. 12), they would be bent or deformed after the manner we have shown in the upper one. The slightest symptom of a distortion like this could not fail to obtrude itself upon an observer’s eye; but in no instance has anything of the kind been seen.

Reverting to the consequences of the terrestrial atmosphere: one of the most striking of these is the phenomenon of diffused daylight, which we need hardly remind the reader is produced by the scattering or diffusion of the sun’s rays among the minute particles of vapour composing or contained in that atmosphere. Were it not for this reflexion and diffusion of the sun’s light, those parts of our earth not exposed to direct sunshine would be hidden in darkness, receiving no illumination beyond the feeble amount that might be reflected from proximate terrestrial objects actually illuminated by direct sunlight. Twilight is a consequence of this reflexion of light by the atmosphere when the sun is below the horizon. If, then, an atmosphere enveloped the moon, we should see by diffused light those parts of the lunar details that are not receiving the direct solar beams; and before the sun rose and after it had set upon any region of the moon, that region would still be partially illuminated by a twilight. But, on the contrary, the shadowed portions of a lunar landscape are pitchy black, without a trace of diffused-light illumination, and the effects that a twilight would produce are entirely absent from the moon. Once, indeed, one observer, Schroeter, noticed something which he suspected was due to an effect of this kind: when the moon exhibited itself as a very slender crescent, he discovered a faint crepuscular light, extending from each of the cusps along the circumference of the unenlightened part of the disc, and he inferred from estimates of the length and breadth of the line of light that there was an atmosphere about the moon of 5376 feet in height. This is the only instance on record, we believe, of such an appearance being seen.

Spectrum analysis would also betray the existence of a lunar atmosphere. The solar rays falling on the moon are reflected from its surface to the earth. If, then, an atmosphere existed, it is plain that the solar rays must first pass through such atmosphere to reach the reflecting surface, and returning from thence, again pass through it on their way to the earth; so that they must in reality pass through virtually twice the thickness of any atmosphere that may cover the moon. And if there be any such atmosphere, the spectrum formed by the moon’s light, that is, by the sun’s light reflected from the moon, would be modified in such a manner as to exhibit absorption-lines different from those found in the spectrum of the direct solar rays, just as the absorption-lines vary according as the sun’s rays have to pass through a thinner or a denser stratum of the terrestrial atmosphere. Guided by this reasoning, Drs. Huggins and Miller made numerous observations upon the spectrum of the moon’s light, which are detailed in the “Philosophical Transactions” for the year 1864; and their result, quoting the words of the report, was “that the spectrum analysis of the light reflected from the moon is wholly negative as to the existence of any considerable lunar atmosphere.”

Upon another occasion, Dr. Huggins made an analogous observation of the spectrum of a star at the moment of its occultation, which observation he records in the following words:—“When an observation is made of the spectrum of a star a little before, or at the moment of its occultation by the dark limb of the moon, several phenomena characteristic of the passage of the star’s light through an atmosphere might possibly present themselves to the observer. If a lunar atmosphere exist, which either by the substances of which it is composed, or by the vapours diffused through it, can exert a selective absorption upon the star’s light, this absorption would be indicated to us by the appearance in the spectrum of new dark lines immediately before the star is occulted by the moon.”

“If finely divided matter, aqueous or otherwise, were present about the moon, the red rays of the star’s light would be enfeebled in a smaller degree than the rays of higher refrangibilities.”

“If there be about the moon an atmosphere free from vapour, and possessing no absorptive power, but of some density, then the spectrum would not be extinguished by the moon’s limb at the same instant throughout its length. The violent and blue rays would lay behind the red rays.”

“I carefully observed the disappearance of the spectrum of η Piscium at its occultation of January 4, 1865, for these phenomena; but no signs of a lunar atmosphere were detected.”

But perhaps the strongest evidence of the non-existence of any appreciable lunar atmosphere is afforded by the non-refraction of the light of a star passing behind the edge of the lunar disc. Refraction, we know, is a bending of the rays of light coming from any object, caused by their passage through strata of transparent matter of different densities; we have a familiar example in the apparent bending of a stick when half plunged into water. There is a simple schoolboy’s experiment which illustrates refraction in a very cogent manner, but which we should, from its very simplicity, hesitate to recall to the reader’s mind did it not very aptly represent the actual case we wish to exemplify. A coin is placed on the bottom of an empty basin, and the eye is brought into such a position that the coin is just hidden behind the basin’s rim. Water is then poured into the basin and, without the eye being moved from its former place, as the depth of water increases, the coin is brought by degrees fully into view; the water refracting or turning out of their course the rays of light coming from the coin, and lifting them, as it were, over the edge of the basin. Now a perfectly similar phenomenon takes place at every sunrise and sunset on the earth. When the sun is really below the horizon, it is nevertheless still visible to us because it is _brought up_ by the refraction of its light by the dense stratum of atmosphere through which the rays have to pass. The sun is, therefore, exactly represented by the coin at the bottom of the basin in the boy’s experiment, the atmosphere answers to the water, and the horizon to the rim or edge of the basin. If there were no atmosphere about the earth, the sun would not be so brought up above the horizon, and, as a consequence, it would set earlier and rise later by about a minute than it really does. This, of course, applies not merely to the sun, but to all celestial bodies that rise and set. Every planet and every star remains a shorter time below the horizon than it would if there were no atmosphere surrounding the earth.

To apply this to the point we are discussing. The moon in her orbital course across the heavens is continually passing before, or occulting, some of the stars that so thickly stud her apparent path. And when we see a star thus pass behind the lunar disc on one side and come out again on the other side, we are virtually observing the setting and rising of that star upon the moon. If, then, the moon had an atmosphere, it is clear, from analogy to the case of the earth, that the star must disappear later and reappear sooner than if it has no atmosphere: just as a star remains too short a time below the earth’s horizon, or behind the earth, in consequence of the terrestrial atmosphere, so would a star remain too short a time behind the moon if an atmosphere surrounded that body. The point is settled in this way:—The moon’s apparent diameter has been measured over and over again and is known with great accuracy; the rate of her motion across the sky is also known with perfect accuracy: hence it is easy to calculate how long the moon will take to travel across a part of the sky exactly equal in length to her own diameter. Supposing, then, that we observe a star pass behind the moon and out again, it is clear that, if there be no atmosphere, the interval of time during which it remains occulted ought to be exactly equal to the computed time which the moon would take to pass over the star. If, however, from the existence of a lunar atmosphere, the star disappears too late and reappears too soon, as we have seen it would, these two intervals will not agree; the computed time will be greater than the observed time, and the difference, if any there be, will represent the amount of refraction the star’s light has sustained or suffered, and hence the extent of atmosphere it has had to pass through.

Comparisons of these two intervals of time have been repeatedly made, the most recent and most extensive was executed under the direction of the Astronomer-Royal several years ago, and it was based upon no less than 296 occultation observations. In this determination the measured or telescopic semidiameter of the moon was compared with the semidiameter deduced from the occultations, upon the above principle, and it was found that the telescopic semidiameter was greater than the occultation semidiameter by two seconds of angular measurement or by about a thousandth part of the whole diameter of the moon. Sir George Airy, commenting on this result, says that it appears to him that the origin of this difference is to be sought in one of two causes. “Either it is due to irradiation[3] of the telescopic semidiameter, and I do not doubt that a part at least of the two seconds is to be ascribed to that cause; or it may be due to refraction by the moon’s atmosphere. If the whole two seconds were caused by atmospheric refraction this would imply a horizontal refraction of one second, which is only 1/2000 part of the earth’s horizontal refraction. It is possible that an atmosphere competent to produce this refraction would not make itself visible in any other way.” This result accords well, considering the relative accuracy of the means employed, with that obtained a century ago by the French astronomer Du Séjour, who made a rigorous examination of the subject founded on observations of the solar eclipse of 1764. He concluded that the horizontal refraction produced by a possible lunar atmosphere amounted to 1″·5—a second and a half—or about 1/1400 of that produced by the earth’s atmosphere. The greater weight is of course to be allowed to the more recent determination in consideration of the large number of accurate observations upon which it was based.

But an atmosphere 2,000 times rarer than our air can scarcely be regarded as an atmosphere at all. The contents of an air-pump receiver can seldom be rarefied to a greater extent than to about 1/1000 of the density of air at the earth’s surface, with the best of pneumatic machines; and the lunar atmosphere, if it exist at all, is thus proved to be twice as attenuated as what we are accustomed to recognise as a vacuum. In discussing the physical phenomena of the lunar surface, we are, therefore, perfectly justified in omitting all considerations of an atmosphere, and adapting our arguments to the non-existence of such an appendage.

And if there be no air upon the moon, we are almost forced to conclude that there can be no water; for if water covered any part of the lunar globe it must be vapourised under the influence of the long period of uninterrupted sunshine (upwards of 300 hours) that constitutes the lunar day, and would manifest itself in the form of clouds or mists obscuring certain parts of the surface. But, as we have already said, no such obliteration of details ever takes place; and, as we have further seen, no evidence of aqueous vapour is manifested upon the occasion of spectrum observations. Since, then, the effects of watery vapour are absent, we are forced to conclude that the cause is absent also.

Those parts of the moon which the ancient astronomers assumed, from their comparatively smooth and dusky appearance, to be seas, have long since been discovered to be merely extensive regions of less reflective surface material; for the telescope reveals to us irregularities and asperities covering well nigh the whole of them, which asperities could not be seen if they were covered with water; unless, indeed, we admit the possibility of seeing to the bottom of the water, not only perpendicularly, but obliquely. Some observers have noticed features that have led them to suppose that water was at one time present upon the moon, and has left its traces in the form of appearances of erosive action in some parts. But if water ever existed, where is it now? One writer, it is true, has suggested as possible, that whatever air, and we presume he would include whatever water also, the moon may possess, is hidden away in sublunarean caves and hollows; but even if water existed in these places it must sometimes assume the vapoury form, and thus make its presence known.

Sir John Herschel pointed out that if any moisture exists upon the moon, it must be in a continual state of migration from the illuminated or hot, to the unilluminated or cold side of the lunar globe. The alternations of temperature, from the heat produced by the unmitigated sunshine of 14 days’ duration, to the intensity of cold resulting from the absence of any sunshine whatever for an equal period, must, he argued, produce an action similar to that of the _cryophorus_ in transporting the lunar moisture from one hemisphere to the other. The cryophorus is a little instrument invented by the late Dr. Wollaston; it consists of two bulbs of glass connected by a bent tube, in the manner shown in the annexed illustration, fig. 13. One of the bulbs, A, is half-filled with water, and, all air being exhausted, the instrument is hermetically sealed, leaving nothing within but the water and the aqueous vapour which rises therefrom in the absence of atmospheric pressure. When the empty bulb, B, is placed in a freezing mixture, a rapid condensation of this vapour takes place within it, and as a consequence the water in the bulb A gives off more vapour. The abstraction of heat from the water, which is a natural consequence of this evaporation, causes it to freeze into a solid mass of ice. Now upon the moon the same phenomenon would occur did the material exist there to supply it. In the accompanying diagram let A represent the illuminated or heated hemisphere of the moon, and B the dark or cold hemisphere; the former being probably at a temperature of 300° above, and the latter 200° below Fahrenheit’s zero. Upon the above principle, if moisture existed upon A it would become vapourised, and the vapour would migrate over to B, and deposit itself there as hoarfrost; it would, therefore, manifest itself to us while in the act of migrating by clouding or dimming the details about the boundary of the illuminated hemisphere. The sun, rising upon any point upon the margin of the dark hemisphere, would have to shine through a bed of moisture, and we may justly suppose, if this were the case, that the tops of mountains catching the first beams of sunlight would be tinged with colour, or be lit up at first with but a faint illumination, just as we see in the case of terrestrial mountains whose summits catch the first, or receive the last beams of the rising or setting sun. Nothing of this kind is, however, perceptible: when the solar rays tip the lofty peaks of lunar mountains, these shine at once with brilliant light, quite as vivid as any of those parts that receive less horizontal illumination, or upon which the sun is almost perpendicularly shining.

All the evidence, then, that we have the means of obtaining, goes to prove that neither air nor water exist upon the moon. Two complicating elements affecting all questions relating to the geology of the terraqueous globe we inhabit may thus be dismissed from our minds while considering the physical features of the lunar surface. Fire on the one hand and water or the other, are the agents to which the configurations of the earth’s surface are referrable: the first of these produced the igneous rocks that form the veritable foundations of the earth, the second has given rise to the superstructure of deposits that constitute the secondary and tertiary formations: were these last removed from the surface of our planet, so as to lay bare its original igneous crust, that crust, so far as reasoning can picture it to us, would probably not differ essentially from the visible surface of the moon. In considering the causes that have given birth to the diversified features of that surface, we may, therefore, ignore the influence of air and water action and confine our reasoning to igneous phenomena alone: our task in this matter, it is hardly necessary to remark, is materially simplified thereby.