NOTE C.

HABITABILITY OF THE MOON.

THE question of the moon's habitability-interesting to astronomers on its own account-acquires an additional interest if we consider that on its solution depends the opinion we shall form of the habitability of the important secondary systems attending on Saturn, Jupiter, and Uranus. I propose to consider in this note some points connected with the inquiry.

The physical conditions and peculiarities of the moon are undoubtedly in striking contrast to those prevailing on the earth. The lunar year consists of little more than twelve lunar days, each day lasting more than four of our weeks. Our seasons, due to an inclination of 23 degrees, are also very different from the lunar seasons due to an inclination of 11⁄2 degrees; if, indeed, we can apply the term seasons to intervals in which the sun rises and sets only three times. Again, our earth (considered as a satellite of the moon) is altogether invisible to three-sevenths of the moon's surface; to the remaining four-sevenths the earth does not rise and set as the moon does to us, but moves within narrow limits round a fixed point on the celestial concave, such motions being the exact converse of the lunar librations; the earth also passes through all her phases in a lunar day and night, the half set of phases passed through in the lunar night varying for each point of the moon's surface.

That the moon has not an atmosphere corresponding in extent and density to our own is undoubted; it has not been considered so certain, however, that the moon's surface is absolutely devoid of atmospheric envelope. The first and most obvious argument against the presence of a lunar atmosphere, is that the lunar disc, even when examined with the most powerful telescopes, exhibits no indication of clouds. Owing to the slow

ness of the moon's rotation we should hardly expect that belts of clouds would be formed, as on the swiftly rotating planets Saturn and Jupiter, but irregularly dispersed clouds, even if not separately visible, must produce effects very easily traceable from the earth. The distinctness of the outlines of mountains, plains, and valleys, on the moon's surface, would vary with the aggregation and dispersion (due to variations of temperature) of clouds

and mists about them. No such changes are observable: as long as the clearness of our own atmosphere remains unchanged, the irregularities of the lunar surface are seen with unvarying distinctness. It appears reasonable, then, to conclude that the visible lunar hemisphere is either devoid of air or of water.

Secondly, if the moon were surrounded by an atmosphere, even of limited extent, the effects of refraction could not fail to be traced in the occultations of stars. The refractive effects of the atmosphere surrounding Saturn are, as we have seen, traceable from the earth, which is removed fully 3,500 times as far from Saturn as from the moon-a disproportion in the distances that would compensate an immense disproportion in the extent and density of the atmospheric envelopes surrounding the two bodies.*

Lastly, there is not the slightest trace of a twilight-circle on the moon, nor do the horns of the new moon extend beyond the semicircle. When it is considered that Venus, though removed so much farther than the moon, and though she is one of the most difficult objects of telescopic observation in the heavens, distinctly presents both these phenomena, their absence in the case of the moon appears the more remarkable. If the moon had an atmosphere, even of small extent and density, the powerful telescopes that have been directed towards her could not have failed to exhibit the phenomena considered. On the other hand, arguments are not wanting in support of the hypothesis

The stars are not always instantaneously occulted by the moon. Some disappear by sudden diminutions of brilliancy (as the star Cancri)-a phenomenon that may be accounted for by supposing such stars to be close double or multiple stars; others, after disappearing, reappear for a brief interval—a phenomenon that appears to indicate the existence of vast irregularities upon the moon's surface. But the phenomena that would result from the presence of a lunar atmosphere are altogether different. Thus, suppose an observer on the moon to witness a central occultation of a star by the earth:-The star as it entered (apparently) the confines of our atmosphere would move more and more slowly; instead of appearing as a point it would assume the form of a circular arc gradually extending farther and farther round the earth's disc; and when actually behind the centre of the earth, the star would appear as a circle of light concentric with the outline of the earth's disc. Passing beyond this point the star would present similar appearances in reverse order. That even in such a central passage a star would not be actually occulted, is clear from the consideration that the horizontal refraction of the earth's atmosphere is upwards of 33', which would be doubled for an object seen beyond the earth from the moon; but the earth's semi-diameter seen from the moon subtends an arc of only 57′ 6′′. Since the moon's semi-diameter viewed from the earth never exceeds 16′ 45′′ it is evident that an atmospheric envelope of much less extent than that of the earth would suffice to render the occultation of a star by the moon impossible.

In the article referred to at page 177, note*, Professor Challis omits to notice the phenomena considered above. It seems clear, however, that they would be the most marked phenomena attending an occultation, if the moon had an atmosphere. In a similar manner it may be shown that the phenomena attending an eclipse of the sun would be very different from those actually presented, if the moon had an atmosphere.

that the moon has an atmosphere. In the first place, we might infer from the analogy of our earth, and of the larger planets, that all the members of the solar system are surrounded by atmospheres of greater or less density and extent. In the second place, the traces of past volcanic action on the lunar surface, leave little doubt that while such action went on the moon must have had an atmosphere capable of supporting combustion; and further, must have been enveloped by the gases distributed during tremendous and long-continued eruptions.

An attempt has been made to reconcile these contradictory evidences by the hypothesis that an atmosphere originally surrounding the visible lunar surface has been attracted to the opposite hemisphere.

The moon's centre of gravity is undoubtedly nearer to us than her centre of figure. In the first place, we have in such a displacement the only possible explanation of the peculiarity of the moon's rotation referred to at p. 50.* Secondly, Professor Hansen has proved that an observed discrepancy between the actual lunar inequalities and the results of the theoretical examination of the lunar motions, is removed, if the centre of gravity of the moon is assumed to be 33 miles farther from the earth than the centre of figure. This result has been confirmed by the comparison of photographic pictures of the moon, taken at the times of her extreme eastern and western librations. In the year 1862, M. Gussew, Director of the Imperial Observatory at Wilna, carefully examined two such pictures taken by Mr. Delarue. The result of the examination may be thus stated :-The outer parts of the visible lunar disc belong to a sphere having a radius of 1,082 miles, the central parts to a sphere having a radius of 1,063 miles; the centre of the smaller sphere is about 79 miles nearer to us than the centre of the larger; the line joining the centres is inclined at an angle of about 5° to the line from the earth at the epoch of mean libration; thus the central

* The question of the moon's rotation has frequently aroused controversy. Bentley and Keill disputed over it in 1690, and so recently as 1855 the columns of the daily press were occupied with its discussion. The question is altogether a verbal one. The moon's motions may be described as being compounded of a motion of revolution around the earth and a motion of rotation in the same time about an axis through the moon's centre (the moon not being a spheroid it is incorrect to speak of the moon's own axis'). Now, if it were not for certain irregularities we might simply say that the moon rotates about an axis near the earth, just as a globe rigidly attached to an arm moving on a central stem would be said to rotate about the stem, though to an eye from which stem and arm were concealed the globe would appear to revolve around a centre and rotate in the same time about its axis. In the case of the moon's motions no such simple rotation exists; but it is a question whether the lunar movements would not be expressed more simply, and (taking dynamical considerations into account) more accurately, by saying that the moon rotates about an axis near the earth, and that this axis is subject to such and such motions, than by the mode of expression generally adopted.

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part of the moon's disc is about 60 miles nearer to us than it would be if the moon were a sphere of the dimensions indicated by the disc's outline. If we suppose the invisible part of the moon's surface to belong to the larger sphere, and the density of the moon's substance uniform, it would follow from this conformation, that the centre of gravity of the moon is about 30 miles nearer to the earth than is the centre of the larger spherethat is, than is the centre of the moon's apparent figure.

*

But although the moon's centre of gravity is thus displaced, it is very doubtful whether we have in such displacement a satisfactory explanation of the observed peculiarities of the lunar disc. The visible hemisphere in all probability was originally clothed with an atmosphere and partially covered by oceans. Now, it is hardly conceivable that a displacement of the moon's centre of gravity should be followed by the departure even of all the inelastic fluids from the nearer to the further hemisphere, far less of all the elastic atmospheric envelope. Assuming, however, that the atmosphere had thus been displaced, and the fluids dissipated by evaporation in vacuo from all the depressions on the visible lunar surface, it is inconceivable that no traces should be visible of the atmosphere and the oceans thus collected on the further lunar hemisphere. Even if the exact half of the moon's surface were invisible to us, some of the oceans would extend into the cavities and depressions visible round the edge of the lunar disc, and the atmosphere would be traceable (by its effect in occultations) completely round that edge; and when we consider that owing to the moon's librations only three-sevenths of the lunar surface are actually hidden from us, we are compelled to reject the notion that the distribution of air and water on the moon's surface is such as has been suggested.

It appears to me that the simplest of all the phenomena presented by the moon-namely, her colour-will serve to guide us to an explanation of the contradictions we are considering. Imagine our earth stripped of air and water, and all verdure destroyed from its surface: what would be the appearance of such a globe removed to the distance of the moon? Bathed in the sun's light it would doubtless be a brilliant object, but its brilliancy would differ altogether from the silvery effulgence of the moon. The various strata which rise to the surface at different parts of the earth might not, in general, be separately visible; but the commingling of the colours that mark such strata would certainly produce warmer tints than are observed on any part of the lunar disc. The vast deserts, steppes, llanos, savannahs, and prairies of the earth would be distinctly visible as streaks and patches of uniform colour. The icy polar regions and the

* That oceans once covered parts of the visible lunar hemisphere seems evidenced by the traces of past volcanic action upon the moon's surface. See note ‡, p. 205..

snow-covered mountain ranges would alone reflect the kind of light that we receive from the moon. *

But now let us imagine our globe subjected to another change. We have plain evidence that the climate of the earth was in past ages far warmer than at present. Animals and plants now found only in the tropics were found in the temperate zones, and many forms of life existed on the earth for which even the tropics would now form but a bleak and unsuitable residence. It seems reasonable to conclude that this change of climate is due to the loss of internal heat by slow radiation, and that the change is still proceeding.† Now, imagine this change to proceed until the whole of the water on the earth's surface should be frozen. Then if this dismal globe were removed to the moon's distance, its brilliancy would no longer present a very marked contrast to the lunar light. The frozen surface of the ocean would present precisely such vast level tracts as the so-called lunar seas; ‡ the glacial regions on land would resemble the rough and mountainous districts of the lunar surface; while, if we conceive the continents on our earth gradually covered with snow as the process we have imagined went on, they would correspond exactly to the vast tracts of brilliant white so conspicuous upon the lunar disc.

It is obvious that some of the difficulties before considered disappear if we suppose all fluids on the moon's surface to be frozen. As the atmosphere would in such a case be perfectly free from clouds or mists, all the irregularities of the lunar surface would be distinctly visible, and such distinctness would not be liable to any perceptible variations. Yet, if the lunar atmosphere bore any proportion to the atmosphere of the earth as regards extent, the outlines of the lunar irregularities would be softer than

Even at the immense distances to which the planets are removed the colours of their surfaces can be traced whenever the actual surfaces are visible, as in the cases of Mars, Jupiter, and Saturn. When a dense atmosphere supports heavy masses of vapour the light reflected is brilliantly white, as in the cases of Mercury and Venus, and the white belts on Saturn and Jupiter. Such whiteness, however, obviously differs altogether from the surface colours of the moon.

† Such a change would be accompanied by a gradual diminution in the dimensions of the earth, and it has been argued that since the period of the earth's rotation has not perceptibly altered during three or four thousand years, no such change can be taking place. It may be remarked, however, that periods of ten or twenty thousand years are but as seconds when compared with the interval necessary to effect perceptible changes of climate in the manner considered. That the dimensions of the earth were once far greater than at present seems evidenced by the cumbrous forms of the animals and reptiles that inhabited it of old. Such animals could have moved with freedom and activity only under the diminished attraction of gravity resulting from greater dimensions of the earth's globe.

Independently of their colour these level spaces in the moon seem scarcely explicable on any other supposition than that they are frozen seas, for while absolutely level they are clearly solid. In every respect save in fluidity they correspond to our terrestrial system of oceans.