CHAPTER IV
CLOSE TO THE MOON--I GIVE SOME INFORMATION ABOUT IT
We were now moving at a comparatively slow speed, yet the size of themoon's disc was very rapidly expanding as we approached nearer andnearer to it. In the course of a little over half-an-hour we were withinten miles of its surface, which now seemed to fill the whole space belowus; and its rotundity was most impressive. The shadows of the mountainsand other elevated portions near the terminator[4] were jet black, owingto the absence of an atmosphere; and, seen contrasted with the brilliantlighting of the parts exposed to the full glare of the sun, appearedalmost like deep holes in the lunar surface.
John now remarked, "Professor, you are aware that I have only a rathervague general knowledge of astronomy, although I take an interest in thesubject, and that I know still less about the dimensions and physicalcharacter of the moon and planets; so perhaps you will be good enough togive us a little detailed information respecting this beautiful orb.Most of it will be news to me, and probably it will all be fresh toM'Allister."
"Heh mon," the latter replied, "just put me among machinery and I'lltell you what's what, but I never learned anything about astronomy, sowill not pretend to any knowledge of it, but now I should be very gladto hear what the Professor has to say about it."
"Well, friends," I replied, "it is not my wish to lecture you upon thesubject, so I will merely just run over a few of the bare facts.
"To begin with--the moon is very much smaller than the earth, itsdiameter being only 2160 miles, while the earth's diameter is 7918miles. Being a smaller globe its mass is much more loosely compactedthan that of the earth, so, although it would take nearly fifty globesthe same as the moon to make one globe as large as the earth, it wouldrequire nearly eighty such globes to make one as heavy as the earth.
"The moon's distance from the earth is generally given as being about238,000 miles, but this is its mean distance. When farthest away fromour world its distance is about 260,000 miles, but at its nearest it isless than 220,000 miles distant. This difference of course arises fromthe eccentricity of the moon's orbit, and it explains why we sometimessee the moon a trifle larger than it appears at other times. By this Imean that it really is seen larger, because it is closer to us. But youhave no doubt often noticed that when the moon is near the horizon itseems to be very large indeed. This apparent increase of size is,however, an illusion, owing to our unconsciously comparing it with theapparent size of terrestrial objects.
"The surface of the moon shows evidence of very violent volcanic actionhaving occurred in every part of it, and astronomers in the past weremuch puzzled to account for the excessive volcanic energy which wasindicated by what they saw, as such a small globe as the moon would not,in the ordinary course of events, have ever possessed sufficient heat tohave developed such violent action. A theory of later years has,however, provided a reasonable explanation. It is that the moon was atone time a part of the same mass as the earth, which became separatedfrom it before the earth had quite cooled down and solidified into itspresent form, and was then gradually driven farther and farther awayfrom the earth by natural forces. It was therefore originally as hot asthe rest of the mass which formed the earth, but being formed into asmaller globe of much less gravity--only one-sixth of that of theearth--volcanic action of the same intensity as that on the earth wouldhave a much more far-reaching effect. A force which on the earth wouldproject volcanic lava and scoriae a distance of three miles would, on themoon, project it a distance of eighteen miles. This accounts for thevery high mountains we see on the moon, some of which are comparatively,for the size of the globe, much higher than those on the earth. It alsoaccounts for the vast size of the lunar craters, ring-plains, andring-mountains.
"These latter are formations quite unknown upon our earth, but on themoon they are numbered by hundreds of all sizes, from a few miles up toone hundred and fifty miles in diameter. They are large plains, roughlycircular in shape, and surrounded by mountains; in a few cases the ringis in some parts a double range of mountains. Sometimes the plain (or'floor,' as it is termed) is many thousands of feet below the generallevel of the lunar surface; in a few cases it is raised considerablyabove it, and in one or two instances, instead of being flat, the flooris convex. Some of the mountain rings are comparatively low, but inother cases the mountains are fifteen to twenty thousand feet in height,or even higher. Frequently a mountain rises near the centre of thefloor, some rings containing more than one such mountain, whilst othershave none at all.
_Drawn by M. Wicks_ Plate II
IDEAL VIEW OF LUNAR SCENERY
As there is no atmosphere on the moon, the sky is a dense black, and thestars shine brilliantly in the daytime. The view is a typical one,showing numerous craters and cracks, and a small ring-mountain withterracing. Ring-mountains and plains vary from a few miles to 150 milesdiameter, some mountains being nearly 20,000 feet in height.]
"There are numerous instances where one mountain ring has overlapped orcut into another, thus indicating that it was a later formation; and inmany cases the mountains are 'terraced,'[5] as it is termed, eitherowing to a series of landslips or to the rise and fall of a sea of lava,which cooled as it sank down, thus forming terraces. Small cratersabound all over the surface of the moon and on the floors of the rings;cracks in the lunar surface are also numerous.
"As regards the lunar mountains, it may truly be said that we have afairly accurate knowledge of peaks and mountains which would either betoo precipitous to be climbed, or quite inaccessible to us, if we couldactually land upon the moon; and the whole visible surface has been morecarefully and thoroughly mapped out and studied than is the case withmany parts of our own earth.
"If the moon has any atmosphere it must be so very attenuated indeedthat human beings could not possibly live in it at all; but nothing hasyet been detected which would enable us to say positively that anyatmosphere does exist there, although there have been some indicationsobserved which support the supposition that there may be an extremelythin air.
"Nor does it appear possible that there is any water upon its surface atthe present; in fact, many astronomers are of opinion that the moonnever did have any water upon it. Personally, from a study of many ofthe formations as seen through the telescope, it seems to me quiteimpossible that they could owe their existence in their present state toanything but the action of water. They present much the same appearanceas formations on our own earth which we know have been fashioned by thatmeans. There is no water upon the moon now, I think, though severallarge depressions are still called oceans, seas, lakes, or marshes,because at one time they were believed to be such. Probably in some ofthose places, if not in all, water existed millions of years ago; butages since they must have lost it either by evaporation or by absorptioninto the soil.
"I will not say any more just now, but as we pass above the lunarsurface I will point out a few of the natural features that may be ofinterest to you."
M'Allister here paid me the compliment of saying, "Well, Professor, Ialways thought astronomy was a very dry and difficult subject; but yourremarks were really very interesting, and quite easy to understand.There is only one thing that seemed to me rather strange as coming froma scientific man, and I would like you to explain that."
"Certainly; if there is anything you do not quite understand, you haveonly to ask and I will try to clear the matter up," I answered. "Whatis it you wish to know?"
"Well," he answered, "I noticed that when you were speaking about thedistance of the moon you always said it was _about_ so far away. Whydidn't you tell us the exact distance? I'm not a scientific man by anymeans, but if any one were to ask me the length of a connecting rod onone of my machines I should say '25 inches,' not 'about 25 inches,' forthat would not do for a practical man!"
"It's like this, M'Allister," I said. "You measure things with atwo-foot rule, which is something you can actually handle, and you knowit is made according to a standard measure and must contain exactly 24inches
. If, however, your rule was 24-1/4 inches long, yet still dividedinto twenty-four equal parts, you could measure work with it just thesame, but would know that every measurement was just a little bit out.If you had no possible means of obtaining another rule, you would haveto put up with a little inexactitude.
"That is just the position in which astronomers are placed; they have toput up with a measure which they know is not perfectly accurate, yet itis the best which can be secured.
"Their two-foot rule, so to speak, may be the distance from the earth tothe sun, or the length of the whole diameter of the earth's orbit, andthese cannot be handled like your rule; and although we know themeasurements of these are nearly correct, they are not quite so. Yet thedistances of the moon, planets, stars, &c., have to be measured by theserules, so it is clear we can only know those distances with a nearapproximation to accuracy.
"For this reason astronomers are always trying different means ofascertaining the sun's exact distance from the earth in order to obtaina perfectly correct measure; but there are so many difficulties andcomplications which affect the result, that it will be a long time yetbefore they succeed in their work.
"You will therefore understand that all these figures as to distancesand dimensions of planets and stars are only as near approaches tocorrectness as is possible to attain in our present circumstances. Theymust not be regarded as literally exact, although they are usuallysufficiently accurate for all general purposes. Astronomers know thisand allow for it; but general readers of books, when they find figureswhich do not agree with others they have seen, are apt to regard them asall being mere guesses, and in this they are doing an injustice to thepainstaking labours of generations of astronomers and mathematicians.
"I shall presently be mentioning the heights of mountains, the size ofring-plains, craters, &c., but the same reasoning applies to them; thedimensions given are averages of measurements made by differentobservers, and, though not quite accurate, are as near the truth as thedifficult conditions under which they have to be measured will allow."
"Thank you, Professor," said M'Allister as I concluded. "I'm glad Idon't have to work with such rules as those you mention, formeasurements a little bit out of correctness would ruin any machine inthe world."
"Still, M'Allister," I said, "you would have the advantage overastronomers with your two-foot rule, because you would know that it wasa quarter of an inch too long. Their difficulty is that they do notknow exactly how much their rule is out of correctness, so cannot obtainabsolute accuracy however they may try."
We now set the machines going very slowly and moved toward the northernpart of the moon, where I pointed out the position of the lunar northpole, and explained that, owing to the very slight inclination of thelunar axis, there can be but very little variation of seasons in any oneparticular part of the moon. Thus, if at one place it were spring, itwould practically always be spring there, but with very cold nights allthrough the lunar year. Where it was summer it would practically alwaysbe summer, also with very cold nights, and so on.
I further explained that, as the moon revolves on its axis in the sametime that it takes to make one revolution round the earth, those on theearth always see the same side of the moon, except when occasionally,owing to inequalities in the lunar motions, they are afforded a peepjust round portions of the edges at different periods. The remainder ofthe other side of the moon has never yet been seen from the earth byhuman eyes, and in all probability never will be seen for millions ofyears to come.
John, who as usual was smoking like a factory chimney, here removed hispipe from his mouth and said, "Professor, you stated just now that thenights on the moon would always be intensely cold, and I should like toknow whether there is any really reliable information respecting thetemperature of the lunar days and nights. I have seen so manycontradictory theories on the subject that I scarcely know what tobelieve."
"In that respect," I answered, "I fear that my position is much thesame as yours, for I have absolutely no certain knowledge on the point,but will just state shortly how the matter stands at present.
"During the past century many investigations have been made byscientific men respecting the temperatures on the moon, and theirresults have differed to an amazing extent. It would take too long, andbe too wearisome, to quote all the authorities, so a few must suffice.Lord Rosse, who used a thermopile in his experiments, found that inorder to produce the results he obtained, the sunlit surface of the moonmust be heated to a temperature of 500 degrees on Fahrenheit's scale.Sir John Herschel had previously concluded that the temperature must bemuch greater than that of boiling water. On the other hand Ericsson and,more recently, Professor Langley--who used a bolometer of his owninvention for measuring the heat of the sun's invisible rays--came tothe conclusion that even under continued sunshine the temperature wouldrarely, if ever, rise above the freezing point of water.
"Professor Very, however, who has continued the delicate experimentswith the bolometer, and also made other experiments and calculations ofquite recent date, has obtained results more nearly in accordance withthose first quoted, for he found that near the end of the second week'ssunshine on the moon the temperature of the rocks, soil, &c., must riseto at least 80 degrees Centigrade above the heat of boiling water."
"My word!" said M'Allister, "that's hotter than a ship's engine-room,and I shouldn't care for such a very high temperature."
"As this is so recent," I proceeded, "and the work of one of ourhighest authorities, I think we must accept it as being more correct,especially as Professor Very has taken into consideration some factorswhich had not previously been allowed due weight.
"In connection with this matter of temperature it is necessary toremember that the days and nights upon the moon are both very long, forthe full lunar 'day' is equal to a month, so the actual lunar day isequal to fourteen of our days, and the lunar night is of the sameduration. Our 'day' of twenty-four hours is divided into day and nightin unequal proportions, according to the changes in the seasons; but, asI before remarked, the seasonal changes on the moon are very slight, sothe variations in the lengths of the days and nights are very small.
"But, whatever may be the difference of opinion as to the heat of thelunar day, there seems to be a pretty general agreement that, owing tothe absence of an atmosphere, the nights must be so intensely cold as tobe almost beyond our conception--probably approaching nearly to theabsolute zero of outer space. Even with an atmosphere the long nights inour polar regions are so cold that only very strong people can endurethem, notwithstanding every device for obtaining warmth.
"You will gather from this that although the moon appears so beautifulfrom a distance, it must be anything but a desirable place of residenceeven from a climatic point of view, for we should practically be friedat midday, while at midnight--or even in the daytime when out of thedirect rays of the sun--we should soon be frozen stiff."
As I said this John chimed in with: "Professor, all things considered,I think I could smoke my pipe more comfortably upon the earth than uponthe moon. I really don't like such extremes of temperature."
"I am of the same mind," I replied, "and it is because I prefer a moreequable temperature that I have carefully kept our martalium blindsdrawn over those windows of our vessel upon which the sun is shining."