Part 10
“Certainly it is not true. The moon is changing its apparent form all the time. There is no sudden alteration at any phase. The popular belief, however, has always been so firmly fixed that many investigations have been made to ascertain whether there is, in reality, any foundation for it. These investigations have shown that no measurable effect of the kind exists.”
“And the Full Moon does not drive away clouds, as some assert?”
“Surely she does not. I will now tell you something that the persons who plant and sow and cut timber according to the phases of the moon, and who believe that she exercises a kind of magic control over the clouds, probably have never heard of, although if they knew it they might use it as an argument in favor of lunar influences. It is this: The alternate approach and retreat of the moon with respect to the earth, as she travels round her elliptical orbit, produce measurable, although slight, disturbances of the magnetism of our planet. The distance of the moon varies to the extent of about 30,000 miles. Now, if it could be shown that these magnetic disturbances were reflected in the character of the weather, then the supposed influence of the moon would be established. But that has not been shown, and if it were shown it would still be found that the phases of the moon had no relation to the fact, for the moon may be at its greatest or its least distance from the earth, or at any intermediate distance during any possible phase.”
“You will, perhaps, think me very persistent in asking foolish questions, but there is one other on my mind that I should like to put, now that we have gone so far. It is this: I have read, since the great earthquakes at San Francisco and Valparaiso, and the great eruption of Vesuvius in the same year, 1906, that the moon has an influence over such things. Is this another unfounded popular superstition?”
“It is not a notion of _popular_ origin at all,” I replied. “It originated rather from scientific considerations, and there may possibly be a germ of truth in it, although it yet remains to be demonstrated, and the evidence concerning it is confusingly contradictory. You will recall, I trust, what has been said about the sun and the moon producing tides in the oceans. We have also seen that before our globe had assumed its present condition, while it was yet more or less plastic throughout its whole mass, and before the birth of the moon, great tides were produced in the body of the earth. The _tendency_ to the production of such bodily tides still exists, and now that the moon has become a near-by attendant of the earth, she acts more effectively in this regard than does the sun. If the earth were still plastic the moon would produce bodily tides in it. In other words the earth would be deformed by the attraction of the moon. The question has arisen whether or not the tendency to the production of such tides, now that the earth has become rigid, may not disturb its crust sufficiently to induce earthquakes and volcanic eruptions. Some students of the subject have thought that they could detect evidence that this is the case. It has frequently happened that such phenomena have occurred on a large scale, at or very close to, the periods of New and of Full Moon. Those are the times, as we saw when we were talking of the oceanic tides, when the sun and the moon pull together. If all great eruptions and earthquakes occurred at these conjunctions there would be little doubt of the correctness of the theory. But, unfortunately for the clearness of our conceptions, this is by no means the case. There have been many earthquakes and volcanic outbursts when the sun and the moon were not thus combining their tidal attractions. Thus the evidence is found to be contradictory or inconsistent, and the question remains unsettled. It is, however, a very interesting one, and the time will come, it is to be hoped, when it will be answered decisively one way or the other.”
After this digression we returned to the study of the photographs.
“Photograph No. 20, which we have just been examining,” I said, “represents the moon at the age of about twenty-four days and twenty hours. The next, and the last of the series showing the moon in progressive phases, is No. 21. Here the age of the moon is about twenty-six days and twenty hours. It is the fast waning sickle of the Old Moon which we behold. You perceive that it is relatively uninteresting when compared with No. 20, because very little except the eastern limb is illuminated. Nearly all the great circular and oval formations and craters, and all the ‘seas,’ have passed into the lunar night. Only the eastern verge of the _Oceanus Procellarum_ remains in sight, dulling the brilliance of the inner curve of the sickle. The dark walled plain above the center is Riccioli, and just below it appears Hevel, a smaller, but yet large formation, with a low central mountain. It is hardly worth our while to attempt to identify the other features shown in the photograph. They include none that we have previously studied. Yet this picture has an interest all its own because it is an excellent representation of the moon at a time when she is so near to the sun. Do not forget that, as I warned you when we began with the crescent of the New Moon, in these photographs the moon appears reversed top for bottom. Seen in the sky in the early morning this sickle would have its rounded edge toward the left hand and directed more or less downward, according to the position of the sun. A great deal of confusion exists in the minds of well-educated people concerning the position of the sickle of the New and the Old Moon. You have, of course, heard of the classic instances in which artists have drawn the New Moon with the concave side toward the sun! It is only necessary to remember that a line drawn straight from the center of the convex side of the sickle, whether it be the New Moon or the Old Moon, always extends directly toward the place occupied by the sun.”
“There is,” said my friend, “an interesting old superstition which I have often heard—I suppose it must of course be a superstition—concerning ‘wet moons’ and ‘dry moons.’ As I recall it they say that when the sickle of the New Moon appears nearly upright in the sky that is a sign of dry weather, because the moon is then like an overturned cup, but when the sickle has its ends turned upward that is a sign of wet weather, because then the cup can hold water. I suppose that these various positions of the moon actually occur, but I do not know how they are brought about.”
“The supposed influence of the position of the New Moon on the weather,” I replied, “is too gross a superstition to be worthy of any notice, but the different attitudes of the sickle are interesting. They arise from the changes in the position of the moon as seen from the earth with respect to the direction of the sun, and these changes depend in turn on the inclination of the moon’s path in the sky to the plane of the earth’s equator as well as to the plane of the ecliptic or the earth’s orbit. The ecliptic has an inclination of about 23½° to the plane of the equator, and the moon’s orbit is inclined a little over 5° to the ecliptic. The moon may, in consequence, appear more than 28° above or below the equator. But since, as I told you in the beginning, the orbit of the moon itself turns slowly about in space, the distance of the moon above or below the equator is not constant. It may be only a little more than 18°. In consequence of these changes of relative position the situation of the horns of the crescent moon varies. But you need never be in doubt as to what position they will occupy at any time if you will simply remember that a straight line drawn from the point of one horn to that of the other must always form a right angle with the direction of the sun.
“There is another very interesting fact about the position of the moon in the sky which we should not neglect to notice. Did you ever observe the superior brilliancy of the light of the Full Moon in winter? It is one of the compensations that nature offers us. Since the Full Moon is necessarily situated opposite to the point occupied by the sun, and since the sun is far south of the equator in midwinter, it follows that at the same season the Full Moon appears high above the equator in the northern hemisphere. You will, perhaps, permit me to show you a diagram intended to explain this phenomenon.
You observe that the sun being south of the equator, in the direction indicated by the dotted line, the Full Moon is correspondingly situated north of the equator, and must necessarily appear high in the sky at midnight, when the sun is at its lowest declination. This is the reason why the winter Full Moons are so brilliant, making the snow-clad hills gleam with a splendor that sometimes dazzles the eyes of the beholders. In the Arctic regions the long winter night, when the sun does not rise for months, is periodically brightened by the presence of the Full Moon. Just the opposite condition of affairs exists in summer. Then the sun being north of the equator the Full Moon is south of it, and ‘runs low,’ appearing in high latitudes to skim along the southern horizon.”
“Thank you, and now I will ask you one more question,” said my friend. “I have often heard of the ‘Harvest Moon’ and the ‘Hunter’s Moon.’ Will you not kindly explain what is meant by these terms and when the ‘Harvest Moon’ can be seen? There is a poetic suggestiveness in the name that appeals to me.”
“I will try with pleasure,” I said, “but I fear that I shall have to trouble you with another diagram, or perhaps with two.”
“Oh, I shall not mind that at all. I have grown used to diagrams as well as to the nomenclature of the moon.”
“Well, if my diagrams conduct your thoughts to things as interesting as many that lie concealed behind the prosaic names on the moon I shall be content. To begin, then, I must remind you that in her monthly journey around the earth the moon moves from west toward east in her orbit, and thus she gets a little over 12° farther east every twenty-four hours, as reckoned from the position of the sun. The earth turning on its axis in the same direction causes the moon to appear to rise in the east and set in the west once every twenty-four hours. But in consequence of the constant eastward motion of the moon she rises at a later hour every night. Here is a graphic representation of what I mean:
“The earth is turning on its axis in the direction represented by the arrows, and simultaneously the moon is moving in its orbit in the same direction, as is shown by the large arrow. Suppose that some night the moon is seen rising at a particular hour from the point A on the earth. Then, the following night, when the observer has again arrived at A, with the rotation of the earth, the moon will have advanced from M^1 to M^2, and will not be seen rising until the point occupied by the observer has arrived at B. This retardation of the hour of moonrise is variable on account of changes in the position of the moon, arising from the inclination of her orbit to the plane of the equator, and from the inequalities of her motion, to which I have before referred. On the average it amounts to fifty-one minutes daily. It varies also with the distance of the observer from the equator, the variation being greater in high latitudes. In the latitude of New York the retardation of moonrise may be as great as an hour and a quarter, or as little as twenty-three minutes.
“Now it is upon this variation that the phenomenon of the ‘Harvest’ and the ‘Hunter’s Moon’ depends. If I had a celestial globe here I could show you that at the time of the Autumn Equinox, September 22d, when the sun crosses the equator moving southward, the apparent path of the moon in the sky intersects the eastern horizon at a comparatively small inclination. In other words the moon at that time instead of rising steeply from the horizon rises on a long slope almost parallel with the horizon. The consequence is that for several evenings in succession the Full Moon near the time of the Autumn Equinox may be seen rising just after sunset at almost the same hour. Look at this second diagram and you will see why this is so.
“The little circles M show the moon at several successive positions in her orbit, just twenty-four hours apart. You perceive that in consequence of the slight inclination to the eastern horizon the sinking of the latter caused by the earth’s rotation will bring the moon into view night after night at almost the same hour. In fact, in high northern latitudes like those of Norway and Sweden the moon’s path at this time of the year may actually coincide with the horizon, so that for several evenings she will rise at exactly the same hour. The name ‘Harvest Moon’ explains itself, since it always occurs at the time of the autumn harvests and the vintage, and seems to supplement the fading daylight for the benefit of late laborers in the fields. The ‘Harvest Moon’ does not occur every year at precisely the same date. It is very rare that Full Moon happens to fall just on September 22d. It usually either precedes or follows that date. The ‘Harvest Moon’ is the Full Moon which occurs nearest to the Autumn Equinox, either before or after. The ‘Hunter’s Moon’ is the first Full Moon which follows the ‘Harvest Moon.’ Like the former it rises for several successive evenings near the same hour, but this phenomenon is less marked in the case of the ‘Hunter’s Moon,’ because it is farther from the Equinox.”
“Thank you, again,” said my friend. “I shall never henceforth look at the moon without thinking of circles, straight lines, and arrows as well as of ‘ring mountains’ and ‘seas.’”
“Then you are making good progress toward science,” I replied. “One last look, now, at the photograph of the Old Moon’s sickle, and then we had better postpone our examination of the large photographs, showing certain particularly interesting districts on the moon, until to-morrow morning. There is here another interesting point for artists to note. The convex side of the sickle of the Old Moon, or the New Moon, is always an arc of a circle, but the concave side is never circular although it is often thus represented. The concave side, neglecting its irregularities arising from the differences of level and of brilliancy of the lunar surface, is elliptical in outline, that is to say, it is a semicircle viewed obliquely.”
“Whatever its geometry may be,” replied my friend, “it is certainly very beautiful. Good night, and I shall demand to see those large photographs before the sun is very high to-morrow.”
IV
GREAT SCENES ON THE MOON
IV
GREAT SCENES ON THE MOON
MY friend did not leave me in doubt on the following morning as to the genuineness of her interest in her new studies. The shadows of the trees in the park were yet as long drawn out as the silhouettes of lunar peaks at sunrise, when we resumed our place under the elm, and, at her request, I opened once more my portfolio.
“The series of photographs that we are now about to examine,” I began, “are on so large a scale that only a selected part of the moon is seen in each of them. But within the restricted limits of these pictures the amount of detail shown is truly astonishing, far more indeed than can be found on the most elaborate lunar charts. These photographs were made by Mr. Ritchey with the great 40-inch telescope of the Yerkes Observatory. Many more besides those that we are going to look at were taken by him, but I have selected, where choice was difficult, six which seemed to me to be of special interest. We shall begin with one which covers the larger part of the _Mare Nubium_, in the southeastern quarter of the moon. You certainly must remember the _Mare Nubium_, for it forms the head of the ‘dark woman’ whom you discovered in the moon last evening, and if you will hold this photograph at arm’s length you will see that her face is unmistakably stamped upon it.”
“I am greatly flattered,” she replied, “that you should remember my discovery so well. I begin to feel hopeful that it may yet find a place in the books.”
“It certainly is as deserving of such a place as many things that get into books. You ought to find a suitable name for this woman in the moon.”
“If I believed myself capable of rivaling the man who christened the ‘Marsh of a Dream,’ I should surely try my hand at lunar nomenclature, but I fear that I should fall too far short of the ideal he has set up, and so I shall leave her nameless.”
“Permit me then to continue to call her the ‘dark woman’ whenever a reference to her may seem useful in fixing the localities that we shall talk about in this photograph. The most striking object shown in the picture is the great ring mountain Bullialdus which forms an extraordinary ornament on the top of the ‘dark woman’s’ ear. This photograph was taken when the line of sunrise ran just along the border between the _Mare Nubium_ and the _Oceanus Procellarum_. The _Mare Humorum_ is yet buried in night beyond the upper right-hand edge of the picture, but some of its bordering mountains and craters have been touched by the morning sunbeams. You will observe that a little more than half of the interior of Bullialdus—which, by the way, I did not mention by name when we were studying the series of phase photographs—is yet filled with shadow, but its double-headed central peak rises clear and bright in the sunlight. The shadow of this central mountain can be seen projecting toward the east over the floor. The east wall, which is distinctly terraced, lies in full sunshine, and the light streaming over the lofty crest of the western wall touches the floor on its eastern half. The steep outer slopes that lead up to the western rampart, and the deep parallel ravines cut near the crest are clearly shown. The distance across the ring from the summit of the wall on one side to that on the other is 38 miles. The depth of the depression is 8,000 feet below the crest of the walls, but the latter rise only 4,000 feet above the level of the _Mare Nubium_ outside, so that Bullialdus is an excellent example of the characteristic form of the lunar volcano, which I tried to illustrate for you last evening. The central mountain is 3,000 feet high. East of the south point of the ring a shadow shows the existence of a profound cleft in the wall, while a little west of south appears a smaller crater ring very black with shadow, except on its eastern side. If we stood on the _Mare Nubium_ and looked toward Bullialdus and its neighbor from a distance of 25 or 30 miles they would resemble a double, flat-topped mountain, with its serrated crests connected by a high neck. The summit of one of the little peaks shown in the photograph in the plain just west of Bullialdus would form an excellent point of observation. Still farther south stands another crater ring most of whose interior is also, at present, filled with shadow. East of this, and a little farther south, is still a third ring of similar aspect, from which a curious range of hills runs southward. Returning to Bullialdus you will notice the radiating lines of hills that surround it, and particularly a more lofty and broken range which runs eastward.”
“Bullialdus verily frightens me!” exclaimed my friend. “What an unearthly look it has! The longer I regard it the stronger becomes the indescribable impression that it produces. I begin to understand now what you meant when you promised to find a history in the moon. Truly there never can have been such another history. I almost feel that I do not care whether the moon ever had inhabitants or not. Its own story is more fascinating than that of any puny race of beings, passing their ephemeral lives upon its wonderful surface, could possibly be.”
“I am glad,” I replied, “that you have begun to enter into the spirit of those who long and carefully study the earth’s satellite. You see now, that it is not necessary to the astronomer to find evidences either of former or of present life upon the moon in order to stimulate his zeal. For him, as you have yourself intimated, the relics of its past history, which this little world in the sky exhibits so abundantly, are of higher interest than any story of human empire, for they have an incomparably vaster theme. But to lighten our labor a little, let me once more refer to the ‘dark woman,’ whose features, like the outlines of a constellation, serve for points of reference. I began by remarking that Bullialdus seems to be placed just over her ear. Observe now that, taken together with its immediate surroundings, the great crater ring forms a kind of barbaric ear-ornament of most extraordinary form and richness of detail. The line of hills east of Bullialdus, of which I spoke a few minutes ago, connects the ring with a tumbled mass of mountains on the border of the _Mare Humorum_. These mountains run northward, or downward in the picture, for a distance of perhaps 150 miles, and then turn abruptly westward for a like distance; after which, in the form of a broken chain, constituting the eastern walls of a row of half-submerged ring plains, they change direction once more and run southward in the _Mare Nubium_. The whole system bears some resemblance to a gigantic buckle.”
“What is that curious object below Bullialdus which resembles an old-fashioned gold earring?”
“I was about to speak of that. It is a ring plain named Lubiniesky, about 23 miles in diameter with a wall a thousand feet in height, except in the direction of Bullialdus where it is broken down. The interior is very flat, and it forms a fine example of the half-submerged lunar volcanoes which abound in this hemisphere. It may have had a central mountain like Bullialdus, but if so it has been completely buried under the influx of molten lava or whatever it was that covered this part of the moon. The perfect form of Bullialdus in all its details when compared with the mere outline that remains of Lubiniesky indicates that the former probably burst forth after the inundation of liquid rock that drowned the latter. Thus we have in these two neighboring formations two chapters of lunar history which, like the monuments of Egypt, tell the story of widely separated epochs. The row of still more completely submerged crater rings westward from Lubiniesky and Bullialdus show by their condition that the depth of the lava flood was probably greater in their vicinity than it was farther eastward.
“Now look southward from Bullialdus, at a distance about twice as great as that of Lubiniesky and you will see another partially submerged ring, with a more serrated crest. The name of this is Kies. It is remarkable for the lofty mountain spur which sets off from its southern wall, and also for the fact that one of the bright streaks from Tycho—one of a parallel pair that I pointed out to you last evening—traverses its flat floor and continues on, broadening as it goes, to a deep crater ring which we have already noticed, southeast of Bullialdus.