Part 18

“And have all these mountains and craters names?”

“Not all of them, but many more, perhaps, than you suppose. On the whole visible surface of the moon about 500 objects, not including the ‘seas,’ have received names. It may surprise you to learn that the position of the most important of these objects has been ascertained with an accuracy which is still lacking in our determination of positions on the earth. In other words our charts of the moon are more exact than those of our own planet.”

“That does indeed surprise me. I should have thought that, living on the earth, we could make very correct maps of it, while, as for the moon, two or three hundred thousand miles away, it seems to me not so easy to do that.”

“It is mainly because we are on the earth that we find such great difficulty in making accurate maps of it. We cannot look at the earth as a whole, but we have to crawl over its surface, making measurements as we go, and afterwards translating those measurements into lines and angles on paper. Thus we are still uncertain about the precise distance between many important points on our globe, while for points on the moon no corresponding uncertainty exists. The moon hangs before us in the sky, with no clouds except those in our own atmosphere to obscure it, and it is only necessary carefully to observe the position of particular points, and with the proper instruments to measure their distance and directions from one another. But even this is not a thing that can be accomplished without much pains and much knowledge. The astronomer, no matter what field he chooses, is necessarily a hard worker, and his motto, above everything else, is accuracy. No one is more tempted than he by the sublimity and the extraordinary character of the objects of his study, to give rein to the imagination, and yet imagination is the thing of all others from whose vagaries he must most carefully guard himself. So you must not blame him too severely if he has not dotted the shores of the moon with cities, and populated its plains with industrious farmers.”

“If you will permit me to wander a little aside from our photographic studies for a few minutes,” said my friend, “I should like to ask you about two or three things concerning the moon which have long puzzled me. From my earliest days, living the greater part of the time in the country, I have heard that the moon exercises a decided influence over the weather, and over the growth of vegetation. I have neighbors who would never think of planting certain things except ‘in the New of the moon’! Some will not cut timber except ‘in the Old of the Moon,’ as they say that the sap is drawn up by the moon’s influence when she is growing. Is there really any truth in all this?”

“Not the least. At any rate there is no scientific evidence whatever for such statements, and no probability that they are based on facts. They are the result of faulty observation, misled by coincidences. It is imaginable that the light of the moon might have some influence upon vegetable growth if it were an original kind of light coming from the moon herself. But moonlight is only reflected sunlight, and when we examine it with the spectroscope we do not find that the rays of light in visiting the moon and returning thence to the earth have had either anything added to or anything taken away from them, except intensity. The total amount of light reflected from the moon upon the earth is estimated to be about 1/618000 of the total amount that comes to us from the sun. Curiously enough the moon appears to reflect proportionally more heat than light, the amount of lunar heat received by the earth being reckoned at 1/185000 of the amount coming from the sun. The popular idea that the moon affects the movement of sap in plants is equally illusory.”

“But about the weather? I know people who believe that a change of the moon from one phase to another brings about a change of weather. Is that true?”

“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.”

NO. 21. AUGUST 19, 1903; MOON’S AGE 26.89 DAYS. ]

“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.

Diagram Showing Why the Winter Moon Runs High. ]

“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.

Diagram Showing Why Moon Rises Later Every Night. ]

“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.

Diagram Illustrating the Harvest Moon. ]