Astronomy for Everybody/Part 3/Chapter 4

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About one hundred years ago there was an unpopular professor in the Government Polytechnique School of Paris, still the great school of mathematics for the French public service, who loved to get his students into difficulties. One morning he addressed one of them the question:

"Monsieur, have you ever seen the moon?"

"No, sir," replied the student, suspecting a trap.

The professor was nonplussed. "Gentlemen," said he, "see Mr. ———, who professes never to have seen the moon!"

The class all smiled.

"I admit that I have heard it spoken of," said the student, "but I have never seen it."

I take it for granted that the reader has been more observant than the French student professed to be, and that he has not only seen the moon, but knows the phases through which it goes and is familiar with the fact that it describes a monthly course around the earth. I also suppose that he knows the moon to be a globe, although, to the naked eye, it seems like a flat disk. The globular form is, however, very evident when we look at it with a small telescope.

Various methods and systems of measurement all agree ​in placing the moon at an average distance of a little less than two hundred and forty thousand miles. This distance is obtained by direct measure of the parallax, as will be explained hereafter, and also by calculating how far off the moon must be in order that, being projected into space, it may describe an orbit around the earth in the time that it actually does perform its round. The orbit is elliptic, so that the actual distance varies. Sometimes it is ten or fifteen thousand miles less, at other times as much more, than the average.

The diameter of the moon's globe is a little more than one fourth that of the earth; more exactly, it is two thousand one hundred and sixty miles. The most careful measures show no deviation from the globular form except that the surface is very irregular.

The moon accompanies the earth in its revolution round the sun. To some the combination of the two motions seems a little complex; but it need not offer any real difficulty. Imagine a chair standing in the centre of a railway car in rapid motion, while a person is walking around it at a distance of three feet. He can go round and round without varying his distance from the chair and without any difficulty arising from the motion of the car. Thus the earth moves forward in its orbit, and the moon continually revolves around it without greatly varying its distance from us.

The actual time of the moon's revolution around the earth is twenty-seven days eight hours; but the time ​from one new moon to another is twenty-nine days thirteen hours. The difference arises from the earth's motion around the sun; or, which amounts to the same thing, the apparent motion of the sun along the ecliptic. To

show this, let AC be a small arc of the earth's orbit around the sun. Suppose that at a certain time the earth is at the point E, and the moon at the point M, between the earth and the sun. At the end of twenty-seven days eight hours the earth will have moved from E to F. ​While the earth is making this motion the moon will have moved around the orbit in the direction of the arrows, so as to have reached the point N. At the moment when the lines EM and FN are parallel to each other, the moon will have completed her actual revolution, and will seem to be in the same place among the stars as before. But the sun is now in the direction FS. The moon therefore has to continue its motion before it catches up to the sun. This requires a little more than two days, and makes the whole time between two new moons twenty-nine and a half days.

The varying phases of the moon depend upon its position with respect to the sun. Being an opaque globe, without light of its own, we see it only as the light of the sun illuminates it. When it is between us and the sun its dark hemisphere is turned toward us, and it is entirely invisible. The time of this position in the almanacs is called "new moon," but we cannot commonly see the moon for nearly two days after this time, because it is lost in the bright twilight of evening. On the second and third day, however, we see a small portion of the illuminated globe, having the familiar form of a thin crescent. This crescent we commonly call the new moon, although the time given in the almanac is several days earlier.

In this position, and for several days longer, we may, if the sky is clear, see the entire face of the moon, the dark parts shining with a faint gray light. This light is that which is reflected from the earth to the moon. An inhabitant of the moon, if there were such, would ​then see the earth in the sky like a full moon, looking much larger than the moon looks to us. As the moon advances in its orbit day after day, this light diminishes, and about the time of first quarter disappears from our sight owing to the brightness of the illuminated portion of the moon.

Seven or eight days after the almanac time of new moon, the moon reaches its first quarter. We then see half of the illuminated disk. During the week following, the moon has the form called gibbous. At the end of the second week the moon is opposite the sun, and we see its entire hemisphere like a round disk. This we call full moon. During the remainder of its course the phases recur in reverse order, as we all know.

We might regard all these recurrences as too well known to need description, yet, in the Ancient Mariner, a star is described as seen between the two horns of the moon as though there were no dark body there to intercept our view of the star. Probably more than one poet has described the new moon as seen in the eastern sky, or the evening full moon as seen in the west.

We can see with the naked eye that the moon's surface is variegated by bright and dark regions. The latter are sometimes conceived to have a vague resemblance to the human face, the nose and eyes being especially prominent. Hence the "man in the moon." Through even the smallest telescopes we see that the surface has an immense variety of detail; and the more powerful the tele​

​scope the more details we see. The first thing to strike us on a telescopic examination will be the elevations, or mountains as they are commonly called. These are best seen about the time of the first quarter, because they then cast shadows. At full moon they cannot be so well made out, because we are looking straight down and see everything illuminated. Although these elevations and depressions are called mountains they are different in form from the ordinary mountains of the earth. There is, however, an almost exact resemblance between them and the craters of our great volcanoes. A very common form is that of a circular fort, one or more miles in diameter, with walls which may be thousands of feet high. The inside of this fort may be saucer shaped, a large portion of the surface being flat. At first quarter we can see the shadow of the walls cast upon the interior flat surface. In the centre a little cone is frequently seen. The interior surface is by no means perfectly flat and smooth. The higher power the more details we shall see. Just what these consist of it is impossible to say; they may be solid rock or they may be piles of loose stone. As we can see no object on the moon, even with the most powerful telescope, unless it is more than a hundred feet in diameter, we cannot say what the exact nature of the surface is in its minutest portions.

The early observers with the telescope supposed that the dark portions were seas and the brighter portions continents. This notion was founded on the fact that the darker portions looked smoother than the others. Names were therefore given to these supposed oceans, such ​as Mare Procellarum, the Sea of Storms; Mare Serenitatis, the Sea of Calms, etc. These names, fanciful though they be, are still retained to designate the large dark regions on the moon. A very slight improvement in the telescope, however, showed that the idea of these dark regions being oceans was an illusion. They are all covered with inequalities, proving that they must be composed of solid matter. The difference of aspect arises from the lighter or darker shade of the materials which compose the lunar surface. These are distributed over the surface of the moon in a very curious way. One of the most remarkable features are the long bright lines which radiate from certain points on the moon. A very low telescopic power will show the most remarkable of these; a good eye might even perceive it without a telescope. On the southern part of the moon's hemisphere, as we see it, is a large spot or region known as Tycho, and from this radiate a number of these bright streaks. The appearance is as if the moon had been cracked and the cracks filled up with melted white matter.

Whether we accept this view or not, it is impossible to examine the surface of the moon without the conviction that in some former age it was the seat of great volcanic activity. In the centre of all the great circular mountains we have described are craters which, it would seem, must have been those of volcanoes. Indeed, a hundred years ago it was supposed by Sir William Herschel that there was an active volcano on the moon, but it is now known that this appearance is due to the light of the earth reflected from a very bright spot on the moon's ​surface. It can be easily seen about the time of the new moon with a telescope of moderate size.

One of the most important questions connected with the moon is whether there is any air or water on its surface. To these the answer of science up to the present time is in the negative. Of course this does not mean that there can absolutely not be a drop of moisture nor the smallest trace of an atmosphere on our satellite; all we can say is that if any atmosphere surrounds the moon it is so rare that we have never been able to get any evidence of its existence. If the latter had such an appendage of even one hundredth of the density of the earth's atmosphere, its existence would be made known to us by refraction of the light from a star seen alongside the moon. But not the slightest trace of any such refraction can be discovered. If there is any such liquid as water, it must be concealed in invisible crevices, or diffused fused through the interior. Were there any large sheets of water in the equatorial regions they would reflect the light of the sun day by day, and would thus become clearly visible. The water would also evaporate and form more or less of an atmosphere of watery vapour.

All this seems to settle another important question; namely, that of the habitability of the moon. Life, in the form in which it exists on our earth, requires water at least for its support, and in all its higher forms air also. We can hardly conceive of a living thing made of mere sand or other dry matter such as forms the lunar ​surface. If we supposed animals to walk about on the moon, it is difficult to imagine what they could eat. Our general conclusion must be that there is no life on the moon subject to the laws which govern life on the surface of this earth.

The total absence of air and water results in a state of things on the moon such as we never experience on the earth. So far as can be ascertained by the most careful examination, not the slightest change ever takes place on its surface. A stone lying on the surface of the earth is continually attacked by the weather and in the course of years is gradually disintegrated or washed away by the wind and water. But there is no weather on the moon, and a stone lying on its surface might rest there for unknown ages undisturbed by any cause whatever. The lunar surface is heated up when the sun shines on it and it cools off when the sun has set. Except for these changes of temperature there is absolutely nothing going on over the whole surface of the moon, so far as we can see. A world which has no weather and on which nothing ever happens—such is the moon.

The rotation of the moon on its axis is a subject on which some are frequently so perplexed that we shall explain it. Anyone who has carefully examined this body knows that it always presents the same face to us. This shows that it rotates on its axis in the same time that it revolves around the earth. An idea frequently entertained is that this shows that it does not rotate at all, ​and many chapters have been written on this subject. The whole difficulty arises from the different ideas which people have of motion. In physics we say that a body does not rotate when, if a rod were passed through it, that rod always maintained the same direction when the
Fig. 21.—Showing how the Moon would Move if it did not Rotate on its Axis. body moved about. Now let us suppose such a rod passed through the moon; then, if the latter did not rotate on its axis the rod would maintain its same direction while the moon, revolving around the earth, would appear at different points in its orbit as we see it in Figure 21. A very little study of this figure will show that as the moon went around we should successively see every part of its surface in succession if it did not rotate on its axis.

All of us who live on the seashore know that there is a rise and fall of the ocean which in the general average occurs about three quarters of an hour later every day, and which keeps pace with the apparent diurnal motion of the moon. That is to say, if it is high tide to-day when ​the moon is in a certain position in the heavens, it will be high tide when the moon is in or near that position day after day, month after month, and year after year. We have all heard that the moon produces these tides by its attraction on the ocean. We readily understand that when the moon is above any region its attraction tends to raise the waters in that region; but the circumstance that most perplexes those who are not expert in the subject is that there are two tides a day, high tide occurring not only under the moon, but on the side of the earth opposite the moon. The explanation of this is that the moon really attracts the earth itself as well as it does the water. It continually draws the entire earth and everything upon it toward itself. As it goes round the earth in its monthly course, it thus keeps up a continual motion of the latter. If it attracted every part of the earth equally, the ocean included, there would then be no tides, and everything would go on on the earth's surface as if there were no attraction at all. But as the attraction is as the inverse square of the distance, the moon attracts the regions of the earth and oceans which are nearest to it more than the average, and those that are farthest from it less than the average.

To show the effect of these changes let A, C, and H be the three points on the earth attracted by the moon. Since the moon attracts C more than A, it tends to pull C away from A and increase the distance between A and C. At the same time pulling H more than C it tends to increase the distance between H and C. If the whole earth was a fluid, the attraction of the moon would be simply to ​draw this fluid out into the form of an ellipsoid, of which the long diameter would be turned toward the moon. But the earth itself, being solid, cannot be drawn out into this shape, while the ocean, being fluid, is thus drawn out. The result is that we have high tides at the two ends of the ellipse into which the ocean is drawn, and low tides in the mid-region.

The complete explanation of the subject requires a statement of the laws of motion which cannot be made

here. I will, however, remark that if the attraction of the moon on the earth were always in the same direction, the two bodies would be drawn together in a few days. But owing to the revolution of the moon round the earth the direction of the pull is always changing, so that the earth is, in the course of a month, only drawn about three thousand miles from its mean position by the moon's pull.

It might be supposed that if the moon produces the tides in this way we should always have high tide when the moon is on the meridian and low tide when the moon is in the horizon. But such is not the case, for two reasons. In the first place it takes time for the moon to draw ​the waters out into the form of an ellipsoid, and when it once gives them the motion necessary to keep this form, that motion keeps up after the moon has passed the meridian, just as a stone continues to rise after it has left the hand or a wave goes forward by the momentum of the water. The other cause is found in the interruption of the motion by the great continents. The tidal wave, as it is called, meeting a continent, spreads out in one direction or the other, according to the lay of the land, and may be a long time in passing from one point to another. Thus arise all sorts of irregularities in the tides when we compare those in different places.

The sun produces a tide as well as the moon, but a smaller one. At the times of new and full moon the two bodies unite their forces and cause the highest and lowest tides. These are familiar to all dwellers on the seacoast and are called spring tides. About the time of the first and last quarters the attraction of the sun opposes that of the moon and the tides do not rise so high or fall so low, and these are called neap tides.