THE MOON

Star-land: Being Talks With Young People About the Wonders of the Heavens by Robert S. Ball is part of the HackerNoon Books Series. You can jump to any chapter in this book here. THE PHASES OF OUR ATTENDANT, THE MOON

LECTURE II. THE MOON

THE PHASES OF OUR ATTENDANT, THE MOON.

The first day of the week is related to the greatest body in the heavens—the sun—and accordingly we call that day Sun-day. The second day of the week is similarly called after the next most important celestial body—the moon—and though we do not actually say Moon-day, we do say Monday, which is very nearly the same. In French, too, we have lune for moon, and Lundi signifies our Monday. The other days of the week also have names derived from the heavens, but of these we shall speak hereafter. We are now going to talk about the moon.

We can divide the objects in this room into two classes. There are the bright faces in front of me, and there are the bright electric lights above. The electric lights give light, and the faces receive it. I can see both lights and faces; but I see the electric lamps by the light which they themselves give. I see the faces by the illumination which they have received from the electric lights. This is a very simple distinction, but it is a very important one in Star-land. Among all these bodies which glitter in the heavens there are some which shine by their own light, like the lamps. There are others only brilliant by reflected light, like the faces. It seems impossible for us to confuse the brightness of a pleasant face with the beam from a pretty lamp, but it is often not very easy to distinguish in the heavens between a body which shines by its own light and a body which merely shines by some other light reflected from it. I think many people would make great mistakes if asked to point out which objects on the sky were really self-luminous and which objects were merely lighted up by other bodies. Astronomers themselves have been sometimes deceived in this way.

The easiest example we can give of bodies so contrasted is found in the case of the sun and the moon. Of course, as we have already seen, the sun is the splendid source of light which it scatters all around. Some of that light falls on our earth to give us the glories of the day; some of the sunbeams fall on the moon, and though the moon has itself no more light than earth or stones, yet when exposed to a torrent of sunbeams, she enjoys a day as we do. One side of her is brilliantly lighted; and this it is which renders our satellite visible.

Hence we explain the marked contrast between the sun and the moon. The whole of the sun is always bright; while half of the moon is always in darkness. When the bright side of the moon is turned directly towards us, then, no doubt, we see a complete circle, and we say the moon is full. On other occasions a portion only of the bright surface is directed to us, and thus are produced the beautiful crescents and semicircles and other phases of the moon.

Fig. 28.—To show that the Moon is lighted by Sunbeams.

A simple apparatus (illustrated in Fig. 28) will explain their various appearances. The large india-rubber ball there shown represents the moon, which I shall illuminate by a beam from the electric light. The side of the ball turned towards the light is glowing brilliantly, and from the right side of the room you see nearly the whole of the bright side. To you the moon is nearly full. From the centre of the room you see the moon like a semicircle, and from the left it appears a thin crescent of light. I alter the position of the ball with respect to the lamp, and now you see the phases are quite changed. To those on my left our mimic moon is now full; to those on my right the moon is almost new, or is visible with only a slender crescent. From the centre of the room the quarter is visible as before. We can also show the same series of changes by a little contrivance of Figs. 29 and 30.

Thus every phase of the moon (Fig. 31), from the thinnest beautiful crescent of light that you can just see low in the west after sunset up to the splendor of the full moon, can be completely accounted for by the different aspects of a globe, of which one-half is brilliantly illuminated.

The Phases of the Moon.

We can now explain a beautiful phenomenon that you will see when the moon is still quite young. We fancifully describe the old moon as lying in the new moon’s arms when we observe the faintly illuminated portion of the rest of that circle, of which a part is the brilliant crescent. This can only be explained by showing how some light has fallen on the shadowed side; for nothing which is not itself a source of light can ever become visible unless illuminated by light from some other body.

Fig. 31.—The Changes in the Moon.

Let us suppose that there is a man on the moon who is looking at the earth. To him the earth will appear in the same way as the moon appears to us, only very much larger. At the time of new moon the bright side of the earth will be turned directly towards him, so that the man in the moon will see an earth nearly full, and consequently pouring forth a large flood of light. Think of the brightest of all the bright moonlight nights you have ever seen on earth, and then think of a light which would be produced if you had thirteen moons, all as big and as bright as our full moon, shining together. How splendid the night would then be! You would be able to read a book quite easily! Well, that is the sort of illumination which the lunar man will enjoy under these circumstances; all the features of his country will be brightly lighted up by the full earth. Of course, this earth-lighted side of the moon cannot be compared in brilliancy with the sun-lighted side, but the brightness will still be perceptible, so that when from the earth we look at the moon, we see this glow distributed all over the dark portion; that is, we observe the feebly lighted globe clasped in the brilliant arms of the crescent. At a later phase the dark part of the moon entirely ceases to be visible, and this for a double reason: firstly, the bright side of the earth is then not so fully turned to the moon, and therefore the illumination it receives from earth-shine is not so great; and, secondly, the increasing size of the sun-lighted part of the moon has such an augmented glow that the fainter light is overpowered by contrast. You must remember79 that more light does not always increase the number of things that can be seen. It has sometimes the opposite effect. Have we not already mentioned how the brightness of day makes the stars invisible? The moon herself, seen in full daylight, seems no brighter than a small particle of white cloud.

THE SIZE OF THE MOON.

It is not easy to answer the question which I am sometimes asked, “Is the moon very big?” I would meet that question by another, “Is a cat a big animal?” The fact is, there is no such thing as absolute bigness or smallness. The cat is no doubt a small animal when compared with the tiger, but I think a mouse would probably tell you that the cat was quite a big animal—rather too big, indeed, in the mouse’s opinion. And the tiger himself is small compared with an elephant, while the mouse is large as compared with a fly.

When we talk of the bigness or the smallness of a body, we must always consider what we are going to compare it with. It is natural in speaking of the moon to compare it with our own globe, and then we can say that the moon is a small body.

The relative sizes of the earth and the moon may be illustrated by objects of very much smaller dimensions. Both a tennis ball and a football are no doubt familiar objects to everybody. If the earth be represented by the football, then the moon would be about as large as the lawn-tennis ball. But this proportion is not quite80 accurate, so I will suggest to you an instructive way of making a better pair of models of the earth and the moon. In fact, experiments somewhat similar to those I describe have been actually going on in every kitchen in the land during this festive season. For have not globes and balls of all sorts and sizes been made of plum-pudding, and it will only require a little care on the part of the cook to make a pair of luscious spheres that shall fairly set forth the sizes of the earth and the moon. There is first to be a nice little round plum-pudding, three inches in diameter. It is just a little bigger than a cricket ball. It should, however, only make its appearance at a bachelor’s table. Were it set down before a hearty circle on Christmas Day, dire disappointment would result. One boy of sound constitution could eat it all. Perhaps it would weigh about three-quarters of a pound. This little globe is to represent the moon.

Fig. 32.—Relative Sizes of the Earth and Moon.

Another plum-pudding is to be constructed which81 shall represent the earth (Fig. 32). We must, however, beg the cook to observe the proportions. The width of the earth, or the diameter, to use the proper word, is about four times the diameter of the moon. Hence, as the small plum-pudding was three inches across, the large one must have a diameter of twelve inches. This will be a family pudding of truly satisfactory dimensions; perhaps the cook will be a little surprised to find the alarming quantity of materials that will be required to complete a sphere of plum-pudding a foot in diameter.

These models having been duly made, and boiled, and placed on the table, we are now to propose the following problem:—

“If one schoolboy could eat the small plum-pudding, how many boys would be required to dispose of the large one?”

The hasty person, who does not reflect, will at once dash out the answer, “Four!” He will say, “It is quite plain that, since one of the puddings has four times the diameter of the other, it must be four times as big; and therefore, as one boy is able to eat the small pudding, four boys will be adequate for the large one.” But the hasty person will, as usual, be quite wrong. His argument would be sound if it were merely two pieces of sugar-stick that he was comparing; no doubt there is only four times as much material in a piece twelve inches long as there is in a piece three inches long. But the plum-puddings have breadth and depth, which are in the same proportions as the length, and the consequence is that the large plum-pudding is82 far more than four times as big as the small one. No four boys, however admirable their capacities, would be equal to the task of consuming it. Nor even if four more boys were called in to help would the dish be cleared. Twenty boys, forty boys, fifty boys would not be enough. It would take sixty-four boys to demolish the magnificent plum-pudding one foot in diameter.

If the cook will try the experiment, she will find that by taking the materials sufficient for sixty-four small plum-puddings all of the same size, and mixing them together, she will, no doubt, make a large plum-pudding, but its diameter will only be four times that of the small puddings.

As a matter of fact, the moon is 2160 miles in diameter, and the earth is 7918 miles. These numbers are so nearly 2000 and 8000 respectively, that for simplicity I have spoken of the earth as having a diameter four times as great as the moon. If we want to be very accurate, we ought to determine the ratio of the two quantities from the figures just given. Our illustration of the plum-puddings must, therefore, be a little modified. The earth is not quite so much as sixty-four times as big as the moon; but this figure is sufficiently accurate for our present purpose.

Another interesting question may be proposed, namely: How much land is there on the moon? We might state the answer in acres or in square miles; but it will, perhaps, be more instructive to make a comparison between the moon and the earth.

Here also I shall use an illustration; and we shall again consider two globes which are respectively three83 inches and twelve inches in diameter. The globes I use this time are hollow balls of india-rubber. These will represent the earth and the moon with sufficient accuracy, and the relative surfaces of these two globes is what I want to find. There are different ways in which the comparison might be made. I might, for instance, paint the two globes and see the quantity of paint that each requires. If I did this, I should find that the great globe took just sixteen times as much paint as the small one. We can adopt a simpler plan. The india-rubber in one of these balls has the same thickness as in the other, as they are each hollow, so that the quantity which is required for each ball may be taken to represent its surface. By simply weighing the two balls, I perceive that the large one is sixteen times as heavy as the small one. You notice here the difference between the comparative weights of two hollow balls and two solid ones of the same material. Had these globes been of solid india-rubber, the large one would have weighed sixty-four times as much as the small one, just as in the case of the plum-puddings; but being hollow, the ratio of their weights is only the square of the ratio of their diameters—that is to say, four times four, or sixteen.

We are thus taught that if the moon were exactly one-fourth of the diameter of the earth, its surface would be one-sixteenth part of that of the earth. It would, no doubt, have made our subject a little easier and simpler if the moon had been created somewhat smaller than it is. As, however, the universe has not been solely constructed for the purpose of these talks84 about Star-land, we must take things as we find them. This proportion is not four; it is more nearly 3⅔, and the relative surfaces of the two bodies is the square of 11/3, or about 13½. In other words, the entire extent of the surface of our globe is about thirteen and a half times that of the moon.

The face of the full moon, being half the entire extent of the surface, is, therefore, about one-twenty-seventh part of the earth’s surface—continents, oceans, seas, and islands all taken together. The British Empire and the Russian Empire are each of them as large as the face of the full moon.

HOW ECLIPSES ARE PRODUCED.

The moon is the attendant, or the satellite of the earth, ministering to the wants of the earth by mitigating the darkness of our nights. The earth goes around the sun in its annual journey of 365 days. The moon revolves around the earth once every twenty-seven days. The motion of the moon is thus a very complicated one, for it is, in fact, moving round a body which is itself in constant motion (Fig. 33).

You will see by your almanacs every year that certain eclipses are to take place; and after what we have said about the sun and the moon, it will be easy to understand how eclipses arise. There are two different kinds. You will sometimes see an eclipse of the moon, and sometimes those eclipses of the sun of which we have spoken in the last Lecture. You may be surprised to find with what accuracy the eclipses can be predicted.85 We can tell not only those that will occur this year and next year, but we could also foretell the eclipses that will appear in a hundred or a thousand years to come; or we can, with equal ease, calculate backwards, so as to find the circumstances of eclipses that happened thousands of years ago. This shows how well we have learned the way the moon moves.

Fig. 33.—To show how the Earth goes round the Sun and the Moon round the Earth.

Fig. 34.—A Total Eclipse to the Girl and a Partial Eclipse to the Boy.

Fig. 35.—Different Kinds of Solar Eclipse.

An eclipse of the sun is the simpler occurrence, so we shall describe it first. It happens when the moon comes between the earth and the sun. Look at our little astronomers shown in Fig. 34. A boy and a girl are both gazing at the sun, when the moon comes between. To the boy the moon appears to take a great bite out of the sun, so that it looks like the left-hand86 picture in Fig. 35. (I have drawn a line from the end of the telescope in Fig. 34, which shows how much of the sun is cut off.) This would be called a partial eclipse of the sun. The almanac will sometimes describe the eclipses as visible in London, or visible at Greenwich; but that need not be taken so literally as was supposed by a Kensington gentleman, who, on noticing that the almanac said an eclipse was to be visible in London, called a cab and drove into the city to look for it. His almanac had not mentioned that it would be visible from his own house. You may usually take for granted that when an eclipse is said to be visible from London or Greenwich, it will be more or less visible all over England. Most of these eclipses are only partial, and though they are interesting to watch they do not teach us much. By far the most wonderful kind of eclipse is that in which the whole of the bright part of the sun is blotted out. Then, indeed, we do see wonders. But such eclipses are rare, and even when they do occur they only last a very few minutes. The sights that are displayed are so interesting88 that astronomers often travel thousands of miles to reach a suitable locality for making observations.

The girl in Fig. 34 is placed in the best possible position for seeing the eclipse. There you find her right in the line of the sun and moon; and I think you will agree that she cannot see any part of the sun, for the moon is altogether in the way. I have drawn two dotted lines, one at each side. All that she can see beyond the moon must lie outside these dotted lines, and she will be in the dark as long as the moon stays in the way. When the eclipse is complete, comparative darkness steals over the land. The birds are deceived, and fly home to the trees to roost. The owls and the bats, thinking their time has arrived, venture forth on their nocturnal business. Even flowers close their petals, only to open a few minutes later when the sun again bursts forth. Other flowers that give forth their fragrance at night are also sweetly perceptible so long as the sun remains obscured. An unruly cow, accustomed to break into a meadow at night, was found there after an eclipse was over; while I learn from the same authority that a man rushed over in great excitement to see what his chickens were doing, but came back much disappointed on finding them pecking away as if nothing had happened.

It will sometimes happen that the moon is so placed that the edge of the sun can be seen all round it. A case of the kind is shown in the right-hand picture of Fig. 35. It is called an annular, or ring-shaped eclipse.

The eclipses of which we have been speaking are, of course, only to be seen during the day when the sun89 must be up. The lunar eclipses, which are visible at night, are due to the interposition of the earth between the sun and the moon. The sun is at night-time under our feet at the other side of the earth, and the earth throws a long shadow upwards. If the moon enter into this shadow, it is plain that the sunlight is partly or wholly cut off, and since the moon shines by no light of her own, but only by light borrowed from the sun, it follows that when she is buried in the shadow all the direct light is intercepted, and she must lose her brilliancy. Thus we obtain what is called a lunar eclipse. It is total if the moon be entirely in the shadow. The eclipse is partial if the moon be only partly in the shadow. The lunar eclipse is visible to everybody on the dark hemisphere of the earth if the clouds will keep out of the way, so that usually a great many more people can see a lunar eclipse than a solar eclipse, which is only visible from a limited part of the earth. It thus happens that the lunar eclipse is the more familiar spectacle of the two.

When the moon is entirely in the shadow, one might naturally think that it would become totally invisible. This is not always the case. It is a curious fact that in the depth of a total eclipse the moon is often still visible, for she glows with a copper-colored light, which is bright enough to render some of the chief marks on her surface distinctly discernible.

EFFECT OF THE MOON’S DISTANCE ON ITS APPEARANCE.

We are now about to take a good look at the moon and examine the different objects which are marked90 upon it. There is a peculiar interest attached to this particular orb, because it is much the nearest of all the heavenly bodies to our globe, and therefore the one that we can see the best. Every other object—sun, star, or planet—is hundreds, or perhaps thousands, of times as far off as the moon. It is right that we should desire to learn all we can about the bodies in space. We know that the earth is a great ball, and we see that there are many other such bodies. Some of them are much larger, and some of them are smaller than the globe on which we dwell; some of them are dark bodies like the earth, and among them the moon is one. Is it not reasonable that we should make special efforts to find out all we can about this interesting neighbor?

Though the moon is so close to us relatively to other objects in space, yet when we express its distance in the ordinary methods of measurement it is a very long way off—about 240,000 miles—a length nearly as great as that of all the railways in the world put together. An express train which runs forty miles an hour would travel 240 miles in six hours, and the whole distance to the moon would be accomplished in 6000 hours, so that travelling by night and day incessantly you would accomplish the journey in 250 days. To take another illustration, if you wrapped a thread ten times round the equator of the earth, it would be long enough to stretch from the earth to the moon. Or suppose a cannon could be made sufficiently strong to be fired with a report loud enough to be audible 240,000 miles away. The sound would only91 be heard at that distance a fortnight after the discharge had taken place.

The moon is too far for us to examine the particular features on its surface by the unaided eye. Suppose that there was a mighty city like London on the moon, with great buildings and teeming millions of people, and you went out on a fine night to take a look at our neighbor. What do you think you would be able to see of the great lunar metropolis? Would you be able to see its streets full of omnibuses, or even its great buildings? Would you see St. Paul’s and Westminster—the great parks and the river? Of all these things your unaided eye would show you almost nothing. I can give you a little illustration. Suppose that you made a tiny model of London; imagine this little structure all complete, so that the streets, the buildings, the bridges, the railways, the parks, and the Thames were placed in their true proportions; suppose that the miniature city was so small that it could stand on a penny postage stamp, surely everything would look very insignificant, even if you had the model in your hand and looked at it with the aid of a magnifying glass. But suppose it were put on the other side of the table or on the other side of the room, or the other side of the street. Even St. Paul’s Cathedral itself would have ceased to be distinguishable; but yet the distance is not nearly great enough. You would have to put the little model a quarter of a mile away before it would be in the right position to illustrate the appearance of a lunar London to the unaided eye.

92

A TALK ABOUT TELESCOPES.

The astronomer will not be contented with a mere naked-eye inspection of a world so interesting as the moon. He will get a telescope to help his vision. The word “telescope” means a contrivance for looking at objects which are a long way off. We have explained that the further an object is, the smaller it appears to be. The telescope enables us to largely overcome this inconvenience. It has the effect of making a distant object look larger.

There are great differences in the forms of telescopes; and some instruments are large and some small, according to the purposes for which they are required. Perhaps the most useful practical application of the telescope is by the officer on duty on board a ship. He is generally provided with a pair of these instruments bound together to form the “binocular.”

You are all acquainted with this useful contrivance, or at all events with the opera-glass, that is used for purposes with which landsmen are more familiar. The ship’s telescope, or the binocular, or the opera-glass, is feeble in power when compared with the great instruments of the Observatory. The officer on the ship will generally be satisfied with a telescope which shall show the objects with which he is concerned at about one-third of their actual distance. Thus, suppose his attention is directed to a great steamer three miles away, he wishes to see her more clearly, and accordingly he takes a view through his binocular.93 Immediately the vessel is so transformed that it seems to be only one mile away. The apparent dimensions of the object are increased threefold. The hull is three times as long, the masts and the funnel are three times as high, the sailors are three times as tall; various objects on the ship too small to be seen at three miles would be visible from one mile, and to that apparent distance the ship has now been brought.

If the sailor desires to reduce the apparent distance of objects, how much more keenly does the astronomer feel the same want? At best, the sailor only has to scan a range of a few miles with his glass, but what are a few miles to the astronomer? It is true that he can count the distance of the moon by thousands of miles, a good many thousands, no doubt, but for all other objects he must use millions, while for most bodies in space, millions of millions of miles are the figures we are constrained to employ. Need it be said that the astronomer must resort to every device he can to make the body appear closer. He does not despise the modest binocular. It is often a useful instrument in the Observatory. It gives most beautiful pictures of the celestial scenery, and you would be amazed to find how many thousands of stars you can see with its help which your unaided eye would not show you at all. The binocular will also greatly improve the appearance of the moon, but still its powers fall far short of what we require for the study of lunar landscapes. Even though we can reduce the moon’s apparent distance to one-third its94 actual amount, yet still that third is a very considerable distance. One-third of 240,000 is 80,000, so that we can see the moon no better with a binocular95 than we should see it were it 80,000 miles away, and were we viewing it with the unaided eye.

Fig. 36.—The Dome at Dunsink Observatory.

Fig. 37.—The Equatorial at Dunsink Observatory.

I am not going to enter here upon any detailed account of the telescope, because I shall say a little more on the subject in a later lecture; at present I only describe that form of instrument which is most convenient for studying the moon. I take as an illustration the South Equatorial at Dunsink Observatory, which belongs to Trinity College, Dublin.

96This telescope has a building to itself, which stands on the lawn in front of the house. The site is open and elevated, so as to command an extensive prospect of the heavens. You will see in Fig. 36 a picture of the structure. It is circular in form and is entered by the little porch. The most peculiar feature of an edifice intended to contain this kind of telescope is its roof, or Dome, as we call it. It is of a hemispherical shape with a projecting rim at the bottom. But no one would go to the trouble and expense of making a round dome like that over the Observatory if it were not necessary for a particular purpose. The dome is very unlike ordinary roofs, not only in appearance, but also because it can turn round. In the next figure you will see a section through the building, and the wheels are exposed by which the dome is carried. These wheels run easily on rails, so that when the attendant pulls the rope which you see in his hands, he turns round a large pulley, and that operates a little cogwheel which works into a rack, and thus makes the dome revolve. The roof is built of timber, covered with copper; it weighs more than six tons, but the machinery is so nicely adjusted, that a child four years old can easily set the whole in motion. The object of all this machinery is seen when we learn that there is only one opening in the dome. It is covered by the shutter shown over the doorway in Fig. 36. When opened to the top, it gives a long and wide aperture, through which the astronomer can look out at the heavens. Of course the dome has to be turned until the opening has been brought to face the required97 aspect. The big telescope can thus be directed to any object above the horizon. You see a gentleman using the telescope (Fig. 37), and this shows that the great instrument is nearly three times as long as the astronomer himself! No doubt the telescope seems to be composed of a good many different parts, but the essential portions of the instrument are comparatively few and simple. At the upper end is the object glass, which consists of two lenses, one of flint glass and the other of crown glass. Both of these must be of exceptional purity, and the shape to be given to the lenses is a matter of the utmost importance. It is in the making of this pair of glasses that the skill of the optician has to be specially put forth. So valuable indeed is an object glass which fulfils all the requirements, that it is by far the most costly part of the instrument. There are no glasses in the interior of the tube until you come to the end where the observer is looking in. This is closed by an eyepiece consisting of a lens, or a pair of lenses. There are usually many different eyepieces for a telescope, and they contain lenses of varied powers, to be used according to the state of the atmosphere, or to the particular kinds of observation in progress.

If you point a big telescope to the sky, and see therein the sun or the moon or any of the stars, you will speedily find that the objects pass away out of view. Remember our earth is constantly turning round, and bears, of course, the Observatory with it, so that though the telescope be rightly pointed to the heavens at one moment, by the next it will have been98 turned aside. To you who are using the telescope, the appearance produced is as if the heavenly bodies were themselves moving. We can counteract this inconvenience. The telescope is supported on a pedestal, which is built on masonry, that goes down through the floor to its foundation on the solid rock beneath. In the iron casing at the top of the pedestal you will see a little window, and inside is clockwork driven by a heavy weight. This clockwork turns the whole telescope round in the opposite direction to that in which the earth is moving. The consequence is that the telescope remains constantly pointed to the same part of the heavens.

Fig. 38.—The Yerkes Telescope, University of Chicago.

This instrument is no doubt a large one, but of late years many much greater have been built. The most powerful telescope that has ever been erected is the great Yerkes instrument belonging to the University of Chicago, of which a picture is shown in Fig. 38. The object glass is 40 inches across.

HOW THE TELESCOPE AIDS US IN VIEWING THE MOON.

Those who are in charge of an observatory are often visited by persons who, coming to see the wonders of the heavens, and finding instruments of such great proportions, not unnaturally expect the views they are to obtain of the celestial bodies shall be of corresponding magnificence. So they are, no doubt, but then it frequently happens that the pictures which even the greatest telescope can display will fall far short of the ideal pictures which the visitors have conjured up in their100 own imaginations, so that they are often sadly disappointed. Especially is this true with regard to the moon. I have seen people who, when they had a view of the moon through a great telescope, were surprised not to find vast ranges of mountains which looked to them as big as the Alps, or mighty deserts, over which the eye could roam for thousands of miles. They have sometimes expected to behold stupendous volcanoes that not only were, but that looked to be as big as Vesuvius. Others seem to have thought they ought to see the moon with such clearness that the fields were to be quite visible, and some would not have been much astonished if they had observed houses and farmyards, and, perhaps, even cocks and hens.

There are different ways of estimating the apparent dimensions of an object, but the size the moon appears to me to have in a great telescope may be illustrated by taking an orange in your hand and looking at the innumerable little marks and spots on its surface. The amount of detail that the eye will show on the orange is about equal to the amount of detail that a good telescope will show on the moon. A desert on the moon, which really is a hundred miles across, will then correspond to a mark about an eighth of an inch in diameter on the orange. Some of you may ask what is gained by the use of a telescope, for the moon looks to us as large as a plate with the unaided eye, and now we hear it only looks as big as an orange in the telescope. But where is the plate with which you compare your moon supposed to be held? It is surely not in your hand. It is imagined to be up in the sky, a very long101 way off. Though an orange is much smaller than a plate, yet you will be able to see many more details in the orange by taking it in your hand than you could see on a plate which was at the other side of the street.

Fig. 39.—The Advantage of using a Telescope.

I sometimes find that people will not believe how much the telescope that they are using is magnifying the moon until they use both eyes together, of which one is applied to the telescope, while the other is directed to the moon. It will then be seen, even with a very small instrument, that the telescopic moon is as big as the larger of the two crescents in the adjoining figure (Fig. 39), while the naked-eye moon is like the smaller.

102The greatest telescopes are capable of reducing the apparent distance of an object to about one-thousandth part of its actual amount. If, therefore, a body were a thousand miles away, it would, when viewed by one of these mighty instruments, be seen as large as our unaided vision would show it, were the body only a single mile distant. No doubt this is a large accession to our power, but it often falls far short of what the astronomer would desire. The distances of the stars are all so great that even when divided by one thousand, they are still enormous. If you have a number expressed by 100,000,000,000,000, then dividing it by a thousand merely means taking off three of the ciphers, and there is still a large number left. We are, however, at present concerned with the moon, and, as its distance is about 240,000 miles, the effect of the best telescope is to reduce this distance apparently to 240 miles. Here, then, we find a limit to what the best of all telescopes can do. It can never show us the moon better than, hardly indeed so well as, we could see it with our unaided eye were it only 240 miles over our heads. We cannot expect the most powerful instruments to reveal any object on the moon unless that object were big enough to be seen by the unaided eye when 240 miles away. What could we expect to see at a distance of 240 miles?

Here is a little experiment which I made to study this point. I marked a round black dot on a sheet of white paper. The dot was a quarter of an inch in diameter, and then I fastened this on a door in the garden, and walked backwards until the dot ceased to be visible.103 I found this distance to be about thirty-six yards. I tried a little boy of eight years old, and it appeared that the dot became invisible to him about the same time as it did to me. “What has this to do with the moon?” you will say. Well, we shall soon see. In thirty-six yards there are 5184 quarters of an inch, and as it is unnecessary to be very particular about the figures, we may say, in round numbers, that the distance when we ceased to be able to distinguish the dot was about five thousand times as great as the width of the dot itself. You need not, therefore, expect to see anything on the moon or on anything else which is not at least as wide as the five-thousandth part of the distance from which we are viewing it. The great telescope practically places the moon at a distance of 240 miles, and the five-thousandth part of that is about eighty yards; consequently a round object on the moon about eighty yards in diameter would be just glimpsed as the merest dot in the most powerful telescope. To attract attention, a lunar object should be much larger than this. If St. Paul’s Cathedral stood on a lunar plain, it would be visible in our great telescopes. It is true that we could not see any details. We should not be able to distinguish between a Cathedral and a Town-hall. There would just be something visible, so that the artist who was making a sketch of that part would put down a mark with his pencil to show that something was there. This will show us that we need not expect to see objects on the moon, even with the mightiest of telescopes, unless they are of great size.

104

TELESCOPIC VIEWS OF LUNAR SCENERY.

I have already warned you not to expect too much, even with the biggest of telescopes; and just as a caution, I may, perhaps, tell you a story I once heard of an astronomer who had a great telescope. It was a very famous instrument, and people often came to the Observatory at night to enjoy a look at the heavens. Sometimes these visitors were grave philosophers, but frequently they were not very accomplished men of science. One evening such a visitor came to the Observatory, and sent in his name and an introduction to the astronomer, with a request that he might enter the temple of mystery. The astronomer courteously welcomed the stranger, and asked him what he specially desired to see.

“Oh!” said the visitor, “I have specially come to see the moon—that is the object I am particularly interested about.”

“But,” said the astronomer, “my dear sir, I would show you the moon with pleasure, if you were here at the proper time; but what brings you here now? Look up; the evening is fine. There are the stars shining brightly, but where is the moon? You see it is not up at present. In fact, it won’t rise till about half-past two to-morrow morning, and it is only nine o’clock now. Come back again in five or six hours, and you shall observe the moon with the great telescope.”

But the visitor evidently thought the astronomer was merely trying to get rid of him by a pretext. And105 he was equal to the occasion—he was not going to be put off in that way.

“Of course, the moon is not up,” he replied; “any one can see that, and that is the reason why I have come, for if the moon had been up, I could have seen it without your telescope at all!”

Although no explorer can ever reach our satellite, yet it is hardly an exaggeration to say that in some respects we know the geography of the moon a good deal better than we know the geography of the earth. Think of the continent of Africa. In that great country there are mighty tracts, there are vast lakes and ranges of mountains, of which we know but little. We could make a better map of Africa, so far at least as its broad outlines are concerned, if it were fastened up on our side of the moon than we actually possess at this moment. There is no spot on the nearer side of the moon as large as an ordinary parish in this country which has not been surveyed. There are maps and charts of the moon showing every part of it, which is as big as a good-sized field. Indeed, as there are no lunar clouds, the features of its surface are never obscured whenever our own atmosphere will permit us to make our observation. Artists have frequently sketched the lunar features, and there is plenty of material for them to work on. We have also had photographs taken of the moon, but there is a special difficulty to be encountered in taking photographs of celestial bodies which photographers of familiar objects on this earth do not experience. For a photograph to be successful, everybody knows that the first requisite106 is for the sitter to stay quiet while the plate is being exposed. This is, unhappily, just what the moon cannot do. We endeavor to obviate the difficulty by moving the telescope round so as to follow the moon in its progress. This can be done with considerable accuracy, but, unfortunately, there is another difficulty which lies entirely beyond our control. As the rays of light from the moon perform their journey through hundreds of miles of unsteady air, the rays are bent hither and thither, so that the picture is more affected by the atmosphere than in the case of a photographer’s portrait taken in the studio. If we are merely viewing the moon through the telescope, the quivering effect on the rays of this long atmospheric voyage, though rather inconvenient, does not prevent us from seeing the object, and we can readily detect the true shape of each feature in spite of incessant fluctuations. When, however, these rays fall not on the eye, but on the photographic plate, they produce by their motion a picture which cannot be much magnified without becoming very confused and wanting in sharpness. Nevertheless, for the general outlines of our satellite’s appearance and for the portraiture of its splendid features we have derived the greatest assistance from photography.

Fig. 40.—The Full Moon.

The adjoining picture (Fig. 40) gives a fair idea of what the full moon looks like when viewed through a small telescope. I do not, however, say that the lunar objects can then be observed under favorable conditions; for when the moon is full is the very worst time for making observations of our satellite. In fact,107 at this phase you can hardly see anything except slight differences between the colors of different parts. The best time for observing the moon is at the first quarter; but even then you can only observe satisfactorily those objects which happen to lie along the border between light and shade. To study the moon properly you must, therefore, watch it during several different phases, from the time when it presents a thin and delicate crescent (just after new moon) until it has again waned108 to a thin and delicate crescent (just before the next new moon). We want the relief given by shadows to bring out the full beauty of lunar scenery.

On the map you will first notice the large dark-colored patches which are so conspicuous on the moon’s face. They are, apparently, the empty basins which great seas once filled. But if water was ever there it has at all events now quite disappeared. These dark parts are, no doubt, a good deal smoother than the rest of the surface; but we can see many little irregularities which tell us that we are not looking at oceans. The chief features I want you to observe are the curious rings which you see in the figure; there is a very well-marked one a little below the centre, and in the upper part many rings—large and small—are crowded together. We call them lunar craters. You will see what they are like from the model, of which a picture is shown in Fig. 42. But to realize from this picture the proper scale of the object, you should imagine it to be some miles in width. The cliffs which rise all round to form the wall, as well as the mountain which adorns the centre, are quite as high as any of the mountains in Great Britain.

Fig. 41.—View on the Moon.

(By Lœwy and Puiseux, Paris Observatory.)

The large central crater is Hipparchus and above it is Albategnius.

Fig. 42.—Our Model of a Lunar Crater.

You may desire to know how we are able to measure the heights of mountains on the moon. That is what I am now going to show you; and for this purpose we shall look at our imitation lunar crater. Here is the great ring, or circular enclosure, surrounded by cliffs, and here is a sharp mountain peak rising in the centre. I shall ask to have the beam from the electric110 lamp turned on our model. You see how prettily it is lighted up. I have placed the lamp so that the beams are sloping; and I have done this with the express object of making the shadows long. In fact, as we look at a lunar crater, which lies on the border between light and shade, the sun illuminates the object under the same conditions as those shown in the figure. I dare say you have often noticed what long shadows are cast at sunset. Similar shadows are made to teach the astronomer the altitudes of the lunar mountains; for he measures the length of the shadow, and then by a little calculation he can find the height of the object by which that shadow has been cast. I shall suppose that we want to measure the height of a flagstaff (Fig. 43). It is quite possible to do this by merely measuring the length of the shadow which that flagstaff casts at noon. It would not be correct to say that the height of the flagstaff is the length of its shadow. This will, indeed, be the case if you are fortunate enough to make your measurement at or111 near London on either the 6th of April or the 5th of September. On all other days in the year a little calculation must be made, which I need not now mention, but which the astronomer, with the aid of his Nautical Almanac, can do in a very few minutes. In a similar manner, by measuring the lengths of the shadows on the moon, and by finding the number of miles in the shadow, we are able to calculate the altitudes of the lunar mountains and of the ranges of cliffs by which the walled plains are surrounded.

Fig. 43.—How we found the Height of the Flagstaff.ON THE ORIGIN OF THE LUNAR CRATERS.

We have now to offer an explanation of the curious rings which are the most characteristic features on the moon. To account for them we must look for a moment at some objects on the earth. You have all112 heard of volcanoes or burning mountains, such as Vesuvius or Etna, which occasionally break out into violent eruptions, and send forth great showers of ashes and torrents of molten lava. In the Sandwich Islands there is a celebrated volcano called Kilauea. It is like a vast lake of lava, so hot that it is actually molten, and glows with heat like red-hot iron. The adventurous tourist who visits this crater can climb to the brink of a lofty range of cliffs which surround it, and gaze down upon the fervid sea beneath. Suppose that by some great change the internal heat which keeps this mighty basin glowing were to decline and go out, the sea of lava would cease to be liquid, and would ultimately grow hard and cold, and we should then have an immense flat plain, surrounded by a range of cliffs. Elsewhere in the Sandwich Islands examples of extinct craters may be found at the present day. Those who have studied these interesting localities point out how such terrestrial craters explain the ringed plains in the moon. It seems certain that in ancient days great volcanoes abounded on our satellite, and the rings were often much larger than those on the Sandwich Islands, some of them being one hundred miles or more in diameter. The volcanoes must long ago have been raging on the moon with a fury altogether unknown in any active volcanoes which this earth can now show. We can also surmise how the lofty mountain peak, which so often rises in the centre of a lunar ring, has been upheaved. When the fires had almost subsided, and the floor had grown nearly cold, one last and expiring effort is made by113 which the congealing surface is burst through at the centre, and materials are shot forth which remain as the central mountain to the present day.

I must, however, impress upon you that even our greatest telescopes never exhibit to us any volcanic eruptions at present going on in the moon; in fact, it is most doubtful if any change has been noticed in the features on its surface since the date of the invention of the telescope. The volcanoes sculptured the crust of the moon into the form in which we see it, and that form our satellite has preserved for ages, of which we cannot estimate the duration. All the craters and all the volcanoes in the moon can only be described as extinct.

It would be interesting for us to compare the present condition of the volcanoes in the earth with that of the ringed craters in the moon. The noisy volcanoes on our globe are those most talked about; we often hear of Vesuvius being in eruption, and in August, 1883, there was a terrific eruption at Krakatoa, during which a large quantity of dust was shot up into the air, to such a height that it was borne right round the earth, and produced beautiful sunsets and unwonted sky hues in almost every country in the world. The explosion at Krakatoa made the loudest noise that history has recorded. Fortunately such convulsions of the earth do not often happen, for, on that occasion, the sea rushed in on the land, and thousands of lives were lost. There are said to be one hundred and fifty volcanoes on different parts of the earth, which are more or less active, but there are many others in which the fire has gone out,114 and which seem to be just as cold and just as extinct as any volcanoes in the moon. Even in our own islands there are abundant remains of ancient volcanoes. Masses of lava are found in many places where now there is no trace of an active volcano. Perhaps there is no more remarkable sight in the British Isles than that lofty rock which is crowned by Edinburgh Castle; it is the remnant of a former volcano, while Arthur’s Seat, close by, is another. In the centre of France is the beautiful district of Auvergne, in which ancient volcanoes abound; and the lava streams can be traced for miles across the country. These volcanoes have been extinct for thousands of years, during which time the lava has become largely covered with soil and vegetation, and in some places vineyards are cultivated upon it.

We are now able to contrast the earth with the moon, in so far as volcanoes are concerned. On the earth we have some that are active, and a much greater number that are extinct. On the moon we find no active volcanoes, for there all are extinct. I can explain how this difference has arisen, but first let me show you a simple experiment. My assistant will kindly bring to me from that furnace two iron balls, which we placed there before the commencement of this lecture; there they are, you see, both glowing with a bright red heat, for at present they are equally hot. We will place them on these stands, and allow them to grow cold. One of these balls is a small cannon-ball, four inches in diameter, while the other is only one inch. They are in the same proportion as the earth is to the moon; but look, even while I am speaking the balls115 have ceased to preserve the same temperature, for the little one has become almost black from loss of its heat, while the large one still looks nearly as red as it did at the beginning; this simple experiment will illustrate the principle that two heated bodies will cool at very different rates, if their sizes be different, while the other conditions are the same. The small body will always cool faster than the large one. They need not be globes for this experiment; if you put a poker and a knitting needle into the fire, and leave both there until they are red-hot, and then put them out into the fender, you will speedily find that though they were at the same temperature when drawn from the fire, they do not long remain so; indeed, the knitting needle has become cold enough to handle before the poker has ceased to glow. Our experiments have been made with, no doubt, small objects only, but the law about which they inform us will remain true, even for the greatest objects.

Our earth at the present day shows many indications of being much hotter within than it is on the surface. The volcanoes themselves are mere outbreaks of incandescent material from inside. Then there are hot springs of water at Bath, which gush out from the earth. There are geysers of hot water in Iceland and in the Yellowstone Park in America, and in other places. And there are other indications also, with which every miner is familiar. Wherever a deep pit is sunk into the earth, the rocks below are always found to be warmer than those on the surface, and the deeper the pit the greater is the heat that is encountered. Thus, from all over the world we obtain proofs of the116 present existence of internal heat. Great as is the earth, we must still apply the simple common-sense principles that we use in our everyday life here. Let me give an illustration. Suppose that a servant came into the room and placed a jug of water on the table, and that an hour afterwards you went to the jug of water and found it to be cold, you would not from that fact alone be able to infer anything with certainty, as to whether the water had been warm or cold when it was brought in. It might have been perfectly cold, as it is at present, though on the other hand the water might have been warm at first, and have since cooled down to the temperature of the room during the hour.

Suppose, however, that when you went to the jug of water, which had stood on the table for an hour, you found it tepid, no matter how slightly its temperature might be above that of the room, do you not see the inference you would be able to draw? You would argue in this way: that water has still some heat; it must, of course, be gradually cooling, and therefore it was hotter a minute ago than it is now; it was hotter still two minutes ago, or ten minutes; and must have been very hot and perhaps boiling when it was brought in an hour ago.

I want you to apply exactly the same reasoning to our earth. It is, as I have shown you, still hot and warm inside. Of course, that heat is gradually becoming lost; so that the earth will from year to year gradually cool down, though at an extremely slow rate. But we must look back into what has happened during past ages. Just as we inferred that the jug must have117 contained very hot water an hour before from the mere fact that the water was still warm, so we are entitled to infer, from the fact that the earth still retains some heat, that it must in ages gone by have been exceedingly hot. In fact, the further we look back, the hotter do we see the earth growing, until at last we are constrained to think of a period, in the excessively remote past, long ere life began to dawn on this earth, when even the surface of the earth was hot. Back further still we see the earth no longer covered with the hard, the dark, and the cold surface we now find; we are to think of it in these primitive times as a huge glowing mass, in which all the substances that now form the rocks were then incandescent, and even molten material.

There is good reason for knowing that in those early times the moon also was molten with heat; and thus our reasoning has led us to think of a period when there were two great red-hot globes—one of which had about four times the diameter of the other—starting on their career of gradually cooling down. Recall our little experiment with the two cooling globes of iron; imagine these globes to preserve their relative proportions, but that one of them was 8000 miles and the other 2000 miles across. Ages will, no doubt, elapse ere they part with their heat sufficiently to allow the surfaces to cool and to consolidate. We may, however, be sure that the small globe will cool the faster, that its outside will become hard sooner than will the surface of the large one, and long after the small globe has become cold to the centre, the large one may continue to retain some of its primeval heat. We can thus118 readily understand why all the volcanoes on the moon have ceased—their day is over. It is over because the moon, being so small, has grown so cold that it no longer sustains the internal fires which are necessary for volcanic outbreaks. Our earth, in consequence of its much greater size, has grown cold more slowly. It has no doubt lost the high temperature on the exterior, and its volcanic energy has probably abated from what it once was. But there is still sufficient power in the subterranean fires to awaken us occasionally by a Krakatoa, or to supply Vesuvius with sufficient materials and vigor for its more frequent outbursts. The argument shows us that the time will at last come when this earth shall have parted with so large a proportion of its heat that it will be no longer able to provide volcanic phenomena, and then we shall pass into the exhausted stage which the moon attained ages ago.

THE MOVEMENTS OF THE MOON.

Though the moon is going round and round the earth incessantly, yet it always manages to avoid affording us a view of what is on the other side. Our satellite always directs the same face towards the earth, and we may reasonably conjecture that the other side is covered, like the side we know, with rings and other traces of former volcanoes. In this respect the moon is quite a peculiar object. The other great celestial bodies, such as the sun or Jupiter, turn round on their axes, and show us now one side and then the other, with complete impartiality. The way in which the119 moon revolves may be illustrated by taking your watch and chain, and as you hold the chain at the centre making the watch revolve in a circular path. At every point of its path the ring of the watch is, of course, pointed to the centre where the chain is held. If you imagine your eye placed at the centre, to represent the earth, the movements of the watch would exemplify the way the moon turns round it.

One more point I must explain about the moon before we close this lecture. There is nothing more familiar than the fact that a heavy body will fall to the ground. Indeed, it hardly matters what the material of the body may be, for you see I have a small iron ball in one hand and I hold a cork in the other. I drop them at the same moment, and they reach the ground together. Perhaps you would have expected that the cork would have lagged behind the iron. I try the experiment again and again, and you can see no difference in the times of their falling, though I do not say this would be true if they were dropped from the top of the Monument. In general we may say that bodies let drop will fall sixteen feet in the first second. Even a bit of paper and a penny piece will fall through the same height in the same time if you can get over the difficulty of the resistance of the air. This is easily managed. Cut a small piece of tissue paper which will lie flat on the top of the penny, and hold the penny horizontal with the paper uppermost. Though there is nothing to fasten the paper to the penny, you will find that they fall together. If we could conduct the experiment of dropping the penny and the bit of paper in a vacuum, then,120 whether the paper was laid on the penny or placed in any other way, the two objects would reach the table at the same moment if released at the same moment at equal heights.

Wherever we go we find that bodies will always tend to fall in towards the centre of the earth; thus in New Zealand, at the opposite side of our globe from where we are now standing, bodies will fall up towards us, and this law of falling is obeyed at the top of a mountain as it is down here. No matter how high may be the ascent made in a balloon, a body released will fall towards the earth’s centre. Of course, we can only ascend some five or six miles high, even in the most buoyant of balloons; but we know that the attraction by which bodies are pulled downwards towards the earth extends far beyond this limit. If we could go ten, twenty, or fifty miles up, we should still find that the earth tried to pull us down. Nor, even if you could imagine an ascent made to the height of 1000 miles, would gravitation have ceased. A cork or an iron ball, or any other object dropped from the height of 1000 miles, would assuredly tumble down on the ground below.

Suppose that by some device we were able to soar aloft to a height of 4000 miles. I name that elevation because we should then be as high above the earth as the centre of the earth is below our feet. We should have doubled our distance from the centre of the earth, and the intensity of the gravitation would have decreased to one-quarter of what it is at the surface. A body which at the earth’s surface falls sixteen feet in a second121 would there fall only four feet in a second, and the apparent weight of any body would be so much reduced that it would seem to weigh only a quarter of what it weighs down here. Thus, the higher and higher we go, the less and less does gravity become; but it does not cease, even at a distance of millions of miles. Therefore you might say that as gravity tries to pull everything down, wherever it may be, why does it not pull down the moon? This is a difficulty which we must carefully consider. Supposing that the earth and the moon were simply held apart, both being at rest, and that then the moon were to be let go, it would no doubt drop down directly on the earth. The movement of the moon would, however, be very different if, instead of being merely let fall, it was thrown sideways. The effect of the earth’s pull upon the moon would then be shown in keeping the moon revolving around us instead of allowing it to fly away altogether, as it would have done had the earth not been there to attract it.

Fig. 44.—An Illustration to explain the Movement of the Moon

We can explain this by an illustration. On the top of a mountain I have placed a big cannon (Fig. 44). We fire off the cannon, and the bullet flies away in a curved path, with a gradual descent until it falls to the ground. I have made the mountain look hundreds of times larger than any mountain could possibly be; and now I want you to imagine a cannon far stronger and gunpowder more potent than any powder or cannon that has ever yet been manufactured. Fire off a bullet with a still greater charge than the last time, and now the path is a much longer one, but still the bullet curves down so as ultimately to fall on the earth. But make122 now one final shot with a charge sufficiently powerful, and away flies the bullet, following this time the curvature of the earth, for the earth’s attraction has the effect of bending the path of the bullet from a straight line into this circular form. By the time the bullet has travelled a quarter of the way round, it is no nearer to the earth than it was at first, nor has it parted with any of its original speed. Thus, notwithstanding its long journey, the bullet has practically just as much energy as when it first left the muzzle of the cannon. Away it will fly round another quarter of the earth, and still123 in the same condition it will accomplish the third and the fourth quarters, thus returning to the point from which it started. If we have cleared the cannon out of the way, the bullet will fly again over the mountain top without having lost any of its speed by its voyage round the earth. Therefore it will be in a condition to start again, and thus to revolve around the earth permanently. If, then, from the top of a mountain 240,000 miles high a great bullet 2000 miles in diameter had once been projected with the proper velocity, that bullet would continue forever to circle round and round the earth, and even though the mountain and the cannon disappeared, the motion would be preserved indefinitely. This illustration will, at all events, show how a continuous revolution of the moon round the earth can exist, notwithstanding that the earth is constantly pulling our satellite down towards its surface.

ON THE POSSIBILITY OF LIFE IN THE MOON.

Astronomers are often asked whether any animals can be living on the moon. No observations we can make with the telescope can answer that question directly. There are great plains to be seen on the moon, of course, but even if there were elephants tramping over those plains, our telescopes could not show them. Nor will our instruments pronounce at once whether plants or trees flourish on the moon. The mammoth trees of California are so big that a tunnel has been cut through the trunk of one large enough to give passage for a carriage and pair. Even if there124 were trees as big as this on the moon, they would not be visible from the most famous observatories.

Let us think what we should ourselves experience if we could in some marvellous manner be transferred from the earth to its satellite, and tried to explore that new and wonderful country. Alas, we should find it utterly impossible to live there for an hour, or even for a minute! Troops of difficulties would immediately beset us. The very first would be the want of air. Ponder for a moment on the invariable presence of air around our own globe. Even if you climb to the top of a high mountain, or if you take a lofty voyage in a balloon, you are all the time bathed in air. It is air which supports the balloon, just as a cork is buoyed up by water. In all circumstances, we must have air to breathe. In that air is oxygen gas, and we must have oxygen incessantly supplied to our lungs to reinvigorate our blood. We require, too, that this oxygen shall be diluted with a much larger amount of nitrogen gas, for our lungs and system of circulation are adapted for abode in that particular mixture of gases which we find here. The atmosphere becomes more and more rarefied the higher we ascend, and apparently terminates altogether some two or three hundred miles over our heads. Beyond the limits of the atmosphere it seems as if empty space would be met with all the way from the earth to the moon. We could not procure a single breath of air, and life would be, of course, impossible. Even at a height of three or four miles, respiration becomes difficult, and doubtless life could not possibly be sustained at a height of ten miles.

125It is therefore plain that for a voyage to the moon we should require an ample supply of air, or, at least, of life-giving oxygen, which in some way or other was to be inhaled during the progress of the journey. When at length 240,000 miles had been traversed, and we were about to land on the moon, we would first of all ascertain whether it was surrounded with a coating of air. Most of the globes through space are, so far as we can learn, covered and warmed with an enveloping atmosphere of some kind; but, unhappily, the poor moon has been left entirely, or almost entirely, without any such clothing. She is quite bare of atmosphere at all comparable in density or in volume to that which surrounds us, though possibly we do now and then perceive some traces of air, or of some kind of gas, in small quantities in the lunar valleys.

I am sure each intelligent boy or girl will want to know how we are able to tell all this. We have never been at the moon, and how then can we say that it is nearly destitute of air? Nor can our telescope answer this question immediately, for you could hardly expect to see air, even if it were there. How then are we able to make such assertions? There are many different ways in which we have learned the absence of air from the moon. I will tell you one of the easiest and the most certain of these methods. First let me say that air is not perfectly transparent. No doubt I can see you, and you can see me, though a good many feet of air may lie between us; but when we deal with distances much greater, there is a very simple way in which we can show that air is not quite transparent. In the126 evening, when the sun is setting and the sky is clear, you can look at him without discomfort; but in the middle of the day you know that it is impossible to look at the sun without shading your eyes with smoked glass or protecting them by some similar contrivance. The reason is, that when the sun is either setting or rising we look at it through an immense thickness of air, which not being perfectly transparent stops some of the light. Thus it is that the sun in these circumstances loses its dazzling brilliancy, and we can view it without discomfort.

At the seaside you can notice the same effect in a different manner. Go out on a fine and clear night, when the stars in their thousands are glittering overhead, and then look down gradually towards the horizon, and you will find the stars becoming fainter and fainter. Indeed, even the brightest star cannot be seen when it is at the horizon, because an immense thickness of the atmosphere is not transparent.

We can now state the argument by which we may prove that there is little or no air on our satellite. The moon will frequently pass between the earth and a star, and when the star is a really bright one the observations that can be made are of great interest. Let me first describe what we actually see. The star is shining brightly until the moment when the moon eclipses it. Generally speaking, its disappearance is instantaneous. But this would not be the case if the moon were encircled with an atmosphere. If the moon were coated with air, the light from the star would not be extinguished instantly; it would gradually decline, according127 as it had to pass through more and more of the moon’s atmosphere. Thus you would find that the star dwindled down in brightness before the solid body of the moon had advanced far enough to shut it out. The sudden extinction of the stars demonstrates the airless state of our satellite.

There would be another insuperable difficulty in adopting the moon as a residence, even supposing that you could get there. Water is absent from its surface. We have examined every part of it, and we find no evidences of seas or of oceans, of lakes or rivers; we never see anything like clouds or mists, which are, of course, only water in the vaporous form. We are, therefore, assured that, so far as water is concerned, the moon is an absolute desert. This is, perhaps, the most striking contrast between the aspect of the earth and the aspect of the moon. Were an astronomer on the moon to look at our earth he would find most of its surface concealed beneath clouds, and through the openings in these clouds he would see that by far the greater part of this globe was covered by the expanse of ocean; in fact, when the lunar astronomer had realized the prevalence of water upon this earth, either in the form of ocean or cloud, I feel sure he would come to the conclusion that nothing could live here except seals or other amphibious animals.

Owing to the absence of air and water, the moon would be totally disqualified for the support of life of those types in which we know it. For air and water are necessary to every animal, from the humblest animalcule up to whales or elephants. Air and water are128 necessary for every form of vegetable life, from the lichen which grows on a stone up to the noble old oak of the forest. But even supposing that we could land on the moon, bearing with us an ample supply of oxygen to breathe, and of water to drink, we should find ourselves perplexed and embarrassed, to say the very least of it, by an extraordinary difference that would immediately attract our notice. That familiar experience of gravity, or the weights of things, which we have acquired in our residence on a great globe like the earth, would seem ludicrously altered when we began to walk about on a little globe like the moon. We should be astonished at the transformation by which the weight of everything was much lessened; when you pulled out your watch you would hardly feel it at the end of the chain; it would seem like a mere shell; but yet the watch is all right, it is going as well as ever. Nothing has altered about it except its weight. A big stone attracts your notice, and, to your amazement, you find that it does not weigh so much as a piece of wood of the same size would weigh down here. A stone that you could hardly stir on the earth, you can carry about on the moon. Nor is this to be explained by any peculiarity in the constitution of the lunar stone. Most probably it will be not very dissimilar to some of the rocks on the earth. The relative lightness of a lunar stone is not due to its being formed of some very special material; we must seek for some other explanation. Every object on the moon would be found only one-sixth as heavy as the same object on the earth. A sturdy laborer at one of the docks can129 carry one sack of corn on his back here, and he finds that this load is as much as is convenient. He would, however, discover, were he placed on the moon, that his load had suddenly become lightened to one-sixth part (Fig. 45). The laborer would find that he could carry six sacks of corn on the moon without making a greater effort than the support of a single sack on the earth cost him. To explain how such a change as this has occurred, look at these two pictures: one shows the laborer on a small body like the moon, the other shows him on a great globe like the earth. What the laborer actually does feel is not quite so simple a thing as he imagines. He imagines that it is the weight of the corn, and the corn alone, which produces that pressure on his shoulders which he knows so well. But that is not exactly the manner in which the philosopher will look at the same question. What the laborer does actually feel is the attraction between the earth beneath130 his feet and the corn on his back. It is this force which produces the pressure on his shoulders. Its magnitude no doubt depends upon the quantity of corn in the sack, but it also depends on the quantity of matter on the earth beneath his feet. In fact, the force between two attracting bodies depends upon the masses of both the attracting bodies. When the laborer is transferred to the moon, of which the mass is so much less than that of the earth, the attraction is less there than it is here, even though the corn is the same in the two cases.

Fig. 45.—The Lessened Gravitation on the Moon.

Many odd instances could be given of the extraordinary consequences of life on a world where all weights are reduced to a sixth part. One occurred to me the other day when I saw a postman going his rounds with an amazing load of Christmas presents and parcels. I thought, how much happier must be the lot of a postman on the moon, if such functionaries are wanted there! All the presents of toys or more substantial donations might be the same as before, the only alteration would be that they would not feel nearly so heavy. A box which contains a pound of chocolate bonbons might still contain exactly the same quantity of sweetmeat on the moon, but the exertion of carrying it would be reduced to one-sixth. It would only weigh as much as two or three ounces do on the earth. Our streets provide another admirable illustration of the drawbacks of our life here as compared with the facilities offered by life on the moon. I feel quite confident that no perambulators can be necessary there. I cannot indeed say that there are babies to be found on the131 moon, but of this I am certain, that even if the lunar babies were as plump and as sturdy as ours, they must still only weigh about a sixth as much as ours do. A lunar nurse would scorn to use a perambulator, even for a pair of twins; she might take them both out on her arm for an airing, and even then only bear one-third of the load that her terrestrial sister must sustain if she is carrying but a single child.

The lightness of bodies in the moon would entirely transform many of our most familiar games. In cricket, for instance, I don’t think the bowling would be so much affected, but the hits on the moon would be truly terrific. I believe an exceptionally good throw of the cricket-ball here is about a hundred yards, but the same man, using the same ball and applying the same force to it, would send the ball six hundred yards on the moon. So, too, every hit would in the lunar game carry the ball to six times the distance it does here. Football would show a striking development in lunar play; a good kick would not only send the ball over the cross-bar, but it would go soaring over the houses, and perhaps drop in the next parish.

Our own bodies would, of course, participate in the general buoyancy, so that, while muscular power remained unabated, we should be almost able to run and jump as if we had on the famous seven-league boots. I have seen an athlete in a circus jump over ten horses placed side by side. The same athlete, making the same effort, would jump over sixty horses on the moon.

A run with a pack of lunar foxhounds would indeed be a marvellous spectacle. There need be no looking132 round by timid horsemen to find open roads or easy gaps. The five-barred gate itself would be utterly despised by a huntsman who could easily clear a hay-rick. It would hardly be worth taking a serious jump to clear a canal unless there was a road and a railway or so, which could be disposed of at the same time.

To illustrate this subject of gravitation in another way, suppose that we were to be transferred from this earth to some globe much greater than the earth—to a globe, for instance, as large and massive as the sun. We can then show that the weight of every object would be increased. Indeed, everything would weigh about twenty-seven times as much as we find it does here. To pull out your watch would be to hoist a weight of about five or six pounds out of your pocket. Indeed, I do not see how you could draw out your watch, for even to raise your arm would be impossible; it would feel heavier by far than if it were made of solid lead. It is, perhaps, conceivable that you might stand upright for a moment, particularly if you had a wall to lean up against; but of this I feel certain, that if you once got down on the ground, it would be utterly out of your power to rise again.

These illustrations will at least answer one purpose: they will show how difficult it is for us to form any opinion as to the presence or the absence of life on the other globes in space. We are just adapted in every way for a residence on this particular earth of a particular size and climate, and with atmosphere of a particular composition. Within certain slender limits our vital powers can become accommodated to change, but133 the conditions of other worlds seem to be so utterly different from those we find here, that it would probably be quite impossible for beings constituted as we are to remain alive for five minutes on any other globe in space.

It is, however, quite another question as to whether there may not be inhabitants of some kind on many of the other splendid globes. We have through the wide extent of space inconceivable myriads of worlds, presenting, no doubt, every variety of size and climate, of atmosphere and soil. It seems quite preposterous to imagine that among all these globes ours alone should be the abode of life. The most reasonable conclusion for us to come to is that these bodies may be endowed with life of types which are just as appropriate to the physical conditions around them as is the life, both animal and vegetable, on this globe to the special circumstances in which it is placed.

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This book is part of the public domain. Robert S. Ball (2019). Star-land: Being Talks With Young People About the Wonders of the Heavens. Urbana, Illinois: Project Gutenberg. Retrieved October 2022 https://www.gutenberg.org/cache/epub/60318/pg60318-images.html

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