Star-land: Being Talks With Young People About the Wonders of the Heavens

The Phases of our Attendant, the Moon—The Size of the Moon—How Eclipses are produced—Effect of the Moon’s Distance on its Appearance—A Talk about Telescopes—How the Telescope aids us in Viewing the Moon—Telescopic Views of the Lunar Scenery—On the Origin of the Lunar Craters—The Movements of the Moon—On the Possibility of Life in 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.

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.

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.

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 remember 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 quite 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.

Another plum-pudding is to be constructed which 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 is 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 three 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 talks 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. 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.

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-hand 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 interesting 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 sun 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 marked 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 only 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.

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. 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 its 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 binocular than we should see it were it 80,000 miles away, and were we viewing it with the unaided eye.

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.

This 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 required 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 been 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.

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 their 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 long 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.

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.

The 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. 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.

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. And 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 requisite 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.