Читать книгу Through Magic Glasses and Other Lectures - Arabella B. Buckley - Страница 9

Fig. 3. Chart of the moon.

Craters—

1 Tycho.	2 Copernicus.	3 Kepler.	4 Aristarchus.
5 Eratosthenes.	6 Archimedes.	7 Plato.	8 Eudoxus.
9 Aristotle.	10 Petavius.	11 Ptolemy.

Grey plains formerly believed to be seas—

A Mare Crisium.	O Mare Imbrium.
C —— Frigoris.	Q Oceanus Procellarum.
G —— Tranquillitatis.	X Mare Fœcunditatis.
H —— Serenitatis.	T —— Humorum.

By studying this map, and the pictures, they were able, even in their small telescopes, to recognise Tycho and Copernicus, and the mountains of the moon, after they had once grown accustomed to the strange changes in their appearance which take place as daylight or darkness creeps over them. They could not however pick out more than some of the chief points. Only the magician himself knew every crater and ridge under all its varying lights, and now, as he waited for the eclipse to begin, he turned to a lad who stood behind him, almost hidden in the dark shadow—the one fortunate boy who had earned the right to share this night's work.

Fig. 3a. The full moon. (From Ball's Starland.)

"We have still half an hour, Alwyn," said he, "before the eclipse will begin, and I can show you the moon's face well to-night. Take my place here and look at her while I point out the chief features. See first, there are the grey plains (A, C, G, etc.) lying chiefly in the lower half of the moon. You can often see these on a clear night with the naked eye, but you must remember that then they appear more in the upper part, because in the telescope we see the moon's face inverted or upside down.

"These plains were once thought to be oceans, but are now proved to be dry flat regions situated at different levels on the moon, and much like what deserts and prairies would appear on our earth if seen from the same distance. Looking through the telescope, is it not difficult to imagine how people could ever have pictured them as a man's face? But not so difficult to understand how some ancient nations thought the moon was a kind of mirror, in which our earth was reflected as in a looking-glass, with its seas and rivers, mountains and valleys; for it does look something like a distant earth, and as the light upon it is really reflected from the sun it was very natural to compare it to a looking-glass.

"Next cast your eye over the hundreds of craters, some large, others quite small, which cover the moon's face with pitted marks, like a man with small-pox; while a few of the larger rings look like holes made in a window-pane, where a stone has passed through, for brilliant shining streaks radiate from them on all sides like the rays of a star, covering a large part of the moon. Brightest of all these starred craters is Tycho, which you will easily find near the top of the moon (I, Fig. 3), for you have often seen it in the small telescope. How grand it looks to-night in the full moon (Fig. 3a)! It is true you see all the craters better when the moon is in her quarters, because the light falls sideways upon them and the shadows are more sharply defined; yet even at the full the bright ray of light on Tycho's rim marks out the huge cavity, and you can even see faintly the magnificent terraces which run round the cup within, one below the other."

Fig. 4. Tycho and his surroundings. (From a photograph of the moon taken by Mr. De la Rue, 1863.)

"This cavity measures fifty-four miles across, so that if it could be moved down to our earth it would cover by far the largest part of Devonshire, or that portion from Bideford on the north, to the sea on the south, and from the borders of Cornwall on the east, to Exeter on the west, and it is 17,000 feet or nearly three miles in depth. Even in the brilliant light of the full moon this enormous cup is dark compared to the bright rim, but it is much better seen in about the middle of the second quarter, when the rising sun begins to light up one side while the other is in black night. The drawing on the wall (Fig. 4), which is taken from an actual photograph of the moon's face, shows Tycho at this time surrounded by the numerous other craters which cover this part of the moon. You may recognise him by the gleaming peak in the centre of the cup, and by his bright rim which is so much more perfect than those of his companions. The gleaming peak is the top of a steep cone or hill rising up 6000 feet, or more than a mile from the base of the crater, so that even the summit is about two miles below the rim.

"There is one very interesting point in Tycho, however, which is seen at its very best at full moon. Look outside the bright rim and you will see that from the shadow which surrounds it there spring on all sides those strange brilliant streaks (see Fig. 3a) which I spoke of just now. There are others quite as bright, or even brighter, round other craters, Copernicus (Fig. 6), Kepler, and Aristarchus, lower down on the right-hand side of the moon; but these of Tycho are far the most widely spread, covering almost all the top of the face.

"What are these streaks? We do not know. During the second quarter of the moon, when the sun is rising slowly upon Tycho, lighting up his peak and showing the crater beautifully divided into a bright cup in the curve to the right, while a dense shadow lies in the left hollow, these streaks are only faint, and among the many craters around (see Fig. 4) you might even have some difficulty at first in finding the well-known giant. But as the sun rises higher and higher they begin to appear, and go on increasing in brightness till they shine with that wonderfully silvery light you see now in the full moon."

Fig. 5. Plan of the Peak of Teneriffe, showing how it resembles a lunar crater. (A. Geikie.)

"Here is a problem for you young astronomers to solve, as we learn more and more how to use the telescope with all its new appliances."

The crater itself is not so difficult to explain, for we have many like it on our earth, only not nearly so large. In fact, we might almost say that our earthly volcanoes differ from those in the moon only by their smaller size and by forming mountains with the crater or cup on the top; while the lunar craters lie flat on the surface of the moon, the hollow of the cup forming a depression below it. The peak of Teneriffe (Fig. 5), which is a dormant volcano, is a good copy in miniature on our earth of many craters on the moon. The large plain surrounded by a high rocky wall, broken in places by lava streams, the smaller craters nestling in the cup, and the high peak or central crater rising up far above the others, are so like what we see on the moon that we cannot doubt that the same causes have been at work in both cases, even though the space enclosed in the rocky wall of Teneriffe measures only eight miles across, while that of Tycho measures fifty-four.

"But of the streaks we have no satisfactory explanation. They pass alike over plain and valley and mountain, cutting even across other craters without swerving from their course. The astronomer Nasmyth thought they were the remains of cracks made when the volcanoes were active, and filled with molten lava from below, as water oozes up through ice-cracks on a pond. But this explanation is not quite satisfactory, for the lava, forcing its way through, would cool in ridges which ought to cast a shadow in sunlight. These streaks, however, not only cast no shadow, as you can see at the full moon but when the sun shines sideways upon them in the new or waning moon they disappear as we have seen altogether. Thus the streaks, so brilliant at full moon in Tycho, Copernicus, Kepler, and Aristarchus, remain a puzzle to astronomers still."

Fig. 6. The crater Copernicus. (As given in Herschel's Astronomy, from a drawing taken in a reflecting telescope of 20 feet focal length.)

"We cannot examine these three last-named craters well to-night with the full sun upon them; but mark their positions well, for Copernicus, at least, you must examine on the first opportunity, when the sun is rising upon it in the moon's second quarter. It is larger even than Tycho, measuring fifty-six miles across, and has a hill in the centre with many peaks; while outside, great spurs or ridges stretch in all directions sometimes for more than a hundred miles, and between these are scattered innumerable minute craters. But the most striking feature in it is the ring, which is composed inside the crater of magnificent terraces divided by deep ravines. These terraces are in some ways very like those of the great crater of Teneriffe, and astronomers can best account for them by supposing that this immense crater was once filled with a lake of molten lava rising, cooling at the edges, and then falling again, leaving the solid ridge behind. The streaks are also beautifully shown in Copernicus (see Fig. 6), but, as in Tycho, they fade away as the sun sets on the crater, and only reappear gradually as midday approaches.

"And now, looking a little to the left of Copernicus, you will see that grand range of mountains, the Lunar Apennines (Fig. 7), which stretches 400 miles across the face of the moon. Other mountain ranges we could find, but none so like mountains on our own globe as these, with their gentle sunny slope down to a plain on the left, and steep perpendicular cliffs on the right. The highest peak in this range, called Huyghens, rises to the height of 21,000 feet, higher than Chimborazo in the Andes. Other mountains on the moon, such as those called the Caucasus, south of the Apennines, are composed of disconnected peaks, while others again stand as solitary pyramids upon the plains."

Fig. 7. The Lunar Apennines. (Copied by kind permission of MM. Henri from part of a magnificent photograph taken by them, March 29, 1890, at the Paris Observatory.)

"But we must hasten on, for I want you to observe those huge walled crater-plains which have no hill in the middle, but smooth steel-grey centres shining like mirrors in the moonlight. One of these, called Archimedes, you will find just below the Lunar Apennines (Figs. 3 and 7), and another called Plato, which is sixty miles broad, is still lower down the moon's face (Figs. 3 and 8). The centres of these broad circles are curiously smooth and shining like quicksilver, with minute dots here and there which are miniature craters, while the walls are rugged and crowned with turret-shaped peaks."

Fig. 8. The crater Plato as seen soon after sunrise. (After Neison.)

"It is easy to picture to oneself how these may once have been vast seas of lava, not surging as in Copernicus, and heaving up as it cooled into one great central cone, but seething as molten lead does in a crucible, little bubbles bursting here and there into minute craters; and this is the explanation given of them by astronomers.

"And now that you have seen the curious rugged face of the moon and its craters and mountains, you will want to know how all this has come about. We can only form theories on the point, except that everything shows that heat and volcanoes have in some way done the work, though no one has ever yet clearly proved that volcanic eruptions have taken place in our time. We must look back to ages long gone by for those mighty volcanic eruptions which hurled out stones and ashes from the great crater of Tycho, and formed the vast seas of lava in Copernicus and Plato.

"And when these were over, and the globe was cooling down rapidly, so that mountain ranges were formed by the wrinkling and rending of the surface, was there then any life on the moon? Who can tell? Our magic glasses can reveal what now is, so far as distance will allow; but what has been, except where the rugged traces remain, we shall probably never know. What we now see is a dead worn-out planet, on which we cannot certainly trace any activity except that of heat in the past. That there is no life there now, at any rate of the kind on our own earth, we are almost certain; first, because we can nowhere find traces of water, clouds, nor even mist, and without moisture no life like ours is possible; and secondly, because even if there is, as perhaps there may be, a thin ocean of gas round the moon there is certainly no atmosphere such as surrounds our globe.

"One fact which proves this is, that there are no half-shadows on the moon. If you look some night at the mountains and craters during her first and second quarters, you will be startled to see what heavy shadows they cast, not with faint edges dying away into light, but sharp and hard (see Figs. 6-8), so that you pass, as it were by one step, from shadow to sunshine. This in itself is enough to show that there is no air to scatter the sunlight and spread it into the edges of the shade as happens on our earth; but there are other and better proofs. One of these is, that during an eclipse of the sun there is no reflection of his light round the dark moon as there would be if the moon had an atmosphere; another is that the spectroscope, that wonderful instrument which shows us invisible gases, gives no hint of air around the moon; and another is the sudden disappearance or occultation of a star behind the moon, such as I hope to see in a few minutes.

"See here! take the small hand telescope and turn it on to the moon's face while I take my place at the large one, and I will tell you what to look for. You know that at sunset we see the sun for some time after it has dipped below the horizon, because the rays of light which come from it are bent in our atmosphere and brought to our eyes, forming in them the image of the sun which is already gone. Now in a short time the moon which we are watching will be darkened by our earth coming between it and the sun, and while it is quite dark it will pass over a little bright star. In fact to us the star will appear to set behind the dark moon as the sun sets below the horizon, and if the moon had an atmosphere like ours, the rays from the star would be bent in it and reach our eyes after the star was gone, so that it would only disappear gradually. Astronomers have always observed, however, that the star is lost to sight quite suddenly, showing that there is no ocean of air round the moon to bend the light-rays."

Fig. 9. Diagram of total eclipse of the moon.

S, Sun. E, Earth. M, Moon passing into the earth's shadow and passing out at M´.

R, R´, Lines meeting at a point U, U´ behind the earth and enclosing a space within which all the direct rays of the sun are intercepted by the earth, causing a black darkness or umbra.

R, P and R´, P´, Lines marking a space within which, behind the earth, part of the sun's rays are cut off, causing a half-shadow or penumbra, P, P´.

a, a, Points where a few of the sun's rays are bent or refracted in the earth's atmosphere, so that they pass along the path marked by the dotted lines and shed a lurid light on the sun's face.

Here the magician paused, for a slight dimness on the lower right-hand side of the moon warned him that she was entering into the penumbra or half-shadow (see Fig. 9) caused by the earth cutting off part of the sun's rays; and soon a deep black shadow creeping over Aristarchus and Plato showed that she was passing into that darker space or umbra where the body of the earth is completely between her and the sun and cuts off all his rays. All, did I say? No! not all. For now was seen a beautiful sight, which would prove to any one who saw our earth from a great distance that it has a deep ocean of air round it.

It was a clear night, with a cloudless sky, and as the deep shadow crept slowly over the moon's face, covering the Lunar Apennines and Copernicus, and stealing gradually across the brilliant streaks of Tycho till the crater itself was swallowed up in darkness, a strange lurid light began to appear. The part of the moon which was eclipsed was not wholly dark, but tinted with a very faint bluish-green light, which changed almost imperceptibly, as the eclipse went on, to rose-red, and then to a fiery copper-coloured glow as the moon crept entirely into the shadow and became all dark. The lad watching through his small telescope noted this weird light, and wondered, as he saw the outlines of the Apennines and of several craters dimly visible by it, though the moon was totally eclipsed. He noted, but was silent. He would not disturb the Principal, for the important moment was at hand, as this dark copper-coloured moon, now almost invisible, drew near to the star over which it was to pass.

This little star, really a glorious sun billions of miles away behind the moon, was perhaps the centre of another system of worlds as unknown to us as we to them, and the fact of our tiny moon crossing between it and our earth would matter as little as if a grain of sand was blown across the heavens. Yet to the watchers it was a great matter—would the star give any further clue to the question of an atmosphere round the moon? Would its light linger even for a moment, like the light of the setting sun? Nearer and nearer came the dark moon; the star shone brilliantly against its darkness; one second and it was gone. The long looked-for moment had passed, and the magician turned from his instrument with a sigh. "I have learnt nothing new, Alwyn," said he, "but at least it is satisfactory to have seen for ourselves the proof that there is no perceptible atmosphere round the moon. We need wait no longer, for before the star reappears on the other side the eclipse will be passing away."

"But, master," burst forth the lad, now the silence was broken, "tell me why did that strange light of many tints shine upon the dark moon?"

"Did you notice it, Alwyn?" said the Principal, with a pleased smile. "Then our evening's work is not lost, for you have made a real observation for yourself. That light was caused by the few rays of the sun which grazed the edge of our earth passing through the ocean of air round it (see Fig. 9). There they were refracted or bent, and so were thrown within the shadow cast by our earth, and fell upon the moon. If there were such a person as a 'man in the moon,' that lurid light would prove to him that our earth has an atmosphere. The cause of the tints is the same which gives us our sunset colours, because as the different coloured waves which make white light are absorbed one by one, passing through the denser atmosphere, the blue are cut off first, then the green, then the yellow, till only the orange and red rays reached the centre of the shadow, where the moon was darkest. But this is too difficult a subject to begin at midnight."

So saying, he lighted his lamp, and covering the object-glass of his telescope with its pasteboard cap, detached the instrument from the clockwork, and the master and his pupil went down the turret stairs and past through the room below. As they did so they heard in the distance a scuffling noise like that of rats in the wall. A smile passed over the face of the Principal, for he knew that his young pupils, who had been making their observations in the gallery above, were hurrying back to their beds.