Читать книгу The Story of the Heavens - Robert S. Ball - Страница 12

Table of Contents

The Moon and the Tides—The Use of the Moon in Navigation—The Changes of the Moon—The Moon and the Poets—Whence the Light of the Moon?—Sizes of the Earth and the Moon—Weight of the Moon—Changes in Apparent Size—Variations in its Distance—Influence of the Earth on the Moon—The Path of the Moon—Explanation of the Moon's Phases—Lunar Eclipses—Eclipses of the Sun, how produced—Visibility of the Moon in a Total Eclipse—How Eclipses are Predicted—Uses of the Moon in finding Longitude—The Moon not connected with the Weather—Topography of the Moon—Nasmyth's Drawing of Triesnecker—Volcanoes on the Moon—Normal Lunar Crater—Plato—The Shadows of Lunar Mountains—The Micrometer—Lunar Heights—Former Activity on the Moon—Nasmyth's View of the Formation of Craters—Gravitation on the Moon—Varied Sizes of the Lunar Craters—Other Features of the Moon—Is there Life on the Moon?—Absence of Water and of Air—Dr. Stoney's Theory—Explanation of the Rugged Character of Lunar Scenery—Possibility of Life on Distant Bodies in Space.

If the moon were suddenly struck out of existence, we should be immediately apprised of the fact by a wail from every seaport in the kingdom. From London and from Liverpool we should hear the same story—the rise and fall of the tide had almost ceased. The ships in dock could not get out; the ships outside could not get in; and the maritime commerce of the world would be thrown into dire confusion.

The moon is the principal agent in causing the daily ebb and flow of the tide, and this is the most important work which our satellite has to do. The fleets of fishing boats around the coasts time their daily movements by the tide, and are largely indebted to the moon for bringing them in and out of harbour. Experienced sailors assure us that the tides are of the utmost service to navigation. The question as to how the moon causes the tides is postponed to a future chapter, in which we shall also sketch the marvellous part which the tides seem to have played in the early history of our earth.

Who is there that has not watched, with admiration, the beautiful series of changes through which the moon passes every month? We first see her as an exquisite crescent of pale light in the western sky after sunset. If the night is fine, the rest of the moon is visible inside the crescent, being faintly illumined by light reflected from our own earth. Night after night she moves further and further to the east, until she becomes full, and rises about the same time that the sun sets. From the time of the full the disc of light begins to diminish until the last quarter is reached. Then it is that the moon is seen high in the heavens in the morning. As the days pass by, the crescent shape is again assumed. The crescent wanes thinner and thinner as the satellite draws closer to the sun. Finally she becomes lost in the overpowering light of the sun, again to emerge as the new moon, and again to go through the same cycle of changes.

The brilliance of the moon arises solely from the light of the sun, which falls on the not self-luminous substance of the moon. Out of the vast flood of light which the sun pours forth with such prodigality into space the dark body of the moon intercepts a little, and of that little it reflects a small fraction to illuminate the earth. The moon sheds so much light, and seems so bright, that it is often difficult at night to remember that the moon has no light except what falls on it from the sun. Nevertheless, the actual surface of the brightest full moon is perhaps not much brighter than the streets of London on a clear sunshiny day. A very simple observation will suffice to show that the moon's light is only sunlight. Look some morning at the moon in daylight, and compare the moon with the clouds. The brightness of the moon and of the clouds are directly comparable, and then it can be readily comprehended how the sun which illuminates the clouds has also illumined the moon. An attempt has been made to form a comparative estimate of the brightness of the sun and the full moon. If 600,000 full moons were shining at once, their collective brilliancy would equal that of the sun.

The beautiful crescent moon has furnished a theme for many a poet. Indeed, if we may venture to say so, it would seem that some poets have forgotten that the moon is not to be seen every night. A poetical description of evening is almost certain to be associated with the appearance of the moon in some phase or other. We may cite one notable instance in which a poet, describing an historical event, has enshrined in exquisite verse a statement which cannot be correct. Every child who speaks our language has been taught that the burial of Sir John Moore took place

"By the struggling moonbeams' misty light."

There is an appearance of detail in this statement which wears the garb of truth. We are not inclined to doubt that the night was misty, nor as to whether the moonbeams had to struggle into visibility; the question at issue is a much more fundamental one. We do not know who was the first to raise the point as to whether any moon shone on that memorable event at all or not; but the question having been raised, the Nautical Almanac immediately supplies an answer. From it we learn in language, whose truthfulness constitutes its only claim to be poetry, that the moon was new at one o'clock in the morning of the day of the battle of Corunna (16th January, 1809). The ballad evidently implies that the funeral took place on the night following the battle. We are therefore assured that the moon can hardly have been a day old when the hero was consigned to his grave. But the moon in such a case is practically invisible, and yields no appreciable moonbeams at all, misty or otherwise. Indeed, if the funeral took place at the "dead of night," as the poet asserts, then the moon must have been far below the horizon at the time.[6]

In alluding to this and similar instances, Mr. Nasmyth gives a word of advice to authors or to artists who desire to bring the moon on a scene without knowing as a matter of fact that our satellite was actually present. He recommends them to follow the example of Bottom in A Midsummer's Night's Dream, and consult "a calendar, a calendar! Look in the almanac; find out moonshine, find out moonshine!"

Fig. 23.—Comparative Sizes of the Earth and the Moon.

Among the countless host of celestial bodies—the sun, the moon, the planets, and the stars—our satellite enjoys one special claim on our attention. The moon is our nearest permanent neighbour. It is just possible that a comet may occasionally approach the earth more closely than the moon but with this exception the other celestial bodies are all many hundreds or thousands, or even many millions, of times further from us than the moon.

It is also to be observed that the moon is one of the smallest visible objects which the heavens contain. Every one of the thousands of stars that can be seen with the unaided eye is enormously larger than our satellite. The brilliance and apparent vast proportions of the moon arise from the fact that it is only 240,000 miles away, which is a distance almost immeasurably small when compared with the distances between the earth and the stars.

Fig. 23 exhibits the relative sizes of the earth and its attendant. The small globe shows the moon, while the larger globe represents the earth. When we measure the actual diameters of the two globes, we find that of the earth to be 7,918 miles and of the moon 2,160 miles, so that the diameter of the earth is nearly four times greater than the diameter of the moon. If the earth were cut into fifty pieces, all equally large, then one of these pieces rolled into a globe would equal the size of the moon. The superficial extent of the moon is equal to about one thirteenth part of the surface of the earth. The hemisphere our neighbour turns towards us exhibits an area equal to about one twenty-seventh part of the area of the earth. This, to speak approximately, is about double the actual extent of the continent of Europe. The average materials of the earth are, however, much heavier than those contained in the moon. It would take more than eighty globes, each as ponderous as the moon, to weigh down the earth.

Amid the changes which the moon presents to us, one obvious fact stands prominently forth. Whether our satellite be new or full, at first quarter or at last, whether it be high in the heavens or low near the horizon, whether it be in process of eclipse by the sun, or whether the sun himself is being eclipsed by the moon, the apparent size of the latter is nearly constant. We can express the matter numerically. A globe one foot in diameter, at a distance of 111 feet from the observer, would under ordinary circumstances be just sufficient to hide the disc of the moon; occasionally, however, the globe would have to be brought in to a distance of only 103 feet, or occasionally it might have to be moved out to so much as 118 feet, if the moon is to be exactly hidden. It is unusual for the moon to approach either of its extreme limits of position, so that the distance from the eye at which the globe must be situated so as to exactly cover the moon is usually more than 105 feet, and less than 117 feet. These fluctuations in the apparent size of our satellite are contained within such narrow limits that in the first glance at the subject they may be overlooked. It will be easily seen that the apparent size of the moon must be connected with its real distance from the earth. Suppose, for the sake of illustration, that the moon were to recede into space, its size would seem to dwindle, and long ere it had reached the distance of even the very nearest of the other celestial bodies it would have shrunk into insignificance. On the other hand, if the moon were to come nearer to the earth, its apparent size would gradually increase until, when close to our globe, it would seem like a mighty continent stretching over the sky. We find that the apparent size of the moon is nearly constant, and hence we infer that the average distance of the same body is also nearly constant. The average value of that distance is 239,000 miles. In rare circumstances it may approach to a distance but little more than 221,000 miles, or recede to a distance hardly less than 253,000 miles, but the ordinary fluctuations do not exceed more than about 13,000 miles on either side of its mean value.

From the moon's incessant changes we perceive that she is in constant motion, and we now further see that whatever these movements may be, the earth and the moon must at present remain at nearly the same distance apart. If we further add that the path pursued by the moon around the heavens lies nearly in a plane, then we are forced to the conclusion that our satellite must be revolving in a nearly circular path around the earth at the centre. It can, indeed, be shown that the constant distance of the two bodies involves as a necessary condition the revolution of the moon around the earth. The attraction between the moon and the earth tends to bring the two bodies together. The only way by which such a catastrophe can be permanently avoided is by making the satellite move as we actually find it to do. The attraction between the earth and the moon still exists, but its effect is not then shown in bringing the moon in towards the earth. The attraction has now to exert its whole power in restraining the moon in its circular path; were the attraction to cease, the moon would start off in a straight line, and recede never to return.

Fig. 24.—The Moon's Path around the Sun.

The fact of the moon's revolution around the earth is easily demonstrated by observations of the stars. The rising and setting of our satellite is, of course, due to the rotation of the earth, and this apparent diurnal movement the moon possesses in common with the sun and with the stars. It will, however, be noticed that the moon is continually changing its place among the stars. Even in the course of a single night the displacement will be conspicuous to a careful observer without the aid of a telescope. The moon completes each revolution around the earth in a period of 27·3 days.

Fig. 25.—The Phases of the Moon.

In Fig. 24 we have a view of the relative positions of the earth, the sun, and the moon, but it is to be observed that, for the convenience of illustration, we have been obliged to represent the orbit of the moon on a much larger scale than it ought to be in comparison with the distance of the sun. That half of the moon which is turned towards the sun is brilliantly illuminated, and, according as we see more or less of that brilliant half, we say that the moon is more or less full, the several "phases" being visible in the succession shown by the numbers in Fig. 25. A beginner sometimes finds considerable difficulty in understanding how the light on the full moon at night can have been derived from the sun. "Is not," he will say, "the earth in the way? and must it not intercept the sunlight from every object on the other side of the earth to the sun?" A study of Fig. 24 will explain the difficulty. The plane in which the moon revolves does not coincide with the plane in which the earth revolves around the sun. The line in which the plane of the earth's motion is intersected by that of the moon divides the moon's path into two semicircles. We must imagine the moon's path to be tilted a little, so that the upper semicircle is somewhat above the plane of the paper, and the other semicircle below. It thus follows that when the moon is in the position marked full, under the circumstances shown in the figure, the moon will be just above the line joining the earth and the sun; the sunlight will thus pass over the earth to the moon, and the moon will be illuminated. At new moon, the moon will be under the line joining the earth and the sun.

As the relative positions of the earth and the sun are changing, it happens twice in each revolution that the sun comes into the position of the line of intersection of the two planes. If this occurs at the time of full moon, the earth lies directly between the moon and the sun; the moon is thus plunged into the shadow of the earth, the light from the sun is intercepted, and we say that the moon is eclipsed. The moon sometimes only partially enters the earth's shadow, in which case the eclipse is a partial one. When, on the other hand, the sun is situated on the line of intersection at the time of new moon, the moon lies directly between the earth and the sun, and the dark body of the moon will then cut off the sunlight from the earth, producing a solar eclipse. Usually only a part of the sun is thus obscured, forming the well-known partial eclipse; if, however, the moon pass centrally over the sun, then we must have one or other of two very remarkable kinds of eclipse. Sometimes the moon entirely blots out the sun, and thus is produced the sublime spectacle of a total eclipse, which tells us so much as to the nature of the sun, and to which we have already referred in the last chapter. Even when the moon is placed centrally over the sun, a thin rim of sunlight is occasionally seen round the margin of the moon. We then have what is known as an annular eclipse.

It is remarkable that the moon is sometimes able to hide the sun completely, while on other occasions it fails to do so. It happens that the average apparent size of the moon is nearly equal to the average apparent size of the sun, but, owing to the fluctuations in their distances, the actual apparent sizes of both bodies undergo certain changes. On certain occasions the apparent size of the moon is greater than that of the sun. In this case a central passage produces a total eclipse; but it may also happen that the apparent size of the sun exceeds that of the moon, in which case a central passage can only produce an annular eclipse.

Fig. 26.—Form of the Earth's Shadow, showing the Penumbra, or partially shaded region. Within the Penumbra, the Moon is visible; in the Shadow it is nearly invisible.

There are hardly any more interesting celestial phenomena than the different descriptions of eclipses. The almanac will always give timely notice of the occurrence, and the more striking features can be observed without a telescope. In an eclipse of the moon (Fig. 26) it is interesting to note the moment when the black shadow is first detected, to watch its gradual encroachment over the bright surface of the moon, to follow it, in case the eclipse is total, until there is only a thin crescent of moonlight left, and to watch the final extinction of that crescent when the whole moon is plunged into the shadow. But now a spectacle of great interest and beauty is often manifested; for though the moon is so hidden behind the earth that not a single direct ray of the sunlight could reach its surface, yet we often find that the moon remains visible, and, indeed, actually glows with a copper-coloured hue bright enough to permit several of the markings on the surface to be discerned.

This illumination of the moon even in the depth of a total eclipse is due to the sunbeams which have just grazed the edge of the earth. In doing so they have become bent by the refraction of the atmosphere, and have thus been turned inwards into the shadow. Such beams have passed through a prodigious thickness of the earth's atmosphere, and in this long journey through hundreds of miles of air they have become tinged with a ruddy or copper-like hue. Nor is this property of our atmosphere an unfamiliar one. The sun both at sunrise and at sunset glows with a light which is much more ruddy than the beams it dispenses at noonday. But at sunset or at sunrise the rays which reach our eyes have much more of our atmosphere to penetrate than they have at noon, and accordingly the atmosphere imparts to them that ruddy colour so characteristic and often so lovely. If the spectrum of the sun when close to the horizon is examined it is seen to be filled with numerous dark lines and bands situated chiefly towards the blue and violet end. These are caused by the increased absorption which the light suffers in the atmosphere, and give rise to the preponderating red light on the sun under such conditions. In the case of the eclipsed moon, the sunbeams have to take an atmospheric journey more than double as long as that at sunrise or sunset, and hence the ruddy glow of the eclipsed moon may be accounted for.

The almanacs give the full particulars of each eclipse that happens in the corresponding year. These predictions are reliable, because astronomers have been carefully observing the moon for ages, and have learned from these observations not only how the moon moves at present, but also how it will move for ages to come. The actual calculations are so complicated that we cannot here discuss them. There is, however, one leading principle about eclipses which is so simple that we must refer to it. The eclipses occurring this year have no very obvious relation to the eclipses that occurred last year, or to those that will occur next year. Yet, when we take a more extended view of the sequence of these phenomena, a very definite principle becomes manifest. If we observe all the eclipses in a period of eighteen years, or nineteen years, then we can predict, with at least an approximation to the truth, all the future eclipses for many years. It is only necessary to recollect that in 6,585-1⁄3 days after one eclipse a nearly similar eclipse follows. For instance, a beautiful eclipse of the moon occurred on the 5th of December, 1881. If we count back 6,585 days from that date, or, that is, eighteen years and eleven days, we come to November 24th, 1863, and a similar eclipse of the moon took place then. Again, there were four eclipses in the year 1881. If we add 6,585-1⁄3 days to the date of each eclipse, it will give the dates of all the four eclipses in the year 1899. It was this rule which enabled the ancient astronomers to predict the recurrence of eclipses, at a time when the motions of the moon were not understood nearly so well as they now are.

During a long voyage, and perhaps in critical circumstances, the moon will often render invaluable information to the sailor. To navigate a ship, suppose from Liverpool to China, the captain must frequently determine the precise position which his ship then occupies. If he could not do this, he would never find his way across the trackless ocean. Observations of the sun give him his latitude and tell him his local time, but the captain further requires to know the Greenwich time before he can place his finger at a point of the chart and say, "My ship is here." To ascertain the Greenwich time the ship carries a chronometer which has been carefully rated before starting, and, as a precaution, two or three chronometers are usually provided to guard against the risk of error. An unknown error of a minute in the chronometer might perhaps lead the vessel fifteen miles from its proper course.

PLATE VI. CHART OF THE MOON'S SURFACE.

Fig. 27.—Key to Chart of the Moon (Plate VI.).

It is important to have the means of testing the chronometers during the progress of the voyage; and it would be a great convenience if every captain, when he wished, could actually consult some infallible standard of Greenwich time. We want, in fact, a Greenwich clock which may be visible over the whole globe. There is such a clock; and, like any other clock, it has a face on which certain marks are made, and a hand which travels round that face. The great clock at Westminster shrinks into insignificance when compared with the mighty clock which the captain uses for setting his chronometer. The face of this stupendous dial is the face of the heavens. The numbers engraved on the face of a clock are replaced by the twinkling stars; while the hand which moves over the dial is the beautiful moon herself. When the captain desires to test his chronometer, he measures the distance of the moon from a neighbouring star. For example, he may see that the moon is three degrees from the star Regulus. In the Nautical Almanac he finds the Greenwich time at which the moon was three degrees from Regulus. Comparing this with the indications of the chronometer, he finds the required correction.

There is one widely-credited myth about the moon which must be regarded as devoid of foundation. The idea that our satellite and the weather bear some relation has no doubt been entertained by high authority, and appears to be an article in the belief of many an excellent mariner. Careful comparison between the state of the weather and the phases of the moon has, however, quite discredited the notion that any connection of the kind does really exist.

We often notice large blank spaces on maps of Africa and of Australia which indicate our ignorance of parts of the interior of those great continents. We can find no such blank spaces in the map of the moon. Astronomers know the surface of the moon better than geographers know the interior of Africa. Every spot on the face of the moon which is as large as an English parish has been mapped, and all the more important objects have been named.

A general map of the moon is shown in Plate VI. It has been based upon drawings made with small telescopes, and it gives an entire view of that side of our satellite which is presented towards us. The moon is shown as it appears in an astronomical telescope, which inverts everything, so that the south is at the top and the north at the bottom (to show objects upright a telescope requires an additional pair of lenses in the eye-piece, and as this diminishes the amount of light reaching the eye they are dispensed with in astronomical telescopes). We can see on the map some of the characteristic features of lunar scenery. Those dark regions so conspicuous in the ordinary full moon are easily recognised on the map. They were thought to be seas by astronomers before the days of telescopes, and indeed the name "Mare" is still retained, though it is obvious that they contain no water at present. The map also shows certain ridges or elevated portions, and when we apply measurement to these objects we learn that they must be mighty mountain ranges. But the most striking features on the moon are those ring-like objects which are scattered over the surface in profusion. These are known as the lunar craters.

To facilitate reference to the chief points of interest we have arranged an index map (Fig. 27) which will give a clue to the names of the several objects depicted upon the plate. The so-called seas are represented by capital letters; so that A is the Mare Crisium, and H the Oceanus Procellarum. The ranges of mountains are indicated by small letters; thus a on the index is the site of the so-called Caucasus mountains, and similarly the Apennines are denoted by c. The numerous craters are distinguished by numbers; for example, the feature on the map corresponding to 20 on the index is the crater designated Ptolemy.

A. Mare Crisium.

B. Mare Fœcunditatis.

C. Mare Tranquillitatis.

D. Mare Serenitatis.

E. Mare Imbrium.

F. Sinus Iridum.

G. Mare Vaporum.

H. Oceanus Procellarum.

I. Mare Humorum.

J. Mare Nubium.

K. Mare Nectaris.

a. Caucasus. b. Alps. c. Apennines. d. Carpathians. f. Cordilleras & D'Alembert mountains. g. Rook mountains. h. Dœrfel mountains. i. Leibnitz mountains. 1. Posidonius. 2. Linné. 3. Aristotle. 4. Great Valley of the Alps. 5. Aristillus. 6. Autolycus. 7. Archimedes. 8. Plato. 9. Eratosthenes. 10. Copernicus. 11. Kepler. 12. Aristarchus. 13. Grimaldi. 14. Gassendi. 15. Schickard. 16. Wargentin. 17. Clavius. 18. Tycho. 19. Alphonsus. 20. Ptolemy. 21. Catharina. 22. Cyrillus. 23. Theophilus. 24. Petavius. 25. Hyginus. 26. Triesnecker.

In every geographical atlas there is a map showing the two hemispheres of the earth, the eastern and the western. In the case of the moon we can only give a map of one hemisphere, for the simple reason that the moon always turns the same side towards us, and accordingly we never get a view of the other side. This is caused by the interesting circumstance that the moon takes exactly the same time to turn once round its own axis as it takes to go once round the earth. The rotation is, however, performed with uniform speed, while the moon does not move in its orbit with a perfectly uniform velocity (see Chapter IV.). The consequence is that we now get a slight glimpse round the east limb, and now a similar glimpse round the west limb, as if the moon were shaking its head very gently at us. But it is only an insignificant margin of the far side of the moon which this libration permits us to examine.

Lunar objects are well suited for observation when the sunlight falls upon them in such a manner as to exhibit strongly contrasted lights and shadows. It is impossible to observe the moon satisfactorily when it is full, for then no conspicuous shadows are cast. The most opportune moment for seeing any particular lunar object is when it lies just at the illuminated side of the boundary between light and shade, for then the features are brought out with exquisite distinctness.

Plate VII.[7] gives an illustration of lunar scenery, the object represented being known to astronomers by the name of Triesnecker. The district included is only a very small fraction of the entire surface of the moon, yet the actual area is very considerable, embracing as it does many hundreds of square miles. We see in it various ranges of lunar mountains, while the central object in the picture is one of those remarkable lunar craters which we meet with so frequently in every lunar landscape. This crater is about twenty miles in diameter, and it has a lofty mountain in the centre, the peak of which is just illuminated by the rising sun in that phase of our satellite which is represented in the picture.

A typical view of a lunar crater is shown in Plate VIII. This is, no doubt, a somewhat imaginary sketch. The point of view from which the artist is supposed to have taken the picture is one quite unattainable by terrestrial astronomers, yet there can be little doubt that it is a fair representation of objects on the moon. We should, however, recollect the scale on which it is drawn. The vast crater must be many miles across, and the mountain at its centre must be thousands of feet high. The telescope will, even at its best, only show the moon as well as we could see it with the unaided eye if it were 250 miles away instead of being 240,000. We must not, therefore, expect to see any details on the moon even with the finest telescopes, unless they were coarse enough to be visible at a distance of 250 miles. England from such a point of view would only show London as a coloured spot, in contrast with the general surface of the country.

We return, however, from a somewhat fancy sketch to a more prosaic examination of what the telescope does actually reveal. Plate IX. represents the large crater Plato, so well known to everyone who uses a telescope. The floor of this remarkable object is nearly flat, and the central mountain, so often seen in other craters, is entirely wanting. We describe it more fully in the general list of lunar objects.

The mountain peaks on the moon throw long, well-defined shadows, characterised by a sharpness which we do not find in the shadows of terrestrial objects. The difference between the two cases arises from the absence of air from the moon. Our atmosphere diffuses a certain amount of light, which mitigates the blackness of terrestrial shadows and tends to soften their outline. No such influences are at work on the moon, and the sharpness of the shadows is taken advantage of in our attempts to measure the heights of the lunar mountains.

It is often easy to compute the altitude of a church steeple, a lofty chimney, or any similar object, from the length of its shadow. The simplest and the most accurate process is to measure at noon the number of feet from the base of the object to the end of the shadow. The elevation of the sun at noon on the day in question can be obtained from the almanac, and then the height of the object follows by a simple calculation. Indeed, if the observations can be made either on the 6th of April or the 6th of September, at or near the latitude of London, then calculations would be unnecessary. The noonday length of the shadow on either of the dates named is equal to the altitude of the object. In summer the length of the noontide shadow is less than the altitude; in winter the length of the shadow exceeds the altitude. At sunrise or sunset the shadows are, of course, much longer than at noon, and it is shadows of this kind that we observe on the moon. The necessary measurements are made by that indispensable adjunct to the equatorial telescope known as the micrometer.

This word denotes an instrument for measuring small distances. In one sense the term is not a happy one. The objects to which the astronomer applies the micrometer are usually anything but small. They are generally of the most transcendent dimensions, far exceeding the moon or the sun, or even our whole system. Still, the name is not altogether inappropriate, for, vast though the objects may be, they generally seem minute, even in the telescope, on account of their great distance.

We require for such measurements an instrument capable of the greatest nicety. Here, again, we invoke the aid of the spider, to whose assistance in another department we have already referred. In the filar micrometer two spider lines are parallel, and one intersects them at right angles. One or both of the parallel lines can be moved by means of screws, the threads of which have been shaped by consummate workmanship. The distance through which the line has been moved is accurately indicated by noting the number of revolutions and parts of a revolution of the screw. Suppose the two lines be first brought into coincidence, and then separated until the apparent length of the shadow of the mountain on the moon is equal to the distance between the lines: we then know the number of revolutions of the micrometer screw which is equivalent to the length of the shadow. The number of miles on the moon which correspond to one revolution of the screw has been previously ascertained by other observations, and hence the length of the shadow can be determined. The elevation of the sun, as it would have appeared to an observer at this point of the moon, at the moment when the measures were being made, is also obtainable, and hence the actual elevation of the mountain can be calculated. By measurements of this kind the altitudes of other lunar objects, such, for example, as the height of the rampart surrounding a circular-walled plane, can be determined.

The beauty and interest of the moon as a telescopic object induces us to give to the student a somewhat detailed account of the more remarkable features which it presents. Most of the objects we are to describe can be effectively exhibited with very moderate telescopic power. It is, however, to be remembered that all of them cannot be well seen at one time. The region most distinctly shown is the boundary between light and darkness. The student will, therefore, select for observation such objects as may happen to lie near that boundary at the time when he is observing.

1. Posidonius.—The diameter of this large crater is nearly 60 miles. Although its surrounding wall is comparatively slender, it is so distinctly marked as to make the object very conspicuous. As so frequently happens in lunar volcanoes, the bottom of the crater is below the level of the surrounding plain, in the present instance to the extent of nearly 2,500 feet.

2. Linné.—This small crater lies in the Mare Serenitatis. About sixty years ago it was described as being about 6-1⁄2 miles in diameter, and seems to have been sufficiently conspicuous. In 1866 Schmidt, of Athens, announced that the crater had disappeared. Since then an exceedingly small shallow depression has been visible, but the whole object is now very inconsiderable. This seems to be the most clearly attested case of change in a lunar object. Apparently the walls of the crater have tumbled into the interior and partly filled it up, but many astronomers doubt that a change has really taken place, as Schröter, a Hanoverian observer at the end of the eighteenth century, appears not to have seen any conspicuous crater in the place, though it must be admitted that his observations are rather incomplete. To give some idea of Schmidt's amazing industry in lunar researches, it may be mentioned that in six years he made nearly 57,000 individual settings of his micrometer in the measurement of lunar altitudes. His great chart of the mountains in the moon is based on no less than 2,731 drawings and sketches, if those are counted twice that may have been used for two divisions of the map.

3. Aristotle.—This great philosopher's name has been attached to a grand crater 50 miles in diameter, the interior of which, although very hilly, shows no decidedly marked central cone. But the lofty wall of the crater, exceeding 10,500 feet in height, overshadows the floor so continuously that its features are never seen to advantage.

4. The Great Valley of the Alps.—A wonderfully straight valley, with a width ranging from 3-1⁄2 to 6 miles, runs right through the lunar Alps. It is, according to Mädler, at least 11,500 feet deep, and over 80 miles in length. A few low ridges which are parallel to the sides of the valley may possibly be the result of landslips.

5. Aristillus.—Under favourable conditions Lord Rosse's great telescope has shown the exterior of this magnificent crater to be scored with deep gullies radiating from its centre. Aristillus is about 34 miles wide and 10,000 feet in depth.

6. Autolycus is somewhat smaller than the foregoing, to which it forms a companion in accordance with what Mädler thought a well-defined relation amongst lunar craters, by which they frequently occurred in pairs, with the smaller one more usually to the south. Towards the edge this arrangement is generally rather apparent than real, and is merely a result of foreshortening.

7. Archimedes.—This large plain, about 50 miles in diameter, has its vast smooth interior divided by unequally bright streaks into seven distinct zones, running east and west. There is no central mountain or other obvious internal sign of former activity, but its irregular wall rises into abrupt towers, and is marked outside by decided terraces.