Astronomy of To-day Part 12

As she moves on in her orbit more and more of her illuminated surface is brought into view; and so the crescent of light becomes broader and broader, until we get what is called half-moon, or _first quarter_, when we see exactly one-half of her surface lit up by the sun's rays. As she draws still further round yet more of her illuminated surface is brought into view, until three-quarters of the disc appear lighted up. She is then said to be _gibbous_.

Eventually she moves round so that she faces the sun completely, and the whole of her disc appears illuminated. She is then spoken of as _full_. When in this position it is clear that she is on the contrary side of the earth to the sun, and therefore rises about the same time that he is setting. She is now, in fact, at her furthest from the sun.

[Ill.u.s.tration: Direction from which the sun's rays are coming.

Various positions and illumination of the mooon by the sun during her revolution around the earth.

The corresponding positions as viewed from the earth, showing the consequent phases.

FIG. 14.--Orbit and Phases of the Moon.]

After this, the motion of the moon in her orbit carries her on back again in the direction of the sun. She thus goes through her phases as before, only these of course are _in the reverse order_. The full phase is seen to give place to the gibbous, and this in turn to the half-moon and to the crescent; after which her motion carries her into the neighbourhood of the sun, and she is once more new, and lost to our sight in the solar glare. Following this she draws away to the east of the sun again, and the old order of phases repeat themselves as before.

The early Babylonians imagined that the moon had a bright and a dark side, and that her phases were caused by the bright side coming more and more into view during her movement around the sky. The Greeks, notably Aristotle, set to work to examine the question from a mathematical standpoint, and came to the conclusion that the crescent and other appearances were such as would necessarily result if the moon were a dark body of spherical shape illumined merely by the light of the sun.

Although the true explanation of the moon's phases has thus been known for centuries, it is unfortunately not unusual to see pictures--advertis.e.m.e.nt posters, for instance--in which stars appear _within_ the horns of a crescent moon! Can it be that there are to-day educated persons who believe that the moon is a thing which _grows_ to a certain size and then wastes away again; who, in fact, do not know that the entire body of the moon is there all the while?

When the moon shows a very thin crescent, we are able dimly to see her still dark portion standing out against the sky. This appearance is popularly known as the "old moon in the new moon's arms." The dark part of her surface must, indeed, be to some degree illumined, or we should not be able to see it at all. Whence then comes the light which illumines it, since it clearly cannot come from the sun? The riddle is easily solved, if we consider what kind of view of our earth an observer situated on this darkened part of the moon would at that moment get. He would, as a matter of fact, just then see nearly the whole disc of the earth brightly lit up by sunlight. The lunar landscape all around would, therefore, be bathed in what to _him_ would be "earthlight," which of course takes the place there of what _we_ call moonlight. If, then, we recollect how much greater in size the earth is than the moon, it should not surprise us that this earthlight will be many times brighter than moonlight. It is considered, indeed, to be some twenty times brighter.

It is thus not at all astonishing that we can see the dark portion of the moon illumined merely by sunlight reflected upon it from our earth.

The ancients were greatly exercised in their minds to account for this "earthlight," or "earthshine," as it is also called. Posidonius (135-51 B.C.) tried to explain it by supposing that the moon was partially transparent, and that some sunlight consequently filtered through from the other side. It was not, however, until the fifteenth century that the correct solution was arrived at.

[Ill.u.s.tration: One side of the moon only is ever presented to the earth. This side is here indicated by the letters S.F.E. (side facing earth).

By placing the above positions in a row, we can see at once that the moon makes one complete rotation on her axis in exactly the same time as she revolves around the earth.

FIG. 15.--The Rotation of the Moon on her Axis.]

Perhaps the most remarkable thing which one notices about the moon is that she always turns the same side towards us, and so we never see her other side. One might be led from this to jump to the conclusion that she does not rotate upon an axis, as do the other bodies which we see; but, paradoxical as it may appear, the fact that she turns one face always towards the earth, is actually a proof that she _does_ rotate upon an axis. The rotation, however, takes place with such slowness, that she turns round but once during the time in which she revolves around the earth (see Fig. 15). In order to understand the matter clearly, let the reader place an object in the centre of a room and walk around it once, _keeping his face turned towards it the whole time_, While he is doing this, it is evident that he will face every one of the four walls of the room in succession. Now in order to face each of the four walls of a room in succession one would be obliged _to turn oneself entirely round_. Therefore, during the act of walking round an object with his face turned directly towards it, a person at the same time turns his body once entirely round.

In the long, long past the moon must have turned round much faster than this. Her rate of rotation has no doubt been slowed down by the action of some force. It will be recollected how, in the course of the previous chapter, we found that the tides were tending, though exceedingly gradually, to slow down the rotation of the earth upon its axis. But, on account of the earth's much greater ma.s.s, the force of gravitation exercised by it upon the surface of the moon is, of course, much more powerful than that which the moon exercises upon the surface of the earth. The tendency to tidal action on the moon itself must, therefore, be much in excess of anything which we here experience. It is, in consequence, probable that such a tidal drag, extending over a very long period of time, has resulted in slowing down the moon's rotation to its present rate.

The fact that we never see but one side of the moon has given rise from time to time to fantastic speculations with regard to the other side.

Some, indeed, have wished to imagine that our satellite is shaped like an egg, the more pointed end being directed away from us. We are here, of course, faced with a riddle, which is all the more tantalising from its appearing for ever insoluble to men, chained as they are to the earth. However, it seems going too far to suppose that any abnormal conditions necessarily exist at the other side of the moon. As a matter of fact, indeed, small portions of that side are brought into our view from time to time in consequence of slight irregularities in the moon's movement; and these portions differ in no way from those which we ordinarily see. On the whole, we obtain a view of about 60 per cent. of the entire lunar surface; that is to say, a good deal more than one-half.

The actual diameter of the moon is about 2163 miles, which is somewhat more than one-quarter the diameter of the earth. For a satellite, therefore, she seems very large compared with her primary, the earth; when we consider that Jupiter's greatest satellite, although nearly twice as broad as our moon, has a diameter only one twenty-fifth that of Jupiter. Furthermore, the moon moves around the earth comparatively slowly, making only about thirteen revolutions during the entire year.

Seen from s.p.a.ce, therefore, she would not give the impression of a circling body, as other satellites do. Her revolutions are, indeed, relatively so very slow that she would appear rather like a smaller planet accompanying the earth in its...o...b..t. In view of all this, some astronomers are inclined to regard the earth and moon rather as a "double planet" than as a system of planet and satellite.

When the moon is full she attracts more attention perhaps than in any of her other phases. The moon, in order to be full, must needs be in that region of the heavens exactly opposite to the sun. The sun _appears_ to go once entirely round the sky in the course of a year, and the moon performs the same journey in the s.p.a.ce of about a month. The moon, when full, having got half-way round this journey, occupies, therefore, that region of the sky which the sun itself will occupy half a year later.

Thus in winter the full moon will be found roughly to occupy the sun's summer position in the sky, and in summer the sun's winter position. It therefore follows that the full moon in winter time is high up in the heavens, while in summer time it is low down. We thus get the greatest amount of full moonlight when it is the most needed.

The great French astronomer, Laplace, being struck by the fact that the "lesser light" did not rule the night to anything like the same extent that the "greater light" ruled the day, set to work to examine the conditions under which it might have been made to do so. The result of his speculations showed that if the moon were removed to such a distance that she took a year instead of a month to revolve around the earth; and if she were started off in her orbit at full moon, she would always continue to remain full--a great advantage for us. Whewell, however, pointed out that in order to get the moon to move with the requisite degree of slowness, she would have to revolve so far from the earth that she would only look one-sixteenth as large as she does at present, which rather militates against the advantage Laplace had in mind! Finally, however, it was shown by M. Liouville, in 1845, that the position of a _perennial full moon_, such as Laplace dreamed of, would be unstable--that is to say, the body in question could not for long remain undisturbed in the situation suggested (see Fig. 16, p. 191).

[Ill.u.s.tration: Various positions of Laplace's "Moon" with regard to the earth and sun during the course of a year.

The same positions of Laplace's "Moon," arranged around the earth, show that it would make only one revolution in a year.

FIG. 16.--Laplace's "Perennial Full Moon."]

There is a well-known phenomenon called _harvest moon_, concerning the nature of which there seems to be much popular confusion. An idea in fact appears to prevail among a good many people that the moon is a harvest moon when, at rising, it looks bigger and redder than usual.

Such an appearance has, however, nothing at all to say to the matter; for the moon always _looks_ larger when low down in the sky, and, furthermore, it usually looks red in the later months of the year, when there is more mist and fog about than there is in summer. What astronomers actually term the harvest moon is, indeed, something entirely different from this. About the month of September the slant at which the full moon comes up from below the horizon happens to be such that, during several evenings together, she _rises almost at the same hour_, instead of some fifty minutes later, as is usually the case. As the harvest is being gathered in about that time, it has come to be popularly considered that this is a provision of nature, according to which the sunlight is, during several evenings, replaced without delay by more or less full-moonlight, in order that harvesters may continue their work straight on into the night, and not be obliged to break off after sunset to wait until the moon rises. The same phenomenon is almost exactly repeated a month later, but by reason of the pursuits then carried on it is known as the "hunter's moon."

In this connection should be mentioned that curious phenomenon above alluded to, and which seems to attract universal notice, namely, that the moon _looks much larger when near the horizon_--at its rising, for instance, than when higher up in the sky. This seeming enlargement is, however, by no means confined to the moon. That the sun also looks much larger when low down in the sky than when high up, seems to strike even the most casual watcher of a sunset. The same kind of effect will, indeed, be noted if close attention be paid to the stars themselves. A constellation, for instance, appears more spread out when low down in the sky than when high up. This enlargement of celestial objects when in the neighbourhood of the horizon is, however, only _apparent_ and not real. It must be entirely an _illusion_; for the most careful measurements of the discs of the sun and of the moon fail to show that the bodies are any larger when near the horizon than when high up in the sky. In fact, if there be any difference in measurements with regard to the moon, it will be found to be the other way round; for her disc, when carefully measured, is actually the slightest degree _greater_ when _high_ in the sky, than when low down. The reason for this is that, on account of the rotundity of the earth's surface, she is a trifle nearer the observer when overhead of him.

This apparent enlargement of celestial objects, when low down in the sky, is granted on all sides to be an illusion; but although the question has been discussed with animation time out of mind, none of the explanations proposed can be said to have received unreserved acceptance. The one which usually figures in text-books is that we unconsciously compare the sun and moon, when low down in the sky, with the terrestrial objects in the same field of view, and are therefore inclined to exaggerate the size of these orbs. Some persons, on the other hand, imagine the illusion to have its source in the structure of the human eye; while others, again, put it down to the atmosphere, maintaining that the celestial objects in question _loom_ large in the thickened air near the horizon, in the same way that they do when viewed through fog or mist.

The writer[14] ventures, however, to think that the illusion has its origin in our notion of the shape of the celestial vault. One would be inclined, indeed, to suppose that this vault ought to appear to us as the half of a hollow sphere; but he maintains that it does not so appear, as a consequence of the manner in which the eyes of men are set quite close together in their heads. If one looks, for instance, high up in the sky, the horizon cannot come within the field of view, and so there is nothing to make one think that the expanse then gazed upon is other than quite _flat_--let us say like the ceiling of a room. But, as the eyes are lowered, a portion of the _encircling_ horizon comes gradually into the field of view, and the region of the sky then gazed upon tends in consequence to a.s.sume a _hollowed-out_ form. From this it would seem that our idea of the shape of the celestial vault is, that it is _flattened down over our heads and hollowed out all around in the neighbourhood of the horizon_ (see Fig. 17, p. 195). Now, as a consequence of their very great distance, all the objects in the heavens necessarily appear to us to move as if they were placed on the background of the vault; the result being that the mind is obliged to conceive them as expanded or contracted, in its unconscious attempts to make them always fill their due proportion of s.p.a.ce in the various parts of this abnormally shaped sky.

From such considerations the writer concludes that the apparent enlargement in question is merely the natural consequence of the idea we have of the shape of the celestial vault--an idea gradually built up in childhood, to become later on what is called "second nature." And in support of this contention, he would point to the fact that the enlargement is not by any means confined to the sun and moon, but is every whit as marked in the case of the constellations. To one who has not noticed this before, it is really quite a revelation to compare the appearance of one of the large constellations (Orion, for instance) when high up in the sky and when low down. The widening apart of the various stars composing the group, when in the latter position, is very noticeable indeed.

[Ill.u.s.tration: FIG. 17.--Ill.u.s.trating the author's explanation of the apparent enlargement of celestial objects.]

Further, if a person were to stand in the centre of a large dome, he would be exactly situated as if he were beneath the vaulted heaven, and one would consequently expect him to suffer the same illusion as to the shape of the dome. Objects fixed upon its background would therefore appear to him under the same conditions as objects in the sky, and the illusions as to their apparent enlargement should hold good here also.

Some years ago a Belgian astronomer, M. Stroobant, in an investigation of the matter at issue, chanced to make a series of experiments under the very conditions just detailed. To various portions of the inner surface of a large dome he attached pairs of electric lights; and on placing himself at the centre of the building, he noticed that, in every case, those pairs which were high up appeared closer together than those which were low down! He does not, however, seem to have sought for the cause in the vaulted expanse. On the contrary, he attributed the effect to something connected with our upright stature, to some physiological reason which regularly makes us estimate objects as larger when in front than when overhead.

In connection with this matter, it may be noted that it always appears extremely difficult to estimate with the eye the exact height above the horizon at which any object (say a star) happens to be. Even skilled observers find themselves in error in attempting to do so. This seems to bear out the writer's contention that the form under which the celestial vault really appears to us is a peculiar one, and tends to give rise to false judgments.

Before leaving this question, it should also be mentioned that nothing perhaps is more deceptive than the size which objects in the sky appear to present. The full moon looks so like a huge plate, that it astonishes one to find that a threepenny bit held at arm's length will a long way more than cover its disc.

[Ill.u.s.tration: PLATE VIII. THE MOON

From a photograph taken at the Paris Observatory by M.P. Puiseux.

(Page 197)]

The moon is just too far off to allow us to see the actual detail on her surface with the naked eye. When thus viewed she merely displays a patchy appearance,[15] and the imaginary forms which her darker markings suggest to the fancy are popularly expressed by the term "Man in the Moon." An examination of her surface with very moderate optical aid is, however, quite a revelation, and the view we then get is not easily comparable to what we see with the unaided eye.

Even with an ordinary opera-gla.s.s, an observer will be able to note a good deal of detail upon the lunar disc. If it be his first observation of the kind, he cannot fail to be struck by the fact to which we have just made allusion, namely, the great change which the moon appears to undergo when viewed with magnifying power. "Cain and his Dog," the "Man in the Moon gathering sticks," or whatever indeed his fancy was wont to conjure up from the lights and shades upon the shining surface, have now completely disappeared; and he sees instead a silvery globe marked here and there with extensive dark areas, and pitted all over with crater-like formations (see Plate VIII., p. 196). The dark areas retain even to the present day their ancient name of "seas," for Galileo and the early telescopic observers believed them to be such, and they are still catalogued under the mystic appellations given to them in the long ago; as, for instance, "Sea of Showers," "Bay of Rainbows," "Lake of Dreams."[16] The improved telescopes of later times showed, however, that they were not really seas (there is no water on the moon), but merely areas of darker material.

The crater-like formations above alluded to are the "lunar mountains." A person examining the moon for the first time with telescopic aid, will perhaps not at once grasp the fact that his view of lunar mountains must needs be what is called a "bird's-eye" one, namely, a view from above, like that from a balloon and that he cannot, of course, expect to see them from the side, as he does the mountains upon the earth. But once he has realised this novel point of view, he will no doubt marvel at the formations which lie scattered as it were at his feet. The type of lunar mountain is indeed in striking contrast to the terrestrial type. On our earth the range-formation is supreme; on the moon the crater-formation is the rule, and is so-called from a.n.a.logy to our volcanoes. A typical lunar crater may be described as a circular wall, enclosing a central plain, or "floor," which is often much depressed below the level of the surface outside. These so-called "craters," or "ring-mountains," as they are also termed, are often of gigantic proportions. For instance, the central plain of one of them, known as Ptolemaeus,[17] is about 115 miles across, while that of Plato is about 60. The walls of craters often rise to great heights; which, in proportion to the small size of the moon, are very much in excess of our highest terrestrial elevations.

Nevertheless, a person posted at the centre of one of the larger craters might be surprised to find that he could not see the encompa.s.sing crater-walls, which would in every direction be below his horizon. This would arise not alone from the great breadth of the crater itself, but also from the fact that the curving of the moon's surface is very sharp compared with that of our earth.

[Ill.u.s.tration: PLATE IX. MAP OF THE MOON, SHOWING THE PRINc.i.p.aL "CRATERS," MOUNTAIN RANGES, AND "SEAS"

In this, as in the other plates of the Moon, the _South_ will be found at the top of the picture; such being the view given by the ordinary astronomical telescope, in which all objects are seen _inverted_.

(Page 199)]

We have mentioned Ptolemaeus as among the very large craters, or ring-mountains, on the moon. Its encompa.s.sing walls rise to nearly 13,000 feet, and it has the further distinction of being almost in the centre of the lunar disc. There are, however, several others much wider, but they are by no means in such a conspicuous position. For instance, Schickard, close to the south-eastern border, is nearly 130 miles in diameter, and its wall rises in one point to over 10,000 feet. Grimaldi, almost exactly at the east point, is nearly as large as Schickard.

Another crater, Clavius, situated near the south point, is about 140 miles across; while its neighbour Bailly--named after a famous French astronomer of the eighteenth century--is 180, and the largest of those which we can see (see Plate IX., p. 198).

Many of the lunar craters encroach upon one another; in fact there is not really room for them all upon the visible hemisphere of the moon.

About 30,000 have been mapped; but this is only a small portion, for according to the American astronomer, Professor W.H. Pickering, there are more than 200,000 in all.

Notwithstanding the fact that the crater is the type of mountain a.s.sociated in the mind with the moon, it must not be imagined that upon our satellite there are no mountains at all of the terrestrial type.

There are indeed many isolated peaks, but strangely enough they are nearly always to be found in the centres of craters. Some of these peaks are of great alt.i.tude, that in the centre of the crater Copernicus being over 11,000 feet high. A few mountain ranges also exist; the best known of which are styled, the Lunar Alps and Lunar Apennines (see Plate X., p. 200).