CHAPTER XV.

YET MORE ABOUT THE MOON.

There are two ways of thinking about the moon. One way is to consider her as merely the earth’s attendant satellite. The other way is to consider her as our sister-planet, traveling with us round the central sun.

The first is the more common view; but the second is just as true as the first.

For the sun does actually pull the moon towards himself, with a very much stronger pulling than that of the earth. The attraction of the sun for the moon is more than double the attraction of the earth for the moon. If it were not that he pulls the earth quite as hard as he pulls the moon, he would soon overpower the earth’s attraction, and drag the moon away from us altogether.

People are often puzzled about the orbit or pathway of the moon through the heavens. For in one sense they have to think of her as traveling round and round in a fixed orbit, with the earth in the center. In another sense they have to think of her as always journeying onwards with the earth in her journey round the sun, and thus never returning to the same point.

There are two ways of meeting this difficulty. First of all, remember that the one movement does not interfere with the other. Just as in the case of the earth traveling round the sun, and also traveling onward with him through space; just as in the case of a boy walking round and round a mast, and also being borne onwards by the moving vessel,--so it is here. The two movements are quite separate and independent of each other. As regards the earth alone, the moon journeys round and round perpetually, not in a circle, but in a pathway which comes near being an ellipse. As regards the actual _line_ which the moon’s movements may be supposed to draw in space, it has nothing elliptical about it, since no one point of it is ever reached a second time by the moon.

But according to this last view of the question, nobody ever can or will walk in a circle or an oval. Take a walk round your grass-plot, measuring your distance carefully at all points from the center. Is that a circle? All the while you moved, the surface of the earth was rushing along and bearing you with it, and the whole earth was hurrying round the sun, and was being also carried by him in a third direction. Whatever point in space you occupied when you started, you can _never fill that particular part of space again_. The two ends of your so-called circle can never be joined.

But then you may come back to the same point _on the grass_, as that from which you started. And this is all that really signifies. Practically you have walked in a circle. Though not a circle as regards space generally, it is a circle as regards the earth. So also the moon comes back to the same point _in her orbit round the earth_. Letting alone the question of space, and considering only the earth, the moon has--roughly speaking--journeyed in an ellipse. You may, however, look at this matter in quite another light. Forget about the moon being the earth’s satellite, and think of earth and moon as two sister-planets going round the sun in company.

The earth, it is true, attracts the moon. So, also, the moon attracts the earth; though the far greater weight of the earth makes her attraction to be far greater. If earth and moon were of the same size, they would pull each other with equal force.

But though the pull of the earth upon the moon is strong, the pull of the sun upon the moon is more than twice as strong. And greatly as the earth influences the moon, yet the actual center of the moon’s orbit is the sun, and not the earth. Just as the earth travels round the sun, so also the moon travels round the sun.

The earth travels steadily in her path, being only a little swayed and disturbed by the attraction of the moon. The moon on the contrary, while traveling in her orbit, is very much swayed and disturbed indeed by the earth’s attraction. In fact, instead of being able to journey straight onwards like the earth, her orbit is made up of a succession of delicate curves or scallops, passing alternately backwards and forwards over the orbit of the earth. Now she is behind the earth; now in front of the earth; now between earth and sun; now outside the earth away from the sun. The order of positions is not as here given, but each is occupied by her in turn. Sometimes she moves quickly, sometimes she moves slowly, just according to whether the earth is pulling her on or holding her back. Two hundred and forty thousand miles sounds a good deal. That is the distance between earth and moon. But it is, after all, a mere nothing, compared with the ninety-three millions of miles which separate the sun from the earth and moon.

If we made a small model, with the sun in the center, and the earth and moon traveling a few inches off, only one slender piece of wire would be needed to represent the path of earth and moon together. For not only would the earth and the moon be so small as to be quite invisible, but the whole of the moon’s orbit would have disappeared into the thickness of the single wire. This question of the moon’s motions is in its nature intricate, and in its details quite beyond the grasp of any beginner in astronomy. But so much at least may be understood, that though the earth’s attraction powerfully affects the moon, and causes in her motions _perturbations_, such as have been already spoken about as taking place among the planets, yet that in reality the great controlling power over the moon is the attraction of the sun.

The tides of the ocean are chiefly brought about by the moon’s attraction. The sun has something to do with the matter, but the moon is the chief agent. This action of the moon can best be seen in the southern hemisphere, where there is less land. As the moon travels slowly round the earth, her attraction draws up the yielding waters of the ocean in a vast wave which travels slowly along with her. The same pulling which thus lifts a wave on the side of the earth towards the moon, also pulls the earth gently _away from_ the water on the opposite side, and causes a second wave there. The parts of the ocean between these two huge waves are depressed, or lower in level. These two waves on the opposite sides of the earth sweep steadily onwards, following the moon’s movements,--not real, but seeming movements, caused by the turning of the earth upon her axis.

Once in every twenty-four hours these wide waves sweep round the whole earth in the southern ocean. They can not do the same in the north, on account of the large continents, but offshoots from the south waves travel northwards, bringing high-tide into every sea and ocean inlet. If there were only one wave, there would be only one tide in each twenty-four hours. As there are two waves, there are two tides, one twelve hours after the other. In the space between these two high-tides we have low-tide.

Twice every month we have very high and very low tides. Twice every month we have tides not so high or so low. The highest are called “spring-tides,” and the lowest “neap-tides.” When the moon is between us and the sun, or when she is “new moon,” there are spring-tides; for the pull or attraction of sun and moon upon the ocean act exactly together. It is the same at full moon, when once more the moon is in a straight line with earth and sun. But at the first and last quarters, when the moon has her _sideways_ position, and when the sun pulls in one direction and the moon pulls in another, each undoes a little of the other’s work. Then we only have neap-tides; for the wave raised is smaller, and the water does not flow so high upon our shores.

In speaking of the surface of the moon, we are able only to speak about one side. The other is entirely hidden from us. This is caused by the curious fact that the moon turns on her axis and travels round the earth in exactly the same length of time. One-half of the moon is thus always turned towards us, though of that half we can only see so much as is receiving the light of the sun. But the half turned in our direction is always the same half. One part of the moon--not quite so much as half, though always the same portion--is turned away from us. A small border on each side of that part becomes now and then visible to us, owing to certain movements of the earth and the moon.

What sort of a landscape may lie in the unknown district, it is idle to imagine. Many guesses have been made. Some have supposed it possible that air _might_ be found there; that water _might_ exist there; that something like earthly animals _might_ live there. It is difficult to say what may not be, in a place about which we know nothing whatever. But judging from our earthly experience, nothing seems more unlikely than that air, water, clouds, should be entirely banished from over one-half of a globe, and collected together in the space remaining.

We are on safer ground when we speak about that part of the moon which is turned towards us. For we can say with confidence that if any atmosphere exist there, it must be in thickness less than the two-thousandth part of our earthly atmosphere. It seems equally clear that water also must be entirely wanting. The tremendous heat of the long lunar day would raise clouds of vapor, which could not fail to be visible. But no such mistiness ever disturbs the sharply-defined outline of the moon, and no signs of water action are seen in the craggy mountains and deep craters.

[Illustration: THE LUNAR CRATER COPERNICUS.]

The craters which honeycomb the surface of the moon are various in size. Many of the larger ones are from fifty to a hundred miles in diameter. These huge craters--or, as we may call them, deep circular plains--are surrounded by mighty mountain ramparts, rising to the height of thousands of feet. Usually they have in their center a sugar-loaf or cone-shape mountain, or even two or more such mountains, somewhat lower in height than the surrounding range. The sunset-lights upon certain of these distant mountain-peaks were first watched by Galileo through his telescope, and have since been seen by many an observer--intense brightness contrasting with intense blackness of shadow.

In addition to her great craters, the moon seems to be thickly covered with little ones, many of them being as small as can be seen at all through a telescope. Whether these are all volcano-craters remains to be discovered. It is not supposed that any of them are now active. From time to time, signs of faint changes on the moon’s surface have been noticed, which it was thought might be owing to volcanic outbursts. Such an outburst as the worst eruptions of Mount Vesuvius would be invisible at this distance. But the said changes may be quite as well accounted for by the startling fortnightly variations of climate which the moon has to endure. The general belief now inclines to the idea that the moon-volcanoes are extinct, though no doubt there was in the past great volcanic activity there.

A description has been given earlier of the rain of meteorites constantly falling to our earth, and only prevented by the atmosphere from becoming serious. But the moon has no such protecting atmosphere, and the amount of cannonading which she has to endure must be by no means small. Perhaps in past times, when her slowly-cooling crust was yet soft, these celestial missiles showering upon her may have occasionally made deep round holes in her surface.

This is another guess, which time may prove to be true. Guesses at possible explanations of mysteries do no harm, so long as we do not accept them for truth without ample reason.

[Illustration: LUNAR ERUPTION--BRISK ACTION.]

The origin of the lunar craters must be referred to some ancient epoch in the moon’s history. How ancient that epoch is we have no means of knowing; but in all probability the antiquity of the lunar craters is enormously great. At the time when the moon was sufficiently heated to have these vast volcanic eruptions, of which the mighty craters are the survivals, the earth must have been very much hotter than it is at present. It is not, indeed, at all unreasonable to believe that when the moon was hot enough for its volcanoes to be active, the earth was so hot that life was impossible on its surface. This supposition would point to an antiquity for the moon’s craters far too great to be estimated by the centuries and the thousands of years which are adequate for the lapse of time as recognized by the history of human events. It seems not unlikely that millions of years may have elapsed since the mighty craters of Plato or of Copernicus consolidated into their present form.

[Illustration: LUNAR ERUPTION--FEEBLE ACTION.]

It will now be possible for us to attempt to account for the formation of the lunar craters. The most probable views on the subject are certainly those adopted by Mr. Nasmyth, as represented in the cuts, though it must be admitted that they are by no means free from difficulty. We can explain the way in which the rampart around the lunar crater is formed, and the great mountain which so often adorns the center of the plain. The first of these cuts contains an imaginary sketch of a volcanic vent on the moon in the days when the craters were active. The eruption is here in the full flush of its energy, when the internal forces are hurling forth a fountain of ashes or stones, which fall at a considerable distance from the vent; and these accumulations constitute the rampart surrounding the crater. The second cut depicts the crater in a later stage of its history. The prodigious explosive power has now been exhausted, and perhaps has been intermitted for some time. A feeble jet issues from the vent, and deposits the materials close around the orifice, and thus gradually raises a mountain in the center.

Besides the craters and their surrounding barriers, there are ranges of mountains on the moon, and flat plains which were once named “seas,” before it was found that water did not exist there. Astronomers also see bright ridges, or lines, or cracks of light, hard to explain.

One of the chief craters is called “Ptolemy,” and in size it is roughly calculated to be no less than one hundred and fourteen miles across. Another, “Copernicus,” is about fifty-six miles; and another, “Tycho,” about fifty-four miles. The central cone-mountain of Tycho is five thousand feet high. The crater of “Schickard” is supposed to be as much as one hundred and thirty-three miles in diameter.

Astronomers have agreed to name these craters after the great discoverers who enlarged our knowledge of the solar and sidereal systems. It is fitting that these great names are suggested every time the moon is seen through a telescope. To Ptolemy we are indebted for what is known as “The Ptolemaic System of the Universe,” which makes the earth the center around which the sun, moon, planets, and stars all revolve, and explains the apparently erratic movements of the planets by supposing their orbits to be epicycles; that is, curves returning upon themselves and forming loops. Tycho Brahe was willing to allow, with Copernicus, that the planets all revolved around the sun; but he taught that both sun and planets turned around the earth.

The system known as “The Copernican” is now known to be the only true one, and it is universally accepted. It is so called after Nicolaus Copernicus, who was born in Thorn, Prussia, February 19, 1473. He was educated for the Church, and studied medicine, but devoted himself especially to astronomy.

Without being precisely a great genius, this quiet and thoughtful monk seems to have been wise far beyond the age in which he lived, and remarkable for his independence of mind. He had a “profound sagacity,” and a wide general grasp of scientific subjects.

The extremely complicated and cumbrous nature of the Ptolemaic system appeared to his judgment hardly compatible with the harmony and simplicity elsewhere characteristic of nature. Moreover, he was impressed and perplexed by the very marked changes in the brilliancy of the planets at different seasons. These changes _now_ are no difficulty at all. Venus, Mars, Jupiter, when on the same side of the sun as ourselves, are comparatively near to us, and naturally look much more bright in consequence of that nearness than when they are on the opposite side of the sun from ourselves. But under the Ptolemaic system, each planet was supposed to revolve round our earth, and to be always at about the same distance from us; therefore, why such variations in their brilliancy?

[Illustration: NICOLAUS COPERNICUS.]

During thirty-six long years he patiently worked out this theory, and during part of those thirty-six years he wrote the one great book of his lifetime, explaining the newer view of the Solar System which had taken hold of his reason and imagination. This book, named _De Revolutionibus Orbium Cœlestium_, or, “Concerning the Revolutions of the Celestial Spheres,” which came out only a few hours before his death, was dedicated to the Bishop of Rome--a little touch of worldly wisdom which doubtless staved off for a while the opposition of the Vatican.

Copernicus was not the first who thought of interpreting the celestial motions by the theory of the earth’s motion. That immortal astronomer has taken care to give, with rare sincerity, the passages in the ancient writers from which he derived the first idea of the probability of this motion--especially Cicero, who attributed this opinion to Nicetas of Syracuse; Plutarch, who puts forward the names of Philolaus, Heraclides of Pontus, and Ecphantus the Pythagorean; Martianus Capella, who adopted, with the Egyptians, the motion of Mercury and Venus around the sun, etc. Even a hundred years before the publication of the work of Copernicus, Cardinal Nicolas of Cusa, in 1444, in his great theological and scientific encyclopedia, had also spoken in favor of the idea of the earth’s motion and the plurality of worlds. From ancient times to the age of Copernicus, the system of the earth’s immobility had been doubted by clear-sighted minds, and that of the earth’s motion was proposed under different forms. But all these attempts still leave to Copernicus the glory of establishing it definitely.

Not content with merely admitting the idea of the earth’s motion as a simple arbitrary hypothesis, which several astronomers had done before him, he wished--and this is his glory--to demonstrate it to himself by acquiring the conviction by study, and wrote his book to prove it. The true prophet of a creed, the apostle of a doctrine, the author of a theory, is the man who, by his works, demonstrates the theory, makes the creed believed in, and spreads the doctrine. He is not the creator. “There is nothing new under the sun,” says an ancient proverb. We may rather say, Nothing which succeeds is entirely new. The newborn is unformed and incapable. The greatest things are born from a state of germ, so to say, and increase unperceived. Ideas fertilize each other. The sciences help each other; progress marches. Men often feel a truth, sympathize with an opinion, touch a discovery, without knowing it. The day arrives when a synthetical mind feels in some way an idea, almost ripe, becoming incarnate in his brain. He becomes enamored of it, he fondles it, he contemplates it. It grows as he regards it. He sees, grouping round it, a multitude of elements which help to support it. To him the idea becomes a doctrine. Then, like the apostles of good tidings, he becomes an evangelist, announces the truth, proves it by his works, and all recognize in him the author of the new contemplation of nature, although all know perfectly well that he has not invented the idea, and that many others before him have foreseen its grandeur.

Such is the position of Copernicus in the history of astronomy. The hypothesis of the earth’s motion had been suggested long before his birth on this planet. This theory counted partisans in his time. But he--he did his work. He examined it with the patience of an astronomer, the rigor of a mathematician, the sincerity of a sage, and the mind of a philosopher. He demonstrated it in his works. Then he died without seeing it understood, and it was not till a century after his death that astronomy adopted it, and popularized it by teaching it. However, Copernicus is really the author of the true system of the world, and his name will remain respected to the end of time.

The so-called “seas” on the moon are those large dark spots to be seen on its surface, in the shape of “eyes, nose, and mouth,” or of the famous old man with his bundle of sticks. The brighter parts are the more mountainous parts.

The chief ranges of lunar mountains have been named by astronomers after mountains on earth, such as the Apennines, the Alps, the Caucasian range, the Carpathian and the Altai Mountains.