Astronomy; the sun and his family

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Contents

CHAP. ^GE

I The Fairy Story of the Skies 5

II The Home of the Sun Family 17

Chapter I

THE FAIRY STORY OF THE SKIES "This majestical roof, fretted with golden fire."

The sun has set and a deep, rosy color lies along the western horizon. In the clear blue above this pink band a large star, with a soft, steady white light, shines out, the first of the evening host. Half-way up the eastern sky the moon has climbed, silver bright. The moonlight falls in a broad, glittering, bronze track across the waters.

In this line of light the rowboats and small sailboats rock at anchor, each little fishing vessel with one steadying sail set; the trading schooners lie waiting for a favorable wind ; they look like silver ships with silver sails. Such is the illusion of the moonlight.

We dwellers in the Occident and the North are told that the true glory of night is never revealed to us ; it is in the Orient and over the desert that the splendor of the heavens is fully shown, and there the starry hosts won their first worshipers, almost compelling man to adoration of the created and the seen, because of such surpassing glory.

Nowhere in our Western w^orld can the heavens be more beautiful, more beguiling to our thoughts than near the sea, when the great, blue, scintillating dome bends over and mirrors itself in the wrinkling or foam-flecked waters, reproducing the starry lights a thousand fold. Of such a scene, Flammarion, the Romance Astronomer, writes : " The sky is studded with brilliant stars, the air is calm and slumberous, the silence of profound peace covers the earth, and in the smooth mirror of the w r aters the heavenly bodies are reflected, opening a new w T orld before our eyes. Thought floats between two immensities ; the infinite sky, the w r ater, peopled also with stars."

In such beauty, when the uproar of the day has died almost to silence, fancy revels in the long, bright, wonderful, seemingly endless story of the starry hosts. We look at the white moon and consider how she draws the tides. The sea is brimming like a cup full to the overflow. It seems as if the w r aters rounded up like a bowl turned upside-down; they seem to rise toward the moon, and long to reach her. We

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almost see the silver chains by which she draws the floods to herself. But those silver chains no eye can see. Why does that serene planet covet these oceans? It is that the earth is the mother of the moon. Long ago the moon sprung out from the earth, as a child springs away from its mother's arms, and since then earth's truant moon-child has never returned. Yet twice in each tw r enty-four hours a great longing for her mother seizes the moon, and she draws with all her might to bring that mother to herself. As for six hours she pulls steadily with those silver chains known as '"" attraction " in our mortal speech, the waters feel the drawing and heap up on the side of the earth toward the moon. If the moon stood still and continued to . pull the seas, they might overflow all the astonished lands.; but always at the end of six hours, the moon becomes discouraged and relaxes her pull ; she has moved so far along in her journey around the world that she is pulling other waves.

In this fairy story of the skies, however, we are but telling half facts, and half facts are dangerous ; really the moon is pulling or" attracting the earth all the twenty-four hours, and the waters are as constantly rising to meet her. In her trip around the world she exerts her power at its maximum on different places, and those are the places which are just

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beneath her; as she passes on her way the place of greatest attraction changes.

The moon attracts or draws the land as well as the water, but we do not notice this so much, so we speak freely of her drawing the tides and very seldom about her drawing the land. There is high tide twice each day on each side of the earth ; the low tides are the half-way stations.

It is well to think high thoughts ; it leads us to living higher lives. Let us study for a while the real story of the skies, not the fairy story, but the story that is true. Yet the real story is very like the fairy story. The moon was once a part of the earth. As the earth went whirling round and round, that force which is called tangential, or centre-flying force, caused a portion of the whirling ball to fly away. That flying part was the moon. As tangential force flung it off, after a while another force set to work and kept it from flying on, out of sight forever. This second force is called attraction. Every large body or mass will exert this force more or less strongly upon other bodies. The attraction of the earth holds the moon from moving farther away, and the attraction of the moon creates tides upon the earth.

Let us say that the case of the moon and the earth is like the case of a cup and ball. The ball

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goes a certain distance, and the string hinders it from going farther; the string represents attractive force. If the cup is whirled by the handle, that whirling acts upon the ball, and one can keep it at the end of the tense string, whirling about the cup from which it was flung. There are many points where the cup and ball fail as an example of the case of the earth and the moon, but let these do for the present. Did the moon then break off in a great lump, leaving a deep hole where it tumbled out of the earth ? Was that deep hole the hollow where now the ocean runs ? We are getting back to fairytale talk ; there was no lump, no hole. When the moon flew from the earth the earth was not a solid, not even a liquid ; it was a great globe of gas, thinner and hotter than anything we can imagine.

Suppose that when that flying-off happened some being from afar, some angel or sprite, had been looking on, w T ould that one have felt wild surprise at the evolutions of that moon-piece ? Not at all. That onlooker would have already seen many such violent dividings of matter ; this forming of the moon was far along in the series. What, then, was the first one ? Who can tell ? Of that the wisest knows nothing. All that any one knows is something, and that a very little, of what happened in one little corner of the universe, in what is called our solar system.

We look up into the sky ; since we began to think of this story of the skies the sunset color has faded from the west, the dome is darker blue and all aflame with myriad lights. A broad band of light lies like a ribbon across the heavens. We call it the Milky Way. It is a part of the heavens where the stars seem crowded together, as if they even touched each other, and in so seeming they mingle into one path of effulgence. In very truth they are far, far apart. It is only distance that makes them seem near ; all heavenly bodies are far apart. This must be so because of tangential and attractive force. If they were too near the pull of attraction would cause the smaller body to fall into the larger ; the strong pull would bring the smaller body out of its orbit. It must be just near enough to be held ; not so near as to be drawn from its proper place. There must be a balance of tangential and attractive force.

Each heavenly body has its path in space, its appointed track, called its orbit. Sustained in space, these innumerable worlds travel their eternal ways and never interfere ; each moves responsive to some sublime, unchanging, perfect law. To be able to discover, and in some degree interpret and appreciate such law, is one of the great privileges of the human mind.

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The broad band called, from its white appearance, the Milky Way is one of many nebulae with which space is set. The white appearance is caused by the immense number and distance of the stellar systems which form these clouds of splendor. We speak of stellar systems, and we speak of our solar system: here is a distinction without a difference, for every sun is a star, and every system is a stellar system. This vast nebulae which crosses the sky like a white ribbon, is made up of many of such stellar systems as ours, but most of them in all probability far larger than our own. Each particle of that apparently closely sown star-dust is a sun, and the systems that surround these suns are so distant that we cannot discern them even as star-dust. Let us try to fix our minds on the thought that this nebulae of the Milky Way is only one of myriads of nebulae, each lying like an isle of light in the seas of space, or anchored on the bosom of that vast dark ocean, as the ships are at anchor here in the harbor. This Milky Way we understand is far from the largest of these isles of light, yet for a ray to flash from one extreme of its greater axis to the other would require seventeen thousand years ; yet light flashes seventy thousand leagues a second ! Truly, now, we are thinking great thoughts, thoughts which take away our breath and dazzle our eyes, and leave us feeling

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very insignificant. Yet, it is something to be able to reach such conceptions as these.

For convenience in study astronomers, from the earliest ages, have grouped together certain stars which come within one field of observation. Star-gazers have seen that between certain stars they could trace some rude outline of some known thing, as, for instance, a chair, a dipper, a crown. These stars were set together then as the constellation of the Dipper, Crown, Orion, etc., etc. We must, however, remember that the stars in a constellation may have no real relationship, no nearness except that which we have made for them, in fancy, and on our astronomical maps.

While a constellation is a thing arbitrary, and arranged for human convenience in study, a system is made up of bodies which really belong together, and have interdependence and mutual relationships. Thus our earth is part of the solar system, the centre of which is the sun. All the planets in this system have sprung from the sun, are held in their places by his attraction, are reached by his beams, and have mutual dependence and relations. The laws which govern this system are harmonious and invariable; they have been discovered, and their nature is such that with some degree of certainty we can assume that they are not limited to the solar system, but

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extend through all space, and govern the most distant nebula?. And how distant and how numerous these are we cannot conceive. Also we have equal reason to believe that ,: star stuff " is the same in all nebulas and all systems.

When we think back from distant to near, what shall we seek for and what shall we find in this little system of ours — the solar system — about which we know that little something, — aggregate of the researches, discoveries, deep-thinkings of the wise of many ages? What is the life-story of the solar system ? How are worlds born, and how do they grow ? What is the story of the sun and his family ?

When we begin with the sun where shall we go to find him ? We must needs go to the Milky Way to find the sun and his family. Here we trace thought-ways over which many minds have passed before. Xo doubt the heavenly bodies, distant, mysterious, splendid, were among the first objects which drew the studious thoughts of men. The early years of the race were not the age of the barbarians, but of the thinkers. The thinking may have had its child-like simplicity at first, but it arrived at great results. Doubtless the first careful observers of the skies were the shepherds, who watched the flocks by night; the mariners, whose little craft hugged the shores ; those desert traders, the men of the caravans, who, resting in the heat of the noon-day, traveled on in the quiet coolness of the night. The rising and setting of the sun was matter of common observation to all classes, and it did not take long to learn that there was not a new sun every clay, but that the same orb disappeared in the west, and rose again in the east. The farmers, no doubt, early interested themselves in the varied positions of the sun in the sky during the different periods of the year, marking the changes of the seasons. The mid-summer sun is high overhead at noon, but the mid-winter sun hangs low in the sky even at noon. Thus it was seen that the sun had a yearly as well as a daily movement.

The men whose business gave them opportunity to watch the skies at night, while the day laborers slept, slowly discovered other motions of the heavenly bodies ; those especial stars, or those groups of stars which seemed to have some interdependence, also slowly journeyed toward a setting ; and as the year passed on the positions of these constellations in the sky varied.

Year by year, as the long-lived, patient watchers of the skies recorded their observations, they saw that the sun had in his yearly journey one invariable path, moving through the domain of certain constel-

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lations, and these they named the zodiac, and divided into twelve signs, or stations, for the year. Then, as the heavens must be supposed to dominate the earth, by these zodiacal signs they governed their affairs and employments.

Sir Robert Ball suggests that observations of the journey of the moon among the stars, the discovery of its times and phases were first understood, and the knowledge of the sun's yearly motion came later. But this was not all ; these thoughtful observers of the early time in their close searching of the skies during wakeful night hours, had discovered that there were five bright bodies, unlike all the others in the blue dome ; while other stars held their places, or moved only as the whole concave seemed slowfy to wheel about, these five bright lights were wanderers among the stars. They named them planets or rovers.

Slowly, for many ages observations were made and theories were formed. Once, even the wisest had thought the earth a great ocean-surrounded plain, somehow upheld, around which the heavens revolved ; the earth great, the heavenly bodies, its attendants, smaller. In the second century a. d. Ptolemy, the astronomer, produced a work which for fourteen hundred years ruled the thinking of men in all that concerned astronomy. Ptolemy had found arguments to prove that the earth is a sphere ; he saw it poised in space ; he held that it was there fixed, and that all the starry hosts revolved about it. One great thinker said to him, " Do you not see that if it should be that this earth-ball revolved, then the apparent moving of the whole heavens could be accounted for ?"

The astronomer assented, but rejected the possibility that the earth did move ; he held that it was absolutely fixed in its place, neither rotated nor pursued an orbit. The sun, moon, and planets moved, he said, and they moved in great circles, the proof of which was, that only circular motion could be perfect, and as the heavenly bodies could be imperfect in nothing, therefore each must pursue a circular path through space about the earth. Such was the early astronomical system of those who had arrived at no real conception of the laws and relationships of the sun and his family.

Chapter II The Home Of The Sun Family

" Heaven's ebony vault Studded with stars unutterably bright."

Day is the time to study botany, geology, and many other sciences ; solemn, restful night invites us to look upon the star-set skies, and review what the wise have said and noted after sunset.

We say " up " in the sky, and "down" here on earth, to begin with. Is that right ? Perhaps some one on a far-off star says, " up " of here, and u down " of there. There is no up or down in space, in matter of fact. That is a form of expression, continued for convenience from the times when people believed that the earth was a vast plain, surrounded with water, and that the sky was over it, as a bowl may be turned upside down over a plate.

What we call " sky " is merely the limit of our vision in space. We speak of the horizon line where land and sky seem to meet ; that is merely as far as we can see. Your eyes are better than mine, and are able to see farther off than mine. We both have a limit, determined by the rotundity of the earth. 2 17

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Some certain place is twenty-five miles from us ; we cannot see it, because on the rounding of the earth it falls below our vision.

" I have seen mountains fifty miles off," says some one. True : they were so high that they came into view. You did not see the ground-line of those mountains.

Why are the skies blue ? we ask as we look above us. They are not blue. About our earth is an elastic envelope, from one hundred and twenty to tw o hundred miles thick ; w^e call it the atmosphere. The gas portions moving w r e know as air ; air violently driven we call wdnd, moving at different rates of speed. Thus while we cannot see our atmosphere except for the blueness, we can feel it. The blueness of w 7 hat we call sky is caused by the depth of atmosphere into which we gaze. All clouds are within our atmosphere ; they are caused by the condensation of water which has risen in invisible vapor from the surface of our globe. As we watch these lovely clouds we say to ourselves, " If I had eagle's wings and flew above our atmosphere, would I find it all golden light ?" Then reason replies, " If you had such wings you could go no higher than the eagle. When you rose above a certain distance the air would become so thin, or rare, that you could not breathe. You would suffocate, as a fish does when

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taken out of water— unless you first froze to death. " Yes, we are dealing with fact, not fairy lore. Why should we freeze to death when rising nearer the sun ?

The little gain in nearness to the sun w r ould not make up to us for the loss of heat; reflected from the earth. The glass roof of a greenhouse admits sun heat but does not readily allow it to pass out. It is " a trap to catch a sunbeam." The atmosphere is our glass roof, the heat poured by the sun on the earth is held here by the atmosphere. Close to the earth's surface the atmosphere is most dense, and retains most heat. As we rise higher the atmosphere is thinner and less heat is retained. Of course, if we could get half-wa}^ to the sun let us say, we would find heat enough to scorch us. All mountain tops are cold, often snowcovered the year round, even in the tropics.

Let us fancy, however, that we could rise far beyond our atmosphere. The deep blue would slowly darken — it would not change to golden light, but to intense blackness. Darkness reigns in stellar space. Our atmosphere is full of atoms of dust which seize and reflect the light poured from the sun, or from any light-reflecting object. By means of this reflection and refraction light is rendered visible. If we wandered into starry spaces we should be lost in awful darkness, unless we found something to serve, as does our air and the dust it contains.

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How many stars should we find in space ? No one can tell. Those seen by the naked eye, although we call them countless, are only about five thousand, clearly visible. The telescope brings more into view.

Each of those stars is a sun, perhaps much larger, hotter, brighter, than our sun. Each of these suns is the centre of a system, each turning on its axis, and each journeying along some fixed path, carrying with it its planets, held by its attraction. Those planets we cannot see.

Beyond the stars which we see, with their systems wheeling about them, may be millions more of sun-stars, in a multitude and distance of which it startles the mind to think. It almost overwhelms us even to fancy so many rushing along, worlds on worlds, suns on " suns in golden splendor burning."

The machinery of the universe is perfect. Each sun, each system, moves in a fixed ordered path ; these paths may intersect, but never interfere. God makes no mistakes, and His machinery never breaks down. We may learn something about these distant sun-stars by the use of the telescope and spectroscope ; what we learn shows us that the same laws govern distant space that govern our system.

Where is our system in space ? we ask as we look abroad. We find it hard to believe that we are

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a part of the Milky Way. Although that looks so far off, above us, and entirely out of our range, it is really our part of space, and in it our sun and his family find their home.

How many queries these skies suggest. In regard to the more distant stars, we may learn that they are single, double, or even triple ; that they are variable ; that they are of different colors : purple, red, blue, white, yellow, green. They can be weighed, but not measured. They are weighed by wonderful instruments, and by means of difficult calculations which we need not discuss in these simple studies. When all is told, we find that we know almost nothing about the far off sun-stars and their satellites.

Into our system come wanderers from afar, vagrants of the skies, we call them comets. They may have traveled through those black and silent spaces, and visited suns and systems which the most powerful telescopes have never brought into view. They may have been drawn by the attraction of those other suns for a time, and then, breaking away, have come to us. They come and go, and tell us nothing.

Meanwhile our sun holds in good order his wellgoverned family about him, and does not allow them to stray off in the reckless company of the comets. For this well-disciplined family, the sun generously pours out light and heat. He gives not only the

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light and heat which they need and use, but so much more, that, as a well-known astronomer says, " The sips that a flying swallow takes from a river is as far from exhausting the water in the river, as are the planets from using all the light that streams from the sun." We use the words " stars," " planets ;" can we use them interchangeably? Do they mean the same kind of body? Not at all ; a star and a planet differ widely. Our sun is a star; if all stars are made of the same " stuff " or matter as our sun, then the material that makes planets is the same as that of which stars are formed, for planets are made out of the material of their central suns. The planet is distinguished from a star by its disc and its motion also ; planets reflect light, but do not pour forth light as the sun-stars. Several of the planets were, when first seen, thought to be stars, until their motion through space betrayed them to be individuals of our own solar system. The most distant of our sister planets is immensely nearer to us than the nearest star, and thus even the farthest off of our planets reveals to the telescope a disc, which the nearest star is too distant to show through our most powerful telescopes. Could we bring those far-off stars into as close view as our sun, we should, no doubt, find them possessed of all the characteristics of the sun, light-givers of unutterable, unendurable splendor, immense, slowly

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rotating, their envelopes rent and convulsed as is his, by leaping, struggling fires.

All this that the telescope reveals to us was unknown to those ancients, who, before history began to be recorded, gathered knowledge of the hosts of heaven simply as their unaided eyes could carry on observations. One does not then wonder that these first astronomers knew so little, but that they knew so much. It was much to detect those " sky wanderers,'* the five early known planets, and to name them and understand something of their motions.

The true theory of the universe was unknown until about the time of Columbus. Almost simultaneously the heavens and the earth revealed their secrets. The heavens unfolded to wrapt student minds the marvels of the solar system. The earth showed a new Avorld, cradled on the waters near the gates of the sunset.

Copernicus is the next great light in astronomy in order of time after Ptolemy. Ptolemy took a great forward step in knowledge when he assured himself that the earth was a globe, but he left it fixed in space, the centre of a system, and the greatest body of that system. Copernicus established the great facts that the earth rotated on its axis, and that the seeming daily motions of the heavens was really due to the revolution of the earth itself. After this he

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assigned to the earth its true position in the universe : it was one of the planets revolving about the sun, one of the smallest of these planets. The sun was the shining centre of our system, and about him wheeled six planets, the five known to the ancients, Mercury, Venus, Mars, Jupiter, Saturn, and the Earth.

All this Copernicus demonstrated before a telescope was invented ; that marvelous lens that should bring the distant near and introduce men to the wonders of the heavens was not yet invented. Simple dials and instruments for measuring the length of the shadow cast at noonday by the sun had been used in the calculations of the ancients. Three years after Copernicus died, Tycho Brahe was born, and on him the mantle of Copernicus seemed to have fallen. The King of Denmark built for Tycho Brahe an observatory, and there for twenty years this last of the astronomers without telescopes, studied the motions of the stars.

Galileo, born in Pisa, in 1642, was an astronomer with a happier destiny. When he was thirty-five years old the first astronomical telescope was constructed. In that square, gray Torre Galileo on the low hill above Florence, Galileo, the father of modern astronomy, wandered into new worlds until then " beyond our ken."

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The telescope is an instrument fashioned on the principle of the human eye. When the pupil of the eye is dilated to its utmost, then the most light is admitted, and the image cast on the retina has the greatest brilliancy. Such objects as distant stars may not be bright enough to excite vision. The telescope on its great lens catches beams which are too large to enter the human eye-pupil, and then concentrates these great rays into a strong, vivid, smaller ray, which excites the sensation of vision on the retina.

The larger the object glass in a telescope the more light it receives, and the greater is its power of revealing the hidden and bringing the distant near.

In looking at a telescope we are full of admiration for the scientific ingenuity which fashioned it, and with wonder that any valuable astronomical work was done without instruments, which now seem so indispensable. It is the telescope which has explored for us the sun, his home, the number and manners of his family.

Chapter III The Parent Sun

" The sun, the centre and the sire of light, The keystone of the world-built arch of heaven."

" How can we know anything about the sun," one demands, "when it is too bright to be looked at?" The moon, by hiding the sun, helps us to study it. " How can the small moon hide the great sun ? How can we study that which is hidden?" Look toward a house and hold, say, an oak leaf before your eyes. The oak leaf hides the house. It is not so much the relative bigness as the nearness of the one object to the e) T es which enables it to hide another. The moon is so much nearer to the earth than the sun is that when, in its journey around the earth, the moon comes between the earth and the sun, the sun is hidden. This is called an eclipse.