Lecture on the Sun by William Leitch

From The Space Library

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Revision as of 02:59, 29 November 2016

This lecture on the sun was given by Principal William Leitch D.D. in September 1862 to the students at Queen's University in Kingston Ontario.

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It is remarkable that the most important body in the solar system should be the one whose physical constitution and structure attracted, till lately, least notice. It seemed hopeless to fathom the mystery of this fountain of light and heat. The milder rays of the other bodies of the system allowed us to gaze comfortably on their surface, and to trace resemblances to our own globe; but the sun repelled us by his fierce rays, and astronomers contented themselves with a rapid glance, as if looking into a scorching furnace. The sun was regarded as wholly dissimilar to the other bodies of the system—so dissimilar indeed that it was thought no knowledge of terrestrial conditions would ever enable us to comprehend conditions apparently so different. What difference could be greater than between a fierce furnace, like the sun, and cold, dark, solid bodies, like the planets? The sun appeared to be a mystery so profound that astronomers felt it was irreverence to pry into it too curiously. Recent science has, however, thrown off all delicacy on this subject, and the sun is now treated as familiarly by the chemist as any substance submitted to his analysis. It has been found that the sun is not wholly dissociated from the planets in constitution and structure, that there are links of connection which show that they belong to the same family of bodies, and it is one of the chief charms of astronomy to trace these links.

The first point for consideration in discussing the subject is the measurement of the distance, size and weight of the sun. When the more startling facts of astronomy are stated to an ignorant or illiterate man, they are received of course with incredulity and, it may be, with ridicule. They so far transcend the circle of his own narrow conceptions that he smiles at the credulity of the learned. Now this incredulity is not confined to the ignorant and illiterate. Well educated people have often a secret unbelief as to the facts of astronomy, though they may be ashamed to put their opinions in opposition to that of the whole scientific world. Yet, when told that the earth's surface spins round with the velocity of a cannon ball, that the little prominences that can be seen with the naked eye on the edge of the moon are vast mountains, that the earth is no more to the sun in magnitude than a single stone of St. Paul's to the whole fabric, they are inclined to shake their heads, although positively assured of the facts by the most eminent astronomers.

They read books which give facts and figures, but still they do not bring conviction. And why does this secret unbelief cling to the mind? Simply because we do not understand the rationale or principle by which these astounding facts have been arrived at. If we once comprehend the method, the facts will readily bring conviction. Now it is this comprehension of the methods and principles of a science that constitutes real scientific knowledge. It is not the storing-up in the memory of the facts and figures of astronomy. A clever boy at school will, in the course of a few months' study, become a more profound scholar than Newton or Herschel, if astronomy consists merely in the recollection of its facts. In company Newton sometimes appeared more ignorant than others about his own discoveries, simply because he had not a memory for numbers. And some, who could not in the least comprehend the science, yet appeared in conversation to be superior, because they could at once give the exact distance of the moon or the exact compression of the earth.

In order to derive true enjoyment from the study of astronomy, and really to believe in its facts, it is necessary that you clearly comprehend the methods by which these facts have been arrived at. But you will ask, Is it possible for the popular mind without a special technical training to attain this? I think it is. It is not at all necessary to comprehend the principles of the celestial mechanism that you should be able to handle astronomical instruments or manage mathematical formulae It is just like understanding the principle of a steam engine. It is not necessary, that you should be a practical engineer, and able to calculate the pressure of steam or the strength of materials, to comprehend the principle on which the engine works. So in the celestial mechanism you may have a thorough comprehension of the general principles involved, although you cannot enter into the technical details of calculation. In determining the distance of the sun the astronomer only employs a principle which you daily take advantage of in estimating distance. On looking out from the windows of a railway carriage you observe that near objects flit along the horizon, while distant objects creep very slowly, and you calculate that the slow objects are more distant than the fast ones. The distance is in direct proportion to the slowness of the motion. If the near house is one mile distant, then you conclude that the more remote one in the same line is two miles if its motion is twice slower; three miles if thrice slower, and so on. You have only to measure the distance at the first house, and the distance of the farthest off is at once known by ascertaining its comparative rate of motion. Instead of the most distant house you may take a cloud, or the moon, or any heavenly object. The principle is precisely the same, only you must move farther to see any appreciable change of place. This change of place according to the different position you occupy is called parallax, and on this depends on your knowledge of the distance of the heavenly bodies.

When you have in this way found the distance of the sun it is easy to measure its size you can do so by the rule a simple proportion. Suppose that, when you hold but a sixpence at arm's length, it exactly covers the face of the sun, you say that the sun and the sixpence have the same apparent size, but the sun appears so much less than it in reality is, just in proportion to its greater distance; and, if you wish to know how much larger it is in reality than the sixpence, you must ascertain how much more distant it is, or how many arm's lengths there are between the sixpence and the sun; and that number will be the number of sixpences required to stretch across the sun, and, knowing the diameter of the sixpence, you know the diameter of the sun. Then, as to the weighing of the sun, this appears still more wonderful; and, when the astronomer speaks of weighing a planet, people imagine that it is only in a metaphorical sense that he does so. But he weighs the planets just as really as the grocer weighs his goods over a counter. When you put a letter into a spring balance you think it is only the letter you are weighing, but you are at the same time weighing the earth. You are not apt to think so because the world is always the same, while you change the letters. But suppose you change the world instead of the letter.

Suppose that a letter which weighs an ounce is carried in the spring balance to another planet, and held at the same distance from its centre, would the letter weigh the same? By no means; if the planet is only half the weight of the earth, the letter will be only half an ounce; if it is double the weight of the earth, it will be two ounces. Let us suppose the one ounce letter to be carried to the sun, how much would it weigh there? Eleven tons ; and, just as eleven tons is greater than one ounce, so is the sun greater than the earth. It must be carefully observed in weighing the planets that the balance must be held at the same distance from their centres, not their surfaces. But you will say, How can you get the balance conveyed to the planets and the sun? The answer is that there are nature’s spring balances in the heavens. These are the orbits or circles in which the planets move. They may be compared to bent steel springs and, just as the earth by its weight or gravity pulls and bends the wire of a spring balance so does it bend the path of the moon into a curve. Were it not for this bonding power the moon would move in a straight line: but the earth bends the straight path of the moon as the cooper bends the hoop of a barrel, and exactly in proportion to the bending in a given line is the pulling power or weight of the earth. The weight of the sun is found in the same way. You have only to measure how much it bends the paths of the planets in a given time. Knowing the weight of the earth, we can readily tell how much heavier the sun is from its superior power in bending the orbits of the planets. Let us next attend to the position of the sun in the solar system. It is the centre or the whole. In this way its light and heat are equally distributed throughout the whole year of each planet. Each planet goes round the central fire in a circle during the course of the year. We might conceive a dark body corresponding to the sun in the centre, controlling all the planets by his gravitation; while another body, such as Jupiter, had assigned to it the function of dispensing light and heat to the solar system. But, were this the case, the various planets would experience extremes of heat and cold which would be destructive to all life. Placed as the furnace is in the centre, there is but little variation in temperature in the course of the year. The next point of interest is the structure of the sun. The spots on his surface, which Milton so poetically represents as demons flitting across his disc, reveal the true structure. These spots are not really on the surface, but holes down which you see through the dark body of the sun. When you look down these funnels you see the edges of the concentric shells of which the outer part of the mass of the sun is composed. There is probably a solid core, and round the core there is layer upon layer, like the concentric layers of a bulbous root. These layers strata are separated from one another by intervals which are probably filled with a transparent atmosphere, just as one stratum of clouds is suspended above another by the buoyancy of the earth's atmosphere. Three distinct concentric shells have been discovered by carefully looking down these abysses, which are so large that our earth could easily be projected through them. These strata are not solid, for you see the whole mass in commotion like a boiling cauldron, and its continuity is broken by these openings or holes, which are like breaks in the continuity of a cloud-covered sky. The dark body of the sun appears through them as you see the blue sky through breaks in the cloudy stratum above us. The outer visible stratum is called the photosphere, as it is from it that the light comes. Total eclipses reveal a new stratum which at other times is quite invisible on account of the brighter radiance of the photosphere. It is of a rose-coloured tint and envelops the photosphere. We are in fact looking through it, when we are looking at the bright disc of the sun. The veil is, however, so transparent that we do not suspect that we are looking through it. When the moon in a total eclipse entirely covers the sun, this rose-coloured stratum shines out with very lofty prominences, like the crests of waves in a storm. This rose-colored stratum projects only a very little beyond the limit of the sun, but in a total eclipse there is a corona like the glory round a Saint's head, which extends far beyond the limb. There is still great doubt as to the nature of this corona, whether it belongs to the sun or moon, or is merely an affection of light in passing the edge of the moon there is however no doubt that the red flames belong to the sun. The sun is encircled by rings of zones, corresponding to the rings of Saturn. The rings of Saturn cannot be solid, as was once supposed, at least they cannot form a rigid mass like a rock. They are probably composed of innumerable small masses of matter, each moving independently like a separate planet, but then so closely packed together that the mass appears solid.

One of these rings in fact, is composed of such fine particles of matter that you can see through it—this is the dark ring lately discovered. The others probably differ from it only in being more massive, or composed of coarser material, so that the stratum is too thick to be transparent. The sun has similar rings. The Zodiacal light is probably one of these. The zone of asteroids between Mars and Jupiter is another, for, although we have discovered only 70 distinct bodies there are probably millions more of a smaller size. Leverrier has also indicated 2 other zones, one within the orbit of Mercury, and the other near the orbit of the Earth. He has even approximated the weight of each of these zones or rings. The next point is the work of the sun. It is not only to the heat and light of the sun we are indebted. Almost all the mechanical power on the face of the earth is traced to the sun. The sum of force in the universe is always the same, just as the sum of matter is always the same. The force may change its form, but its amount is always the same. This principle is known by the name of correlation of physical force. When the river leaps over the Niagara Falls and reaches the level beneath, its mechanical force is lost as to form, but it is transmuted into heat. The water at the bottom of the fall is increased in temperature, and were this heat collected, it would be converted into mechanical power, exactly adequate to raise the water to its former level. The heat of explosion is converted into mechanical power when the ball is impelled from a gun. The mechanical power is reconverted into heat when the ball is suddenly arrested in its flight. The ball will be found to be hot exactly in proportion to its velocity when arrested. Now this is the case with the sun's heat. All the mechanical power employed by man can be traced to the sun. The water wheel is turned by the sun. Its heat raises the water from the ocean and deposits it in the form of rain on the mountain's side. The river collects the rain, fills the buckets of the water wheel and by this process the sun indirectly works the machinery of the mill. The steam engine is not an exception. Its power is derived from the heat of the furnace, but the furnace depends for its power on fuel. But how should fuel possess this power? It has derived it from the sun. The fuel as growing wood stored-up the power dispensed by the sun. The tree is the concentrated power of many summers' heat, and, though it may lie for thousands of years as coal in the bowels of the earth, it retains the power till it is evolved by burning. But you will say that animal power is surely different? Such is not the case. Every exercise of animal power costs some waste of tissue. That tissue is ultimately derived from vegetable matter, and the vegetable matter owes its power to the rays of the sun. Volition cannot create mechanical power; it can only direct and apply it. The only power not derived from the sun is that of the rise and fall of the tide, as far as this is due to the moon. The trade winds may also be regarded as an exception. This power is derived from the rotation of the earth, though the heat of the sun is necessary to develop the power.

The next point of interest is the combustion of the sun. It was long thought that the sun's combustion was totally different from that of all other bodies, and that by some mysterious process light and heat could be constantly given out without any loss. The principle of the correlation of physical force tends to the conclusion that there is a real loss of power; that the radiation of heat is like the pouring of water out of a cistern, and that, unless there are some means of supply, it must be exhausted. What is more, recent science has actually discovered well known substances in the incandescent atmosphere of the sun, bringing the flame into close analogy to terrestrial combustion. The following metals have already been detected in the state of vapor in the incandescent atmosphere of the sun:—Sodium, potassium, magnesium, iron, chromium and nickel. This has been accomplished by means of what is called spectrum analyses. The general principle is readily understood. It is the use of color as a test. You can often judge, simply by the color, as to what the nature of any substance is. When certain substances are put into the flame of a lamp, you can guess at the nature of the substances by the color of the flame. (Flames were exhibited of different colors, produced by the mixture of soda, potash, lime, strontia, with spirits of wine.) And by merely marking the shade of color you might form a good idea as to the substances which tinged the flame. Still this test would often fail, as the same color may result from the mixture of various substances. There may be various substances in the flame giving one compound color, and from this one color it would be impossible to discover the various substances. When, however, you view the flame through a prism with proper precautions, admitting the light only through a narrow slit, you find that the spectrum or colored image of the flame of each substance has a distinct pattern—has so many colored bands running across it with dark intervals between. Each substance is known by the color, number and position of the bands. If there are incandescent substances in the flame, the patterns of both are given, so that they may be at once distinguished. If the flame is supposed to become a solid, white, incandescent body, such as platinum, you get a spectrum with all the seven primitive colors, and they are quite continuous. There are no dark gaps, because the light is pure white, and comes from a solid body. There are dark gaps in the spectrum of a flame charged with incandescent particles in it, because the flame has not all the colors of white light. The sodium spectrum has only one yellow band, and all the other colors are wanting. Lithium has only a yellow and an orange band, with all the other colors wanting; and there is a dark gap between these two colored bands, because the intermediate shades of yellow and orange are wanting. The delicacy of this test transcends immeasurably all other tests. The thirty-millionth of a grain of sodium can be detected in a flame. If a bucketful of salt were thrown into Lake Ontario, and equally diffused, it could be detected in a bucketful of water drawn at any part of the lake. But how does all this bear on the chemistry of the sun?

How does this principle enable us to detect the substances in the solar atmosphere? It has been stated that a solid, white, incandescent body gives all the several colors with their innumerable shades. The sun gives this; and, if this were all, we would be entitled to conclude that the illuminating portion of the sun was also solid or fluid, for a fluid comports itself like a solid. But along with the perfect continuous spectrum there is a peculiar structure. The spectrum is striated with innumerable fine black lines, not uniformly distributed, but peculiarly grouped. Every color is thus striated, just as a rainbow would be striated if you held up between it and your eye the warp of a web, the threads running along the ribs of the bow. The interest of Kirchhoff and Bunsen's researches lies in the explanation given of these dark lines. They have shown that they are the reversed spectra of the incandescent substances in the vaporous atmosphere of the sun and that they are reversed or appear dark because they are seen on the brighter background of the white solid or fluid body of the sun. According to this theory, if the solid or fluid body of the sun were obliterated, while the vaporous incandescent atmosphere remained, all the black lines would become colored with their appropriate tints, and we could recognise the patterns with which we are so familiar when analyzing the substances diffused in the flame of a lamp. This theory is verified by actual experiment. When the brighter light of ignited lime or charcoal points is placed behind the flame of a lamp, the colored patterns give way to dark lines, which occupy the same place and preserve the same grouping. The colored bands in the spectrum of the flame extinguish the corresponding colors in the spectrum of the solid source of light, and replace them by corresponding dark lines. The color of the bars of a window is not visible when you look out upon the bright sky; they appear simply as black lines. And so do the colored lines of the spectra of the various substances appear dark when seen against the brighter spectrum of the solid source of light. By carefully examining the grouping of the dark lines in the sun's spectrum, and comparing them with the known colored patterns of various substances, the metals already enumerated have been detected. You might think it impossible to single out from innumerable dark lines the pattern of a certain metal, but the chemist can do this as readily as the sailor can single out the rig of his own ship from a forest of masts in the harbor. This spectrum analysis is one of the most brilliant achievements of our day, and will undoubtedly form an era in the history of chemistry. It has enabled chemistry to extend its dominion to the sun and stars. An interesting question in connection with the combustion of the sun is, How is it supplied with fuel? for it cannot dispense light and heat with undiminished intensity unless replenished with fuel. The old theory that the comets are the sun's fuel is revived in another form. The comet of Encke is gradually approaching the sun in a spiral course, and will ultimately fall into it. And, although no tendency to this result has, as yet, been detected in reference to the planets, there is little doubt that the same fate is reserved for them. This may be caused by a resisting medium, or it may be due to the repelling force exercised by the sun, which all comets show in a striking form, and which the analysis of M. Faye has proved to be explanatory of the shortening periods of Encke's comet. It is believed that the zones of meteorites, approaching the sun in a spiral course, like that of a comet, gradually supply the sun with the necessary material to keep up its heat; and this can be done, though these meteorites be not combustible. Their arrested motion would supply an adequate amount of heat. These zones of meteorites are closing in like the rings of Saturn upon the central body, for M. Struve's observations incontestably show that these rings are stretching out to the body of the planet. This spiral tendency is also illustrated by the spiral form of so many nebulae. And no one can look at these spirals without the conviction that there is progress towards a centre. But the sun's fuel is limited, and the combustion must at last cease. The researches of the German chemists lead to the conclusion that the photosphere is fluid, not gaseous. It cannot be conceived a continuous solid. It is also probable that the region of the incandescent metals in the state of vapor is the rose-colored stratum seen in total eclipses. It will be a matter of intense interest, on the occasion of the next total eclipse of the sun, to ascertain whether the characteristic colored bands of the metals are to be found in the rose-colored prominences and in the corona.

We have seen that science has distinctly traced the doom written on the solar system. It is destined to pass away. The machine is running down. The central fire will at last be exhausted. The planets and satellites in their spiral courses will come to a standstill. But are we to arrive at the conclusion that God's glory shall no longer be manifested in the heavens? or that this system is to rush into annihilation? No, there is no ground in science for the belief that a single particle of matter will ever be annihilated; but there is every ground for the belief that the passing-away of the solar system is only one phase of some grander revolution, and that from the ashes of the present system more glorious worlds and systems may arise. All this is in perfect, almost literal, accordance with the Scriptures, which represent the heavens as passing away as a scroll. "They shall wax old as a garment. As a vesture shalt thou change them, and they shall be changed." It represents the phenomenal world as ever changing—in a state of unceasing fluctuation—while the great absolute I AM remains ever the same. It is with a feeling of regret that we detect anything like imperfection or decay in the heavens. We would fondly cling to the belief that the celestial mechanism is imperishable, while all things change and decay on earth. But why should the heavens be an exception to the rule, that every structure and organism has only certain periods of existence? We do not think the flower that blossoms but for a day less beautiful, or manifesting God's wisdom less wondrously because it has but a brief period of existence. The wisdom of God is displayed in adapting its structure to the period of its existence, whether long or short.

And so in the heavens God's wisdom is displayed in so balancing and adjusting the solar system that it is admirably adapted to serve the temporary purpose for which it is intended. The constituent elements of the flower pass away for a time from view, but only to reappear in some other form, and fulfil perhaps some higher functions; and so it will undoubtedly be with the elements of the solar system. And is there not a great and important lesson taught by this fleeting character of even the grandest systems of the universe? It tells us that we seek in vain for something immutable and eternal in the shadows of material things. Amongst the ceaseless fluctuations of material phenomena it forces us to seek Him who is the same yesterday, to-day, and forever. To confer upon matter the attribute of immutability, and to stamp upon systems the attribute of eternity, would be to make the universe God. It would be to deify matter and material things; whereas the ever-changing character of all created things—of systems of worlds, as well as vegetable and animal organisms—is designed to point to the personal, living, unchangeable God, who is in all, through all, and above all. God spoke the worlds into being, and worlds and systems are but the written thoughts of God. But we have no reason to believe that God has spoken His last word, or that worlds and systems are not still to be evolved from chaos. The solar system may pass away, as a spoken sound fades upon the ear, but it is after all only one articulate utterance of the Almighty. Are there not yet tones to be uttered, chords to be struck, far surpassing any utterances that have yet been heard? The spirit is overwhelmed at the vast period of the solar system, the millions of years that may yet elapse before it reaches its final destiny, but in a higher state of being, and occupying a loftier eminence, this vast period will be only the turning of a single page in the history of the universe. Milton sublimely speaks of the skies as of the book of God wherein to read His glory; but, after all, it is only the hornbook of the beginner. There are other books to be opened, deeper mysteries to be fathomed ; and the heavens above us are only the preface of that greater roll which is to be unfolded to us when suitably prepared by our training on earth. Let us then reverentially read this book, believing that it is purposely designed to fit us for a higher state of being, where we shall see no longer in part, but when with open face we shall behold the full glory of God.

The lecturer in conclusion stated that since the last lecture it had been represented to him that an effort should be made in Kingston to do something to raise the Observatory to one of national importance before an appeal was made to other parts of Canada. He was ready to assent to this; but still, as the Observatory was not to be of a local character, it was but fair that other cities of Canada should contribute. Kingston had already contributed upwards of £300, independently of the recent cost of the building. Were suitable instruments provided, there would be a strong claim on Government to have the present inadequate grant increased, so as to secure a suitable staff of observers. The great interest manifested by the people of Kingston in the subject of this was an assurance that they would have a hand in founding an institution which would not only reflect credit upon the city but give to Canada a scientific position among the nations of the World.