Chapter II: Part 2

Imagine yourself a spectator, standing immovable in space. A luminous body appears in the distance, little by little you see it approach and increase in size; its immense circumference, which exceeds a hundred thousand leagues, is in rapid rotation, which makes each point on its periphery travel through nine miles a second. The globe itself passes before you, carried through space with a velocity twenty-four times greater than that of a cannon-ball. In such a way Jupiter would appear to you traveling in its orbit. This headlong course would banish it forever to the most remote regions of the visible universe, if it were not subdued and held by the powerful attraction of a globe a thousand times larger than its own—by the sun himself. Not only does astronomy show, by undeniable proofs, the reality of these marvelous movements—not only has she arrived at the knowledge of their invariable constancy, at least during thousands of centuries; but she has found in their very rapidity the cause of the stability of all the celestial bodies.

If there is difficulty in imagining such masses freely circulating in the ether, how much more are we impressed when we consider that these rapid movements are not confined to the planets; and when we look upon the sun with all his retinue as moving in an orbit yet unknown, himself attracted no doubt by a more powerful sun, or by a group of suns! All the stars which by reason of their infinite distances appear immovable, move in different directions; and we shall see later, that if these movements are performed with extreme slowness, the slowness is apparent only. In reality, these are the most rapid celestial movements that we know of.

Thousands of centuries will be necessary before these immense sidereal voyages are accomplished. Their vast periods are to the length of our year what the dimensions of the earth are to the distances of the stars; and, according to the happy expression of Humboldt, they make of the universe an eternal timekeeper. Thus, in the contemplation of celestial phenomena, the idea of infinite duration impresses itself on the mind with the same irresistible power as the idea of the infinity of space.

FOOTNOTES:

[1] Via Lactea. It is also called the Galaxy, from the Greek word for the same thing.

SPACE.—RICHARD A. PROCTOR

Although astronomy tells us in the clearest words of the vast depths of space which surround our earth on all sides, we are not thereby enabled to realize their enormous extension. It is not merely that the unknown depths beyond the range of our most powerful telescopes are inconceivable, but that the parts of space which we can examine are on too large a scale for us to conceive their real dimensions. It is hardly going too far to say that our powers of actual conception are limited to the extent of space over which the eye _seems_ to range in the daytime. Of course, in the daytime, at least in clear weather, there is one direction in which the eyesight ranges over a distance of many millions of miles—namely, where we see the sun. But the sense of sight is not cognizant of that enormous distance, and simply presents the sun to us as a bright disk in the sky, or perhaps rather nearer to us than the sky. Even the distance of the sky itself is underestimated. A portion of the light we receive from the sky on a clear day comes from parts of the atmosphere distant more than thirty or forty miles from us; but the eye does not recognize the fact. The blue sky seems a little further off than the clouds, but not much; the light clouds of summer seem a little, but not much, further off than the heavier clouds of a winter sky; a cloud-covered winter sky seems a little further off than heavy rain-clouds. The actual varieties of distance among clouds of various kinds are not much more clearly discerned than the actual varieties of distance among the heavenly bodies. The estimate formed of the distance of a cloud-covered sky overhead probably amounts to little more than a mile, and it is very doubtful whether the mind presents the remotest depths of a blue sky overhead at more than two miles. Toward the horizon the distance seems greater, and probably on a cloudy day the sky near the horizon is unconsciously regarded as at a distance of about five miles, while blue sky near the horizon may be regarded as lying at a distance of six or seven miles, the arch of a blue sky seeming to be far more deeply curved than that of a cloud-covered sky.

It is to distances such as these that the mind unconsciously refers the celestial bodies. We know that the moon is about 2,000 miles in diameter, but the mind refuses to present her to us as other than a round disk much smaller than those other objects in sight which occupy a much larger portion of the field of vision. The sun can not be conceived to exceed the moon enormously in size, seeing that he appears no larger; and all the multitude of stars are judged by the sight to be mere bright points of light in reality as they appear to be.

How, then, can we hope to appreciate the vastness of space whereof astronomy tells us? To the student of science attempting to conceive the immensities of whose existence he is assured, the same lesson might be taught in parable which the child of St. Augustine’s vision taught the Numidian theologian. As reasonably might an infant hope to pour the waters of ocean into a hollow, scooped with his tiny fingers in the sand, as man to picture in his narrow mind the length and breadth and depth of the abysses of space in which our earth is lost.

Yet, as a picture of a great mansion may be so drawn on a small scrap of paper as to convey just ideas of its proportions, so may the great truths which astronomy has taught us about the depths of space be so presented that just conceptions may be formed of the proportions of at least those parts of the universe which lie within the range of scientific vision, though it would be hopeless to attempt to conceive their real dimensions.

When we learn that a globe as large as our earth, suspended beside the moon, would seem to have a diameter exceeding hers nearly four times, so that the globe would cover a space in the heavens about thirteen times as large as the moon covers, we form a just conception of the size of the moon as compared with the earth, though the mind can not conceive such a body as the moon or the earth really is. When, in turn, we are told that if a globe as large as the earth, but glowing as brightly as the sun, were set beside the sun, it would look a mere point of light, we not only learn to picture rightly to ourselves how largely the sun exceeds the earth, but also how enormous must be the real distance of the sun.

Another step leads us to a standpoint whence we can form a correct estimate of the vast distance of the fixed stars; for we can learn that so enormous is the distance of even the nearest fixed star, that the tremendous space separating the earth from that star sinks in turn into the merest point, insomuch that if a globe as bright as the sun had the earth’s orbit as a close-fitting girdle, then this glorious orb (with a diameter of some 184,000,000 of miles) would look very much smaller than such a globe as our earth would look at the sun’s distance—would, in fact, occupy but about one-fortieth part of the space in the sky which she, though she would then look a mere point, would occupy if viewed from that distance.

But there is a way of viewing the immensities of space which, though not aiding us indeed to conceive them, enables the mind to picture their proportions better than any other. The dimensions of the earth’s path around the sun sink into insignificance beside those of the outermost planets; but these in their turn dwindle into nothingness beside those of some among the comets. From the path of these comets, if only sentient and reasoning beings could trace out in a comet’s company those mighty orbits, and could have for the duration of their existence not the brief span of time which measures the longest human life, but many circuits of their comet home around the same ruling orb (as we live during many circuits of our globe around the sun), the dimensions of the star-depths, which even to scientific insight are all but immeasurable, would be directly discernible. Not only would the proportions of that mighty system be perceived, whose fruits and blossoms are suns and worlds, but even the gradually changing arrangement of its parts could be discerned.

Some comets, indeed, do not travel around the sun, but flit from sun to sun on journeys lasting millions of years, paying each sun but a single visit. A being inhabiting such a comet, and having these interstellar journeys as the years of his existence, so that he could live through many of them, would have a wonderful insight into the economy of the stellar system. If his powers of conception as far exceeded ours as the range of his travels and the duration of his existence, he would be able to recognize the proportions of a large part of the stellar universe as clearly as we recognize the proportions of the solar system.

But leaving these wonderful wanderers, whose journeys are as far beyond our powers of conception as the immensity of the regions of star-strewn space, we may find, among the comets belonging to the sun’s domain, bodies whose range of travel would give their inhabitants far clearer views of the architecture of the heavens than even the profoundest terrestrial astronomer can possibly obtain.

Such a comet as Halley’s, for instance, though one of comparatively limited range in space, yet travels so far from the sun that, from the extreme part of its path, it sees the stars displaced nearly twenty times as much (owing to its own change of position) as they are from the earth on opposite sides of her comparatively narrow orbit. And the length of this comet’s year, if it indicated the lives of all creatures traveling along with it, would suggest a power of patiently watching the progress of changes lasting not a few of our years only, but for centuries. Seventy-five or seventy-six years elapse between each return of this comet to the sun’s neighborhood, and one who should have lived during sixty or seventy circuits of this body around its mighty orbit would have been able to watch the rush of stars, with their velocities of many miles per second, until visible displacements had taken place in their positions.

This, however, is as nothing compared with the mighty range in space and the enormous period of the orbit of the great comet of the year 1811. This comet is, on the whole, the most remarkable ever known. It was visible for nearly seventeen months, and though it did not approach the sun within 100,000,000 miles, and was therefore not subject to that violence of action which has caused enormous tails to be thrown out from comets which have come within a few million miles of him, or even within less than a quarter of his own diameter, it flourished forth a tail 120,000,000 of miles in length. Its orbit has, according to the calculations of the astronomer Argelander, a space exceeding the earth’s distance from the sun 211 times, and thus surpassing even the mighty distance of Neptune fully seven times. It occupies in circuiting this mighty path no less than 3,065 of our years (with a possible error either way of about forty-three years). So that, according to Bible chronology, this comet’s last appearance probably occurred during the rule of the Judge Tola, son of Puah, son of Dodo, over the children of Israel, though it may have occurred during the rule of his predecessor Abimelech, or during that of his successor Jair.[2] During one-half of the enormous interval between that time and 1811 the comet was rushing outward into space, reaching the remotest part of its path somewhere about the year 278 (A. D.), and from that time to 1811 it was on its return journey. It is strange to think, however, that though the remotest part of its path lay 211 times further from the sun than the earth’s orbit, yet even this mighty path, requiring more than 3,000 years for a single circuit, can not be said to have carried the comet into the star-depths. If the earth were to shift its position by the some enormous amount, the nearest fixed star would have its apparent position changed only by about an eighth part of the apparent diameter of the sun or moon, or by about one-quarter of the distance separating the middle star of the Bear’s tail from its close companion.

But this fact of itself is most strikingly suggestive of the vast distance of the stars. For consider what it means. Imagine the middle star of the Bear’s tail to be the really nearest of all the stars instead of lying probably twenty or thirty times further away. Conceive a comet belonging to that sun after making its nearest approach to it to travel away upon an orbit requiring 3,000 years for each circuit. _Then_ (supposing that star equal to our sun in mass) the comet, though rushing away from its sun with inconceivable velocity during 1,500 years, would, at the end of that vast period, seem to be no further away than one-fourth of the distance separating the sun from its near companion. Look at the middle star of the Bear’s tail on any clear night, and on its small satellite, remembering this fact, and the awful immensity of the star-depths are strongly impressed upon the mind. But the observer must not fail to remember that the star really is many times more remote than we have here for a moment supposed, and that such a comet’s range of travel would be proportionately reduced. Moreover, many among the stars are doubtless hundreds, even thousands, of times still further away.

Let us turn lastly to the amazing comet of the year 1744. We find that though it had the longest period of any which has ever been assigned to a comet as the result of actual mathematical calculation, yet its range in space would scarcely suffice to change the position of the stars in such sort that the aspect of the familiar constellations would be materially altered. Euler, the eminent mathematician, calculated for this comet a period of 122,683 years, which would correspond, I find, to a distance of recession equal to 2,469 times the distance of the earth from the sun, or about eighty times the distance of Neptune. Yet this is but little more than twelve times the greatest distance of the comet of 1811. Probably the actual range of such an orbit from the middle star of the Bear’s tail would be equal in appearance to the range described above on the supposition that the star is no further from us than the nearest known star (Alpha Centauri). That is, such a comet, if it could be seen and watched during a period of about 122,000 years, would seem to recede from the star to a distance equal to about one-fourth the space separating it from its close companion, and then to return to the point of nearest approach to its ruling sun.

Such are the immensities of star-strewn space! The journey of a comet receding from the sun with inconceivable velocity during hundreds of thousands of years carries it but so small a distance from him compared with the distance of the nearest star as scarcely to change the appearance of the celestial landscape; and yet the distances separating the sun from the nearest of his fellow suns are but as hairbreadths to leagues when compared with the proportions of the scheme of suns to which he belongs. These distances, though so mighty that by comparison with them the inconceivable dimensions of our own earth sink into utter nothingness, do not bring us even to the threshold of the outermost court of that region of space to which the scrutiny of our telescopes extends. Yet the whole of that region is but an atom in the infinity of space.

FOOTNOTES:

[2] It might be suggested that the appearance of this blazing comet among the stars drove the more superstitious of the Israelites at that time to the worship of star-gods, as we read how, during the Judgeship of Jair, they “served Baalim and Ashtaroth, and the gods of Syria and the gods of Moab, and the gods of the Philistines, and forsook the Lord and served not Him.” To a people like the Jews, who seem to have been in continual danger of returning to the Sabaistic worship of their Chaldean ancestors, the appearance of a blazing comet may have been a frequent occasion of backsliding.

EXTENT OF THE SIDEREAL HEAVENS.—SIR ROBERT S. BALL

Of all the discoveries that have ever been made in science there are two which especially baffle our powers of comprehension. They lie at the opposite extremes of nature. One relates to objects which are infinitely small, the other relates to objects which are almost infinitely great. The microscope teaches us that there are animals so minute that if a thousand of them were ranged abreast they would easily swim without being thrown out of line through the eye of the finest cambric needle. Each of those minute creatures is a highly organized number of particles, capable of moving about, of finding and devouring its food, and of behaving in all other respects as becomes an animal as distinguished from an unorganized piece of matter. The mind is capable of realizing the structure of these little creatures, and of fully appreciating their marvelous adaptation to the life they are destined to lead. If these animals excite our astonishment by reason of their extreme minuteness, there is an appeal made to conceptions of an entirely different character when we learn the lessons which the telescope teaches. As the microscope reveals the excessively minute, so does the telescope disclose the sublimely great. In each case myriads of objects are submitted to our astonished view, but while the microscope brings before us creatures of which countless millions could swim about freely in a thimbleful of water, the telescope conducts our vision to uncounted legions of stars, many of them millions of times larger than the earth.

The grandest truth in the whole of nature is conveyed in that first lesson in astronomy which answers the question: What are the stars? This is a question that a child will ask, and I have heard of a child’s pretty idea that the stars were little holes in the sky to let the glory of heaven shine through. The philosopher will replace this explanation by another hardly less poetical, which will enable us to form some more adequate notion of the real magnificence of the universe. Each star that we see is, it is true, only a glittering little point of light, but that is merely because we are a long way from it. An electric light which will dazzle your eye when quite close will be reduced to an agreeable illumination if it is at a little distance, will become a faint light a mile away, and at no great distance will become altogether invisible. We must remember that out in space there is plenty of room—there are no bounds; and therefore when we see light glistening in the far distant depths we can not at once conclude that the light is a faint one because it appears to us to be faint. It may be that the light is only faint because it comes from such a tremendous distance. In fact, the brightest light conceivable could be reduced to the insignificance of a small star if only it were removed sufficiently far.

The most intense light we know of comes, of course, from the light which rules by day, from our sun himself. The sun pours his unrivaled beams around us in all directions with prodigal abundance, notwithstanding his enormous distance of ninety-three millions of miles. Let me describe an experiment with respect to our sun, an experiment, it is needless to say, which could never be performed, but the results to which it leads us are none the less certain. Astronomers have demonstrated them in many other ways.

Suppose that the sun were gradually to be moved away further and further into space; suppose that by this time to-morrow the great luminary should be twice as far as it is now, and the next day should be three times as far, and the day after that four times, and so on until in a year’s time we should find that the sun was 365 times the distance from us that it is at present. Let us now trace the changes which we should see in the brilliancy of our orb of day. When he had reached double his distance from us, we should find that the light had decreased to a quarter of its present amount, and the heat which we derived from his beams would have decreased in the same proportion. In ten days we should find that the light had become so feeble as to be only one-hundredth part of that which we enjoy now. The apparent size of the sun would also be steadily decreasing, for as the distance of a body increases its apparent dimensions diminish. Sometimes the diminution of apparent size with distance is well illustrated on a clock tower. You would hardly believe that the hands and face of a clock like that at Westminster were so large until you happen to see a man cleaning or repairing it, when he appears a mere pigmy in comparison with the mighty dial which points out the hours. In a similar way with every increase of distance, the apparent size of the sun would decline, and in the lapse of a year the sunlight would be reduced to a feeble twilight. The sun itself would remain visible for many years, even if it were steadily moving away, though its lustre would continually decline, and its size would continually diminish, until at last it would have shrunk to the insignificance of a small point of light, still visible as a glittering object, but too minute to enable any definite form to be perceived.

Further still, the sun might recede until it passed beyond the reach of vision of the unaided eye; the telescope would, however, be able to pursue the retreating luminary until at last it sank into the depths of space beyond the reach of any instrument whatever.

This little argument will prepare us for an explanation of the stars. They merely appear to us to be points of light of varying degrees of brightness, but we have seen that our own sun might be reduced in lustre to that of the very dimmest of the stars if only it were removed sufficiently far. If, therefore, the stars are at a great enough distance from our system, it may indeed be that they also are suns, possibly equaling, or possibly even surpassing, our own sun in magnificence.

Here is indeed an imposing suggestion. Can it be that the host of stars which adorn our midnight sky are actually suns themselves of an importance comparable with that of our own? This is a great thought, and we desire to test it by every means in our power. You will see from the reasoning I have given that the whole question turns simply on one point, and that is: How far off are the stars?

The tiniest point of light that is just seen as a glimmer in the mightiest of telescopes may be indeed a sun as great, or indeed a million times greater, than our sun, if only that star be sufficiently far off. To find the distance of a star is a problem which taxes the utmost powers of the painstaking astronomer; every refinement of skill in making his measurements and of care in the calculation of his observations have to be lavished on the operation. Alas! it but too often happens that the astronomer’s labors prove to be futile. The surveying navigator often has to mark on his chart that no bottom could be found in the depths of the sea. His appliances would not work, or work reliably, in those ocean abysses; so, too, the astronomer, when he tries to sound the depths of space to the distances of the stars, has also to mark, generally speaking, “No bottom here,” as the result of most of his investigations. When this is the case we know for certain that the star on which his calculations have been made must be a gorgeous sun, because we are assured of the greatness of its distance, even though we have not been able to find out what that distance was. There are, however, some few places through the sky where the astronomer’s sounding line can, so to speak, touch bottom; there are a few stars of which we do know the distance, and the result is not a little significant. Were our sun to be withdrawn from us to a distance so great as that of the very nearest of the stars, our magnificent ruler and benefactor would certainly have lost all his splendor; he would, in fact, have shrunk to the similitude of a little star not nearly so bright as many of those which we see over our heads every night. Imagine the sun’s light subdivided into two hundred thousand parts, each of which would give us only a feeble illumination, and then imagine that each of these parts was again divided into two hundred thousand parts more, and it is one of these last fragments that would represent the miserable lustre which the sun would then display.

From these considerations we can enunciate the magnificent truth which astronomy discloses to us. I do not think that in the whole range of nature there is any thought so magnificent or so imposing as that which teaches us to regard every star of every constellation as a sun. We can not indeed assert that they are all so great as our sun, but we can affirm with certainty that many of them are far greater and far more splendid. Considering that our sun presides over a system of worlds of which the earth is one, that it gives light and heat to those worlds, and guides them in their movements, it would greatly enlarge our conceptions of the universe if we were assured that there was even one more sun as large and as splendidly attended as is our own. But now we find that not only is there one additional sun, but that they teem in uncounted thousands through space. Look, for example, on the next fine night at the Great Bear, the best known of all our northern constellations, and there you see seven stars forming the well-known feature. Figure in your mind’s eye each one of those stars in the likeness of a majestic sun, as big, warm, and bright as our sun, and look at other parts of the sky and repeat the process with the other constellations, and your conception of the magnificence of the starry system will begin to assume proper proportions. But this is only the first step, you must next look at the smaller stars, and reflect that they, too, are also suns, only much further off as a general rule than the brighter stars, though this is by no means invariably the case. Thus your estimate of the number of suns in the universe will rise to thousands, but you will not stop there, you will get a telescope to help you, and, to your extreme delight and wonder, you will find that there are hosts of stars—too faint to be visible to the eye, but which the telescope will immediately disclose. You will get a more powerful instrument, and then you will perceive that the stars are to be numbered by tens of thousands, and even by millions, and with every fresh accession of power in your telescope fresh troops and myriads of suns are revealed. Suns in clusters, suns strewn thickly here and sparsely there, so as to give us the notion that the only limit to the number we can see is the power of the telescopes we are using. Attempts at actual numeration are futile, for who can tell the number of the stars?

We can, however, form an estimate, and by taking samples, so to speak, of the sky here and other samples there, we have been enabled to learn the overwhelming fact that our universe does contain at the very least one hundred millions of suns.

In discussing the extent of the visible universe, it must always be borne in mind that the further a source of light is from us the fainter is the illumination which we receive from it. Suppose that a star which just lies on the limits of naked-eye visibility were somehow to be transported to a distance which is twice as great, then the lustre of that star would be diminished to one-fourth of its original amount. It would, therefore, be of course invisible to the unaided eye, but could still be easily perceived by a telescope. Indeed, the very word _telescope_ means an instrument for looking at objects a long way off, and the effect of the telescope is to reduce the apparent distance of the object.

The bulk of a grain of sand as compared with the bulk of a football may illustrate the space accessible to our eyes when compared with the space accessible to one of the great telescopes. The larger of these spaces has a thousand times the diameter of the others; therefore, the relative quantities of these spaces are to be obtained by multiplying 1,000 by 1,000 and by 1,000 again. Thus we finally learn that the amplitude of our vision is augmented to one thousand million times its original extent by the use of our greatest telescopes. It need, therefore, be no matter for surprise that the number of stars visible through our great telescopes or recorded on the sensitive films of photographic plates should number scores of millions. In fact, it would sometimes seem surprising that the number of telescopic stars is not even greater than it actually appears to be. If we are able to explore one thousand million times as much space, we might expect that the number of objects disclosed would be also increased about a thousand million-fold, but this is certainly not the case. The truth seems to be that our sun is but one star of a mighty cluster of stars; we happen to lie near the middle of the cluster, and the rest of the stars belonging to it form what we know as the Milky Way. There are, of course, other clusters scattered through the heavens, some of them, perhaps, as great as that body of stars which forms the Milky Way. Owing to our residence in this cluster we see the neighboring suns in multitudes, and thus we receive the impression that the solar system lies in an exceptionally rich part of the universe in as far as the distribution of stars is concerned.

On the outskirts of the universe lie those faintest and dimmest of objects which we can just perceive through our greatest telescopes. We know that many of the stars around us would still remain visible in great instruments, even though they were removed a thousand times as far off. Among the myriads of faint stars which we see from our observatories, there may be many, indeed there must be many, which are fully a thousand times as distant as the bright stars which twinkle in our comparative neighborhood. We thus obtain some conception of the stupendous distance at which the outskirts of the universe are situated.

There are different ways of illustrating this point, but I think the simplest, as well as the most striking, is that which is founded on the velocity of light. It is a remarkable fact that the beautiful star known as Vega[3] has a distance from us so tremendous that its light must have taken somewhere about eighteen years to travel hither from thence. Notwithstanding that the light dashes along with such inconceivable speed that it will cover 185,000 miles in every second, notwithstanding that a journey at this pace will complete the entire circuit of this globe seven or eight times between two successive ticks of the clock, the light will, nevertheless, take eighteen years to reach our eye from the time it leaves Vega. We do not, therefore, see the star as it is at present; we see it as it was eighteen years ago. For the light which this evening enters our eyes has been all that time on its journey. Indeed, if Vega were actually to be blotted out from existence it would still continue to shine out as vividly as ever for eighteen years before all the light on its way had reached us.

We have been led to the belief that among the more distant stars in the universe there must be many which are fully a thousand times as far from us as is Vega, hence we arrive at the startling conception that the light they emit has been on its journey for 18,000 years before it reached us. When we look at those lights to-night we are actually viewing them as they were 18,000 years ago. In fact, those stars might have totally vanished 17,000 years ago, though we and our descendants may still see them glittering for yet another thousand years.

We shall realize a little more fully what this reasoning involves if we suppose that astronomers dwelt on such a star, and that they had eyes and telescopes sufficiently keen not only to discern our little earth, but even to scrutinize its surface with attention. Let us suppose that the stellar astronomers looked at England: do you think they would see a network of railways joining mighty and populous cities, furnished with immense manufactories and with countless institutions? Such would be the England of to-day. But from the distance at which these astronomers are situated light takes 18,000 years for its journey, and, therefore, what they would see would be England as it was 18,000 years ago. To them England would even now appear as a country mainly covered with forests inhabited by bears and wolves, and totally void of any trace of civilization. This illustration will, at all events, serve to convey some conception of the distance at which the outskirts of our visible universe are plunged in the depths of space.

FOOTNOTES:

[3] Vega is the brightest star in the Lyre and is nearly always at night directly overhead in our latitude.—E. S.

THE STARS.—AMÉDÉE GUILLEMIN

No sight is at once so awe-inspiring and so grand as that of the heavens on a beautiful night. If care be taken to choose as a standpoint for observation an open place, such as a plain or the summit of a hill on land, or, again, the open sea, and if the atmosphere, somewhat charged with dew, possesses all its transparency and purity, we shall see thousands of luminous points twinkling in all directions, accomplishing slowly and together their silent march. The contrast of the obscurity which reigns on the surface of the earth with the brightness of that resplendent vault gives an indefinite depth to the celestial ocean that deepens over our heads. But let us here leave the magnificence of the spectacle to study it in its most minute details.

Let us commence with the appearances. A characteristic common to all the stars is an incessant and very rapid change of brightness, which has received the name of _scintillation_. This is accompanied by variations of color equally rapid, due to the same cause as the successive disappearances and reappearances. All stars scintillate, whatever may be their brilliancy, at least in our temperate regions. But the intensity of this luminous movement is not the same in all, and it varies, moreover, both with the degree of purity of the sky, the elevation of the stars above the horizon, and the temperature of the night.

According to Arago, scintillation is due to the difference of velocity of the various colored rays traversing the unequally warm, unequally dense, unequally humid atmospheric strata. Thus, in tropical regions, where the atmospheric strata are more homogeneous, scintillation is rarely observed in stars the elevation of which above the horizon is more than 15°, or the sixth of the distance of the horizon from the zenith. “This circumstance,” says Humboldt, “gives to the celestial vault of these countries a particularly calm and soft character.”

Another specific character of the stars is that their diameters are without appreciable dimensions. To the naked eye, this distinction would be insufficient, since, the moon and the sun excepted, the most considerable planets have not sensible diameters. But, while the magnifying power of optical instruments shows us the principal planets under the form of clearly defined disks, the most powerful glasses only show a star as a luminous point. The distance which separates us from these bodies is so great that there is nothing to astonish us in such a result.

Wollaston affirms that the apparent diameter of the most brilliant star in the heavens, Sirius, is not more than the fiftieth part of a second of an arc. But let us hasten to say that this result still leaves a good margin as to the real dimensions of the star, since, at the distance of Sirius, an apparent diameter would represent a real diameter of 11,000,000 miles; that is, twelve times the diameter of our sun.

Let us add, lastly, that the absence of appreciable dimensions does not suffice to distinguish absolutely the stars from the planets, since a certain number of the latter, as we have before seen, appear in telescopes only as simple luminous points. Let us come, then, to a permanent specific characteristic, the knowledge of which will always prevent us from confounding a star with one of the known or unknown bodies which form part of our solar group. This characteristic is as follows:

The stars, properly so called, preserve among themselves—nearly enough for our present purpose—the same relative distances. They form, then, on the celestial vault apparent groups, the configuration of which is nearly invariable. Centuries must elapse to show a change of form, unless we employ extremely delicate measures. A planet, on the contrary, moves rapidly across these groups, to such a degree that, in the interval of a night, or at most of a few nights, this displacement is very perceptible; hence the old denomination of _fixed stars_, in opposition to the _wandering_ ones, or planets.

We must be careful, however, to guard against assigning to this word a rigidity which it does not possess, for the stars really move with a velocity not inferior to that which animates the members of our system. Their immense distance is the only cause of their apparent immobility, which vanishes when precise observations, embracing a sufficient interval of time—some years, for example—are made.

A fact which strikes every one is the great diversity of brightness in the stars which people the heavens. All degrees of intensity are remarked, from the resplendent light of Sirius to the scarcely perceptible glimmer of those hardly visible to the naked eye.

Whence arises this difference of brightness? This question we can not answer for any star in particular, but it is easy to imagine that it may result from various circumstances, such as their less or greater distance, the real and various dimensions of the bodies, and, lastly, the intrinsic brightness of the light peculiar to each. However this may be, astronomers without regard to the unknown causes which may influence the intensity of the stellar light, have divided stars into _classes_ or _magnitudes_; and when we speak of a star of the first, second, or fifth magnitude, it is understood that this way of speaking refers only to the apparent brightness, and that nothing is affirmed either as to the real dimensions or distance, or even intrinsic brightness.

Besides, as the stars, arranged in the order of their brightness, would form a progression decreasing by imperceptible degrees, the classes adopted are themselves conventional and arbitrary. The first six magnitudes comprise all stars visible to the naked eye. But the use of the most powerful telescopes brings to view stars of feebler light, descending to the sixteenth and seventeenth magnitudes. In truth, the progression has no inferior limit: it extends more and more in proportion as the progress of the optician’s art increases the penetrating power of our instruments.

To gain an idea of the respective intensities of the light emitted by the stars of the first six magnitudes, following the scale adopted by astronomers, the accompanying illustration (Fig. 1), should be inspected; in it the stars are figured by disks, the surfaces of which are in proportion to their brilliancy.

But, we repeat, it must not be thought that the stars ranked in the same class are, on that account, of the same brightness. Thus the light of Sirius is estimated at four times the star Alpha Centauri; but both, nevertheless, are included by astronomers in the number of the stars of the first magnitude.

We here give the names of the twenty most brilliant stars of the two hemispheres which it is usual to consider as forming the first class. They are here arranged in the order of their brightness:

1. Sirius
2. Eta Argus
3. Canopus
4. Alpha Centauri
5. Arcturus
6. Rigel
7. Capella
8. Vega
9. Procyon
10. Betelgeuse
11. Achernar
12. Aldebaran
13. Beta Centauri
14. Alpha Crucis
15. Antares
16. Altair
17. Spica
18. Fomalhaut
19. Beta Crucis
20. Pollux

Lastly, Regulus, a bright star in the constellation of the Lion, is also ranked by some astronomers in the first magnitude, while others only admit in this class the first seventeen stars in the above list. These divergences are of no importance.

In proportion as the scale of brilliancy or magnitude is descended, the number of the stars contained in each class rapidly increases. The number of second magnitude stars in the heavens is about 65; of the third, about 200; of the fifth, 1,100; and of the sixth magnitude, 3,200. Adding these numbers together, we obtain a few over 5,000 stars of the first six magnitudes, and these comprise very nearly all those that can be seen with the naked eye.

The smallness of this number nearly always astonishes those who have not tried to form an exact estimate of the number of stars which shine in the celestial vault on the most favorable nights.

The aspect of the multitude of sparkling points which are scattered over the sky makes us disposed to believe that they are innumerable, and to be counted, if not by millions, at all events by hundreds of thousands. This is, nevertheless, an illusion. All observers who have taken the trouble to make an exact enumeration of the stars visible to the naked eye have arrived at a maximum of 3,000 as the mean number which can be observed in every part of the heavens, visible at the same time, at the same place; this, of course, is but half of the entire heavens.

Argelander has published an exact catalogue of the stars visible on the horizon of Berlin during the course of the year. This catalogue comprises 3,256 stars. According to Humboldt, there are 4,146 visible on the horizon of Paris in the whole course of the year; and as this number increases in proportion as we approach the Equator, that is to say, in proportion as the double movement of the earth unfolds to us during a year a more extensive portion of the heavens, 4,638 stars are already visible to the naked eye on the horizon of Alexandria.

We repeat, the maximum number is comprised between 5,000 and 6,000 stars for the entire heavens, including those seen by the most piercing and most accustomed eyes in the best nights for observation. When the atmosphere is lit up by the moon, or by twilight, or, as happens in the great centres of population, by the illumination of the houses and streets, the lowest magnitude stars are effaced altogether, and the number of those visible is consequently much more limited. We may add in conclusion, that the more the scintillation, the more easy it is to distinguish very faint stars.

A word now on the number of stars that can be seen with the help of the telescope. Here we shall find the numbers which our imagination had erroneously led us to believe are visible to the naked eye.

According to the illustrious director of the Observatory of Bonn—Argelander—the seventh magnitude comprises nearly 13,000 stars; the eighth, 40,000; and, lastly, the ninth, 142,000. The calculations of Struve give the total number of stars visible in the entire heavens by the aid of Sir William Herschel’s 20-foot reflector as more than 20,000,000. But, without doubt, these approximate numbers are much below the real ones. It will be seen, besides, that the richness of the heavens in stars is very unequal. The bright zone known under the name of the Milky Way alone contains, according to Herschel, 18,000,000.

THE LUCID STARS.—J. E. GORE

The term “lucid” has been applied to the stars visible to the naked eye, without optical aid of any kind.[4] Many people think that the number of stars visible in this way is very large. But in reality the number visible to the naked eye is comparatively small. Some persons are, of course, gifted with very keen eyesight—“miraculous vision” it is sometimes called—and can see more stars than others; but to average eyesight the number visible in this way, and which can be individually counted, is very limited. The famous Hipparchus formed a catalogue of stars in the year 127 B. C. This presumably contained all the most conspicuous stars he could see in his latitude, and it includes only 1,025 stars. Al-Sûfi, the Persian astronomer, in his _Description of the Fixed Stars_, written in the Tenth Century, describes the positions of only 1,018 stars, although he refers to a number of other faint stars, of which he does not record the exact places. Pliny thought that about 1,600 stars were visible in the sky of Europe.

In modern times, however, a considerable number of fainter stars have been recorded as visible to the naked eye. The famous German astronomer, Heis, who had keen eyesight, records the positions of 3,903 stars north of the Equator, and 1,040 between the Equator and 20 degrees south declination, or a total of 4,943 stars between the North Pole and 20 degrees south of the Equator. This would, I find, give a total of about 7,366 stars for both hemispheres if the stars were equally distributed. Behrmann, in his _Atlas of Southern Stars_, between 20 degrees south declination and the South Pole, shows 2,344 stars as visible to the naked eye. This would give a total of 7,124 for both hemispheres. The actual number seen by Heis and Behrmann in both hemispheres is 4,943 + 2,344, or 7,287 stars. The Belgian astronomer, Houzeau, published a catalogue and atlas of the stars in _both_ hemispheres, made from his own observations in Jamaica and South America, and finds a total of 5,719 stars in the whole sky. As all these observers had good eyesight, we may take a mean of the above results as the total number visible to the naked eye in the whole star sphere. This gives 6,874 stars, or in round numbers we may say that there are about 7,000 stars visible to average eyesight in both hemispheres. This gives, of course, about 3,500 stars to one observer at the same time at any point on the earth’s surface.

As the whole star sphere contains an area of 41,253 square degrees, we have an average of one star to six square degrees. In other words there is, _on an average_, one lucid star in a space equal to about thirty times the area covered by the full moon! This result may seem rather surprising considering the apparently large number of stars visible to the naked eye on a clear night, but the fact can not be denied. The stars are not, of course, equally distributed over the surface of the sky, but are gathered together in some places, and sparsely scattered in others, and this may perhaps help to give the impression of a greater number than there really are.

That the stars are of various degrees of brightness was recognized by the ancient astronomers. Ptolemy divided them into six classes, the brightest being called first magnitude, those considerably fainter the second, those much fainter still the third, down to the sixth magnitude, which were supposed to be the faintest just visible to the naked eye on a clear moonless night. Ptolemy only recorded whole magnitudes, but Al-Sûfi, in the Tenth Century, divided these magnitudes, for the first time, into thirds. Thus a star slightly less than an average star of the second magnitude he called 2—3, that is nearer in brightness to 2 than to 3; one a little brighter than the third he recorded as 3—2, or nearer to 3 than to 2, and so on. This method has been followed by Argelander, Behrmann, Heis, and Houzeau, but in the photometric catalogues of Harvard, Oxford, and Potsdam the magnitudes are measured in decimals of a degree. This has been found necessary for greater accuracy, as the heavens contain stars of all degrees of brightness.

The term “magnitude” means the ratio between the light of a star of a given magnitude and that of another exactly one magnitude fainter. This ratio has been variously estimated by different astronomers, and ranges from 2.155, found by Johnson in 1851, to 3.06, assumed by Pierce in 1878. The value now universally adopted by astronomers is 2.512 (of which the logarithm is 0.4). This number is nearly a mean of all the estimates made, and agrees with the value found by Pogson in 1854 by means of an oil flame, and by Rosen with a Zöllner photometer in 1870. It simply means that an average star of the first magnitude is 2.512 times the brightness of a star of the second magnitude; a star of the second, 2.512 times brighter than one of the third, and so on. This makes a star of the first magnitude just 100 times brighter than one of the sixth.

There are several stars brighter than an average star of the first magnitude, such as Aldebaran. These are Sirius, which is nearly 11 times brighter than Aldebaran (according to the revised measures at Harvard); Canopus, the second brightest star in the heavens, and about two magnitudes brighter than Aldebaran; Arcturus, Capella, Vega, Alpha Centauri, Rigel, Procyon, Alpha Eridani, Beta Centauri, and Alpha Orionis. Al-Sûfi rated 13 stars of the first magnitude, visible at his station in Persia, and Halley enumerates 16 in the whole sky. According to the Harvard photometric measures, there are 13 stars in both hemispheres brighter than Aldebaran, which is rated 1.07.

As average stars of the different magnitudes the following may be taken as examples, derived from the Harvard measures: First magnitude, Aldebaran and Spica; second magnitude, β Aurigæ and β Canis Majoris; third magnitude, ι Aurigæ and β Ophiuchi; fourth magnitude, θ Herculis and ε Draconis; and fifth magnitude, ρ Ursæ Majoris and ω Sagittarii. Stars of about the sixth magnitude are, of course, numerous, and lie near the limit of naked-eye vision for average eyesight, although on clear moonless nights still fainter stars may be “glimpsed” by keen-eyed observers.

The stars have been divided into groups and constellations, now chiefly used for the purpose of reference, but in ancient times they were associated with the imaginary figures of men and animals, etc. The origin of these constellation figures is doubtful, but they are certainly of great antiquity. Ptolemy’s constellations were 48 in number, but different writers from the First Century B. C. give different numbers, ranging from 43 to 62. Bayer’s _Uranometria_, published in 1603, contains 60, 12 new constellations in the Southern Hemisphere having been added by Theodorus to Ptolemy’s original 48.