"During the holy days of the solemnization of Easter, a phenomenon ever fatal, and of gloomy foreboding, appeared in the heavens. As soon as the Emperor, who paid attention to these phenomena, received the first announcement of it, he gave himself no rest until he had called a certain learned man and myself before him. As soon as I arrived, he anxiously asked me what I thought of such a sign; I asked time of him, in order to consider the aspects of the stars, and to discover the truth by their means, promising to acquaint him on the morrow; but the Emperor, persuaded that I wished to gain time, which was true, in order not to be obliged to announce anything fatal to him, said to me: 'Go on the terrace of the palace and return at once to tell me what you have seen, for I did not see this star last evening, and you did not point it out to me; but I know that it is a comet; tell me what you think it announces to me.' Then scarcely allowing me time to say a word, he added: 'There is still another thing you keep back; it is that a change of reign and the death of a prince are announced by this sign.' And as I advanced the testimony of the prophet, who said: 'Fear not the signs of the heavens as the nations fear them,' the prince with his grand nature, and the wisdom which never forsook him, said, 'We must not only fear Him who has created both us and this star. But as this phenomenon may refer to us, let us acknowledge it as a warning from Heaven."
Louis le Debonnaire gave himself and his court to fasting and prayer, and built churches and monasteries. He died three years later, in 840, and historians have profited by this slight coincidence to prove that the appearance of the comet was a harbinger of death. The historian, Raoul Glader, added later: "These phenomena of the universe are never presented to man without surely announcing some wonderful and terrible event."
Halley's comet again appeared in April, 1066, at the moment when William the Conqueror invaded England. It was pretended that it had the greatest influence on the fate of the battle of Hastings, which delivered over the country to the Normans.
A contemporary poet, alluding probably to the English diadem with which William was crowned, had proclaimed in one place, "that the comet had been more favorable to William than nature had been to Caesar; the latter had no hair, but William had received some from the comet." A monk of Malmesbury apostrophized the comet in these terms: "Here thou art again, thou cause of the tears of many mothers! It is long since I have seen thee, but I see thee now, more terrible than ever; thou threatenest my country with complete ruin!"
In 1455, the same comet made a more memorable appearance still. The Turks and Christians were at war, the West and the East seemed armed from head to foot—on the point of annihilating each other. The crusade undertaken by Pope Calixtus III. against the invading Saracens, was waged with redoubled ardor on the sudden appearance of the star with the flaming tail. Mahomet II. took Constantinople by storm, and raised the siege of Belgrade. But the Pope having put aside both the curse of the comet, and the abominable designs of the Mussulmans, the Christians gained the battle, and vanquished their enemies in a bloody fight. The Angelus to the sound of bells dates from these ordinances of Calixtus III. referring to the comet.
In his poem on astronomy, Daru, of the French Academy, describes this episode in eloquent terms:
"Un autre Mahomet a-t-il d'un bras puissant Aux murs de Constantine arbore le croissant: Le Danube etonne se trouble au bruit des armes, La Grece est dans les fers, l'Europe est en alarmes; Et pour comble d'horreur, l'astre au visage ardent De ses ailes de feu va couvrir l'Occident. Au pied de ses autels, qu'il ne saurait defendre, Calixte, l'oeil en pleurs, le front convert de cendre, Conjure la comete, objet de tant d'effroi: Regarde vers les cieux, pontife, et leve-toi! L'astre poursuit sa course, et le fer d'Huniade Arrete le vainqueur, qui tombe sous Belgrade. Dans les cieux cependant le globe suspendu, Par la loi generale a jamais retenu, Ignore les terreurs, l'existence de Rome, Et la Terre peut-etre, et jusqu'au nom de l'homme, De l'homme, etre credule, atome ambitieux, Qui tremble sous un pretre et qui lit dans les cieux."
This ancient comet witnessed many revolutions in human history, at each of its appearances, even in its later ones, in 1682, 1759, 1835; it was also presented to the Earth under the most diverse aspects, passing through a great variety of forms, from the appearance of a curved sabre, as in 1456, to that of a misty head, as in its last visit. Moreover, this is not an exception to the general rule, for these mysterious stars have had the gift of exercising a power on the imagination which plunged it in ecstasy or trouble. Swords of fire, bloody crosses, flaming daggers, spears, dragons, fish, and other appearances of the same kind, were given to them in the middle ages and the Renaissance.
Comets like those of 1577 appear, moreover, to justify by their strange form the titles with which they are generally greeted. The most serious writers were not free from this terror. Thus, in a chapter on celestial monsters, the celebrated surgeon, Ambroise Pare, described the comet of 1528 under the most vivid and frightful colors: "This comet was so horrible and dreadful that it engendered such great terror to the people, that they died, some with fear, others with illness. It appeared to be of immense length, and of blood color; at its head was seen the figure of a curved arm, holding a large sword in the hand as if it wished to strike. At the point of the sword there were three stars, and on either side was seen a great number of hatchets, knives, and swords covered with blood, amongst which were numerous hideous human faces, with bristling beards and hair."
The imagination has good eyes when it exerts itself. The great and strange variety of cometary aspects is described with exactitude by Father Souciet in his Latin poem on comets. "Most of them," says he, "shine with fires interlaced like thick hair, and from this they have taken the name of comets. One draws after it the twisted folds of a long tail; another appears to have a white and bushy beard; this one throws a glimmer similar to that of a lamp burning during the night; that one, O Titan! represents thy resplendent face; and this other, O Phoebe! the form of thy nascent horns. There are some which bristle with twisted serpents. Shall I speak of those armies which have sometimes appeared in the air? of those clouds which follow as it were along a circle, or which resemble the head of Medusa? Have there not often been seen figures of men or savage animals?
"Often, in the gloom of night, lighted up by these sad fires, the horrible sound of arms is heard, the clashing of swords which meet in the clouds, the ether furiously resounding with fearful din which crush the people with terror. All comets have a melancholy light, but they have not all the same color. Some have a leaden color; others that of flame or brass. The fires of some have the redness of blood; others resemble the brightness of silver. Some again are azure; others have the dark and pale color of iron. These differences come from the diversity of the vapors which surround them, or from the different manner in which they receive the Sun's rays. Do you not see in our fires, that various kinds of wood produce different colors? Pines and firs give a flame mixed with thick smoke, and throw out little light. That which rises from sulphur and thick bitumen is bluish. Lighted straw gives out sparks of a reddish color. The large olive, laurel, ash of Parnassus, etc., trees which always retain their sap, throw a whitish light similar to that of a lamp. Thus, comets whose fires are formed of different materials, each take and preserve a color which is peculiar to them."
Instead of being a cause of fear and terror, the variety and variability of the aspect of comets ought rather to indicate to us the harmlessness of their nature.
THE TOTAL SOLAR ECLIPSE OF 1883
AN ASTRONOMERS VOYAGE TO FAIRY-LAND.
(FROM THE ATLANTIC MONTHLY, MAY, 1890.)
BY PROF. E.S. HOLDEN.
In 1883 calculations showed that a solar eclipse of unusually long duration (5 minutes, 20 seconds) would occur in the South Pacific Ocean. The track of the eclipse lay south of the equator, but north of Tahiti. There were in fact only two dots of coral islands on the charts in the line of totality, Caroline Island, and one hundred and fifty miles west Flint Island (longitude 150 west, latitude 10 south). Almost nothing was known of either of these minute points. The station of the party under my charge (sent out by the United States government under the direction of the National Academy of Sciences) was to be Caroline Islands.
Every inch of that island (seven miles long, a mile or so broad) is familiar now; but it is almost ludicrous to recollect with what anxiety we pored over the hydrographic charts and sailing instructions of the various nations, to find some information, however scanty, about the spot which was to be our home for nearly a month. All that was known was that this island had formerly been occupied as a guano station. There was a landing then.
After the personnel of the party had been decided on, there were the preparations for its subsistence to be looked out for. How to feed seventeen men for twenty-one days? Fortunately the provisions that we took, and the fresh fish caught for us by the natives, just sufficed to carry us through with comfort and with health.
In March of 1883 we sailed from New York, and about the same time a French expedition left Europe bound for the same spot. From New York to Panama, from Panama to Lima, were our first steps. Here we joined the United States steamship Hartford, Admiral Farragut's flagship, and the next day set sail for our destined port,—if a coral reef surrounded by a raging surf can be called a port. About the same time a party of French observers under Monsieur Janssen, of the Paris Academy of Sciences, left Panama in the Eclaireur.
It was an ocean race of four thousand miles due west. The station Caroline Islands was supposed to be more desirable than Flint Island. Admiral Wilkes's expedition had lain off the latter several days without being able to land on account of the tremendous surf, so that it was eminently desirable to "beat the Frenchman," as the sailors put it. With this end in view our party had secured (through a member of the National Academy in Washington) the verbal promise of the proper official of the Navy Department that the Hartford's orders should read "to burn coal as necessary." The last obstacle to success was thus removed. We were all prepared, and now the ship would take us speedily to our station.
Imagine our feelings the next day after leaving Callao, when the commanding officer of the Hartford opened his sealed orders. They read (dated Washington, in February), "To arrive at Caroline Islands (in April) with full coal-bunkers!"
Officialism could go no further. Here was an expedition sent on a slow-sailing ship directly through the regions of calms for four thousand miles. It was of no possible use to send the expedition at all unless it arrived in time. And here were our orders "to arrive with full coal-bunkers."
Fortunately we had unheard-of good-luck. The trade-wind blew for us as it did for the Ancient Mariner, and we sped along the parallel of 12 deg. south at the rate of one hundred and fifty miles a day under sail, while the Eclaireur was steaming for thirty days a little nearer the equator in a dead calm. We arrived off the island just in time, with not a day to spare. It was a narrow escape, and a warning to all of us never to sail again under sealed orders unless we knew what was under the seal.
Here we were, then, lying off the island and scanning its sparse crown of cocoanut palms, looking for a French flag among their wavy tufts. There was none in sight. We were the winners in the long race. Directly a whale-boat was lowered, and rowed around the white fringe of tremendous surf that broke ceaselessly against the vertical wall of coral rock. There was just one narrow place where the waves rolled into a sort of cleft and did not break. Here was the "landing," then.
Landing was an acrobatic feat. In you went on the crest of a wave, pointing for the place where the blue seas did not break into white. An instant after, you were in the quiet water inside of the surf. Jump out everybody and hold the boat! Then it was pick up the various instruments, and carry them for a quarter of a mile to high-water mark and beyond, over the sharp points of the reef.
In one night we were fairly settled; in another the Hartford had sailed away, leaving us in our fairy paradise, where the corals and the fish were of all the brilliant hues of the rainbow, and where the whiteness of the sand, the emerald of the lagoon, and the turquoise of the ocean made a picture of color and form never to be forgotten.
But where are the Frenchmen? The next morning there is the Eclaireur lying a mile or so out, and there is a boat with the bo'sun—maitre d'equipage—pulling towards the surf. I wade out to the brink. He halloes:
"Where is the landing, then?"
"Mais ici"—Right here,—I say.
"Yes, that's all very well for persons, but where do you land les bagages?"
"Mais ici" I say again, and he says, "Diable!"
But all the same he lands both persons and baggage in a neat, sailor-like way. In a couple of days our two parties of fifty persons had taken possession of this fairy isle. Observatories go up, telescopes, spectroscopes, photographic cameras are pointed and adjusted. The eventful day arrives. Everything is successful. Then comes the Hartford and takes us away, and a few days later comes the Eclaireur, and the Frenchmen are gone. The little island is left there, abandoned to the five natives who tend the sickly plantation of cocoa-palms, and live from year to year with no incident but the annual visit of "the blig" (Kanaka for brig), which brings their store of ship biscuit and molasses.
Think of their stupendous experience! For years and years they have lived like that in the marvellous, continuous charm of the silent island. The "blig" had come and gone away this year, and there will be no more disturbance and discord for a twelve-month longer.
"Surely, surely, slumber is more sweet than toil, the shore Than labor in the deep mid-ocean, wind, and wave, and oar, Then rest ye, brother mariners, we will not wander more!"
Not so! for here comes a great warship out of the East under a press of canvas. What event is this? See! she clews up her light sails and fires an eleven-inch gun! One of those guns of Mobile Bay. Then swarms out the starboard watch, one hundred and sixty strong, and a fleet of boats brings ashore these pale astronomers with those useless tubes that they point at the sky every night. But there are useful things too; cooking-stoves, and lumber, and bricks.
What is all this? No sooner are these established than comes another ship and fires its gun! and another set of hardy sailormen pours out, and here is another party of madmen with tubes,—yes, and with cooking-stoves and lumber, too. Then comes the crowning, stupendous, and unspeakable event. The whole sun is hidden and the heavens are lighted up with pearly streamers! In the name of all the Polynesian gods, what is the meaning of all this?
And then in a few days all these are gone. All the madmen. They have taken away the useless tubes, but they have left their houses standing. Their splendid, priceless, precious cook-stoves are here. See! here is a frying-pan! here are empty tin cans! and a keg of nails! They must have forgotten all this, madmen as they are!
And the little island sinks back to its quiet and its calm. The lagoon lies placid like a mirror. The slow sea breaks eternally on the outer reef. The white clouds sail over day by day. The seabirds come back to their haunts,—the fierce man-of-war birds, the gentle, soft-eyed tern. But we, whose island home was thus invaded—are we the same? Was this a dream? Will it happen again next year? every year? What indeed was it that happened,—or in fact, did it happen at all? Is it not a dream, indeed?
If we left those peaceful Kanakas to their dream, we Americans have brought ours away with us. Who will forget it? Which of us does not wish to be in that peaceful fairyland once more? That is the personal longing. But we have all come back, each one with his note-books full; and in a few weeks the stimulus of accustomed habit has taken possession of us again. Right and wrong are again determined by "municipal sanctions." We have become useful citizens once more. Perhaps it is just as well. We should have been poor poets, and we do not forget. So ends the astronomer's voyage to fairyland.
HALOS—PARHELIA—THE SPECTRE OF THE BROCKEN, ETC
(FROM THE ATMOSPHERE.)
BY CAMILLE FLAMMARION.
Treatises on meteorology have not, up to the present day, classified with sufficient regularity the divers optical phenomena of the air. Some of these phenomena have, however, been seen but rarely, and have not been sufficiently studied to admit of their classification. We have examined the common phenomenon of the rainbow and we have seen that it is due to the refraction and reflection of light on drops of water, and that it is seen upon the opposite side of the sky to the sun in day-time, or the moon at night. We are now about to consider an order of phenomena which are of rarer occurrence, but which have this property in common with the rainbow, viz., that they take place also upon the side of the sky opposite to the sun. These different optical effects are classed together under the name of anthelia (from Greek, opposite to, and Greek, the sun). The optical phenomena which occur on the same side as, or around the sun, such as halos, parhelia, etc., will be dealt with later on.
Before coming to the anthelia, properly so called, or to the colored rings which appear around a shadow, it is as well first to note the effects produced on the clouds and mists that are facing the sun when it rises or sets.
Upon high mountains, the shadow of the mountain is often seen thrown either upon the surface of the lower mists or upon the neighboring mountains, and projected opposite to the sun almost horizontally. I once saw the shadow of the Righi very distinctly traced upon Mount Pilate, which is situated to the west of the Righi, on the other side of the Lake of Lucerne. This phenomenon occurs a few minutes after sunrise, and the triangular form of Righi is delineated in a shape very easy to recognize.
The shadow of Mont Blanc is discerned more easily at sunset. MM. Bravais and Martins, in one of their scientific ascents, noticed it under specially favorable circumstances, the shadow being thrown upon the snow-covered mountains, and gradually rising in the atmosphere until it reached a height of 1 deg., still remaining quite visible. The air above the cone of the shadow was tinted with that rosy purple which is seen, in a fine sunset, coloring the lofty peaks. "Imagine," says Bravais, "the other mountains also projecting, at the same moment, their shadows into the atmosphere, the lower parts dark and slightly greenish, and above each of these shadows the rosy surface, with the deeper rose of the belt which separates it from them; add to this the regular contour of the cones of the shadow, principally at the upper edge, and lastly, the laws of perspective causing all these lines to converge the one to the other toward the very summit of the shadow of Mont Blanc; that is to say, to the point of the sky where the shadows of our own selves were; and even then one will have but a faint idea of the richness of the meteorological phenomenon displayed before our eyes for a few instants. It seemed as though an invisible being was seated upon a throne surrounded by fire, and that angels with glittering wings were kneeling before him in adoration."
Among the natural phenomena which now attract our attention, but fail to excite our surprise, there are some which possess the characteristics of a supernatural intervention. The names which they have received still bear witness to the terror which they once inspired; and even to-day, when science has stripped them of their marvellous origin, and explained the causes of their production, these phenomena have retained a part of their primitive importance, and are welcomed by the savant with as much interest as when they were attributed to divine agency. Out of a large and very diverse number, I will first select the Spectre of the Brocken.
The Brocken is the highest mountain in the picturesque Hartz chain, running through Hanover, being three hundred and thirty feet above the level of the sea.
One of the best descriptions of this phenomenon is given by the traveller Hane, who witnessed it on the 23d of May, 1797. After having ascended no less than thirty miles to the summit, he had the good fortune at last to contemplate the object of his curiosity. The sun rose at about four o'clock, the weather being fine, and the wind driving off to the west the transparent vapors which had not yet had time to be condensed into clouds. About a quarter-past four, Hane saw in this direction a human figure of enormous dimensions. A gust of wind nearly blowing off his hat at that moment, he raised his hand to secure it, and the colossal figure imitated his action. Hane, noticing this, at once made a stooping movement, and this was also reproduced by the spectre. He then called another person to him, and placing themselves in the very spot where the apparition was first seen, the pair kept their eyes fixed on the Achtermannshohe, but saw nothing. After a short interval, however, two colossal figures appeared, which repeated the gestures made by them, and then disappeared.
Some few years ago, in the summer of 1862, a French artist, M. Stroobant, witnessed and carefully sketched this phenomenon, which is drawn in full-page illustration, opposite p. 272. He had slept at the inn of the Brocken, and rising at two in the morning, he repaired to the plateau upon the summit in the company of a guide. They reached the highest point just as the first glimmer of the rising sun enabled them to distinguish clearly objects at a great distance. To use M. Stroobant's own words, "My guide, who had for some time appeared to be walking in search of something, suddenly led me to an elevation whence I had the singular privilege of contemplating for a few instants the magnificent effect of mirage, which is termed the Spectre of the Brocken. The appearance is most striking. A thick mist, which seemed to emerge from the clouds like an immense curtain, suddenly rose to the west of the mountain, a rainbow was formed, then certain indistinct shapes were delineated. First, the large tower of the inn was reproduced upon a gigantic scale; after that we saw our two selves in a more vague and less exact shape, and these shadows were in each instance surrounded by the colors of the rainbow, which served as a frame to this fairy picture. Some tourists who were staying at the inn had seen the sun rise from their windows, but no one had witnessed the magnificent spectacle which had taken place on the other side of the mountain."
Sometimes these spectres are surrounded by colored concentric arcs. Since the beginning of the present century, treatises on meteorology designate, under the name of the Ulloa circle, the pale external arch which surrounds the phenomenon, and this same circle has sometimes been called the "white rainbow." But it is not formed at the same angular distance as the rainbow, and, although pale, it often envelops a series of interior colored arcs.
Ulloa, being in company with six fellow-travellers upon the Pambamarca at daybreak one morning, observed that the summit of the mountain was entirely covered with thick clouds, and that the sun, when it rose, dissipated them, leaving only in their stead light vapors, which it was almost impossible to distinguish. Suddenly, in the opposite direction to where the sun was rising, "each of the travellers beheld, at about seventy feet from where he was standing, his own image reflected in the air as in a mirror. The image was in the centre of three rainbows of different colors, and surrounded at a certain distance by a fourth bow with only one color. The inside color of each bow was carnation or red, the next shade was violet, the third yellow, the fourth straw color, the last green. All these bows were perpendicular to the horizon; they moved in the direction of, and followed, the image of the person they enveloped as with a glory." The most remarkable point was that, although the seven spectators were standing in a group, each person only saw the phenomenon in regard to his own person, and was disposed to disbelieve that it was repeated in respect to his companions. The extent of the bows increased continually and in proportion to the height of the sun; at the same time their colors faded away, the spectre became paler and more indistinct, and finally the phenomenon disappeared altogether. At the first appearance the shape of the bows was oval, but toward the end they became quite circular. The same apparition was observed in the polar regions by Scoresby, and described by him. He states that the phenomenon appears whenever there is mist and at the same time shining sun. In the polar seas, whenever a rather thick mist rises over the ocean, an observer, placed on the mast, sees one or several circles upon the mist.
These circles are concentric, and their common centre is in the straight line joining the eye of the observer to the sun, and extended from the sun toward the mist. The number of circles varies from one to five; they are particularly numerous and well colored when the sun is very brilliant and the mist thick and low. On July 23, 1821, Scoresby saw four concentric circles around his head. The colors of the first and of the second were very well defined; those of the third, only visible at intervals, were very faint, and the fourth only showed a slight greenish tint.
The meteorologist Kaemtz has often observed the same fact in the Alps. Whenever this shadow was projected upon a cloud, his head appeared surrounded by a luminous aureola.
To what action of light is this phenomenon due? Bouguer is of opinion that it must be attributed to the passage of light through icy particles. Such, also, is the opinion of De Saussure, Scoresby, and other meteorologists.
In regard to the mountains, as we cannot assure ourselves directly of the fact by entering the clouds, we are reduced to conjecture. The aerostat traversing the clouds completely, and passing by the very point where the apparition is seen, affords one an opportunity of ascertaining the state of the cloud. This observation I have been able to make, and so to offer an explanation of the phenomenon.
As the balloon sails on, borne forward by the wind, its shadow travels either on the ground or on the clouds. This shadow is, as a rule, black, like all others; but it frequently happens that it appears alone on the surface of the ground, and thus appears luminous. Examining this shadow by the aid of a telescope, I have noticed that it is often composed of a dark nucleus and a penumbra of the shape of an aureola. This aureola, frequently very large in proportion to the diameter of the central nucleus, eclipses it to the naked eye, so that the whole shadow appears like a nebulous circle projected in yellow upon the green ground of the woods and meadows. I have noticed, too, that this luminous shadow is generally all the more strongly marked in proportion to the greater humidity of the surface of the ground.
Seen upon the clouds, this shadow sometimes presents a curious aspect. I have often, when the balloon emerged from the clouds into the clear sky, suddenly perceived, at twenty or thirty yards' distance, a second balloon distinctly delineated, and apparently of a grayish color, against the white ground of the clouds. This phenomenon manifests itself at the moment when the sun re-appears. The smallest details of the car can be made out clearly, and our gestures are strikingly reproduced by the shadow.
On April 15, 1868, at about half-past three in the afternoon, we emerged from a stratum of clouds, when the shadow of the balloon was seen by us, surrounded by colored concentric circles, of which the car formed the centre. It was very plainly visible upon a yellowish white ground. A first circle of pale blue encompassed this ground and the car in a kind of ring. Around this ring was a second of a deeper yellow, then a grayish red zone, and lastly as the exterior circumference, a fourth circle, violet in hue, and imperceptibly toning down into the gray tint of the clouds. The slightest details were clearly discernible—net, robes, and instruments. Every one of our gestures was instantaneously reproduced by the aerial spectres. The anthelion remained upon the clouds sufficiently distinct, and for a sufficiently long time, to permit of my taking a sketch in my journal and studying the physical condition of the clouds upon which it was produced. I was able to determine directly the circumstances of its production. Indeed, as this brilliant phenomenon occurred in the midst of the very clouds which I was traversing, it was easy for me to ascertain that these clouds were not formed of frozen particles. The thermometer marked 2 deg. above zero. The hygrometer marked a maximum of humidity experienced, namely, seventy-seven at three thousand seven hundred and seventy feet, and the balloon was then at four thousand six hundred feet, where the humidity was only seventy-three. It is therefore certain that this is a phenomenon of the diffraction of light simply produced by the vesicles of the mist.
The name of diffraction is given to all the modifications which the luminous rays undergo when they come in contact with the surface of bodies. Light, under these circumstances, is subject to a sort of deviation, at the same time becoming decomposed, whence result those curious appearances in the shadows of objects which were observed for the first time by Grimaldi and Newton.
The most interesting phenomena of diffraction are those presented by gratings, as are technically denominated the systems of linear and very narrow openings situated parallel to one another and at very small intervals. A system of this kind may be realized by tracing with a diamond, for instance, on a pane of glass equidistant lines very close together. As the light would be able to pass in the interstices between the strokes, whereas it would be stopped in the points corresponding to those where the glass was not smooth, there is, in reality, an effect produced as if there were a series of openings very near to each other. A hundred strokes, about 1/25th of an inch in length, may thus be drawn without difficulty. The light is then decomposed in spectra, each overlapping the other. It is a phenomenon of this kind which is seen when we look into the light with the eye half closed; the eyelashes in this case, acting as a net-work or grating. These net-works may also be produced by reflection, and it is to this circumstance that are due the brilliant colors observed when a pencil of luminous rays is reflected on a metallic surface regularly striated.
To the phenomena of gratings must be attributed, too, the colors, often so brilliant, to be seen in mother-of-pearl. This substance is of a laminated structure; so much so, that in carving it the different folds are often cut in such a way as to form a regular net-work upon the surface. It is, again, to a phenomenon of this sort that are due the rainbow hues seen in the feathers of certain birds, and sometimes in spiders' webs. The latter, although very fine, are not simple, for they are composed of a large number of pieces joined together by a viscous substance, and thus constitute a kind of net-work.
If the sun is near the horizon, and the shadow of the observer falls upon the grass, upon a field of corn, or other surface covered with dew, there is visible an aureola, the light of which is especially bright about the head, but which diminishes from below the middle of the body. This light is due to the reflection of light by the moist stubble and the drops of dew. It is brighter about the head, because the blades that are near where the shadow of the head falls expose to it all that part of them which is lighted up, whereas those farther off expose not only the part which is lighted up, but other parts which are not, and this diminishes the brightness in proportion as their distance from the head increases. The phenomenon is seen whenever there is simultaneously mist and sun. This fact is easily verified upon a mountain. As soon as the shadow of a mountaineer is projected upon a mist, his head gives rise to a shadow surrounded by a luminous aureola.
The Illustrated London News of July 8, 1871, illustrates one of these apparitions, "The Fog-Bow, seen from the Matterhorn," observed by E. Whymper in this celebrated region of the Alps. The observation was taken just after the catastrophe of July 14, 1865; and by a curious coincidence, two immense white aerial crosses projected into the interior of the external arc. These two crosses were no doubt formed by the intersection of circles, the remaining parts of which were invisible. The apparition was of a grand and solemn character, further increased by the silence of the fathomless abyss into which the four ill-fated tourists had just been precipitated.
Other optical appearances of an analogous kind are manifested under different conditions. Thus, for instance, if any one, turning his back to the sun, looks into water, he will perceive the shadow of his head, but always very much deformed. At the same time he will see starting from this very shadow what seem to be luminous bodies, which dart their rays in all directions with inconceivable rapidity, and to a great distance. These luminous appearances—these aureola rays—have, in addition to the darting movement, a rapid rotary movement around the head.
THE PLANET VENUS
BY AGNES M. CLERKE.
HESPERUS AND PHOSPHOR.
The radiant planet that hangs on the skirts of dusk and dawn
"like a jewel in an Ethiop's ear,"
has been known and sung by poets in all ages. Its supremacy over the remainder of the starry host is recognized in the name given it by the Arabs, those nomad watchers of the skies, for while they term the moon "El Azhar," "the Brighter One," and the sun and moon together "El Azharan," "the Brighter Pair," they call Venus "Ez Zahra," the bright or shining one par excellence, in which sense the same word is used to describe a flower. This "Flower of Night" is supposed to be no other than the white rose into which Adonis was changed by Venus in the fable which is the basis of all early Asiatic mythology. The morning and evening star is thus the celestial symbol of that union between earth and heaven in the vivifying processes of nature, typified in the love of the goddess for a mortal.
The ancient Greeks, on the other hand, not unnaturally took the star, which they saw alternately emerging from the effulgence of the rising and setting sun, in the east and in the west, for two distinct bodies, and named it differently according to the time of its appearance. The evening star they called Hesperus, and from its place on the western horizon, fabled an earthly hero of that name, the son of Atlas, who from the slopes of that mountain on the verge of the known world used to observe the stars until eventually carried off by a mighty wind, and so translated to the skies. These divine honors were earned by his piety, wisdom, and justice as a ruler of men, and his name long shed a shimmering glory over those Hesperidean regions of the earth, where the real and unreal touched hands in the mystical twilight of the unknown.
But the morning star shone with a different significance as the herald of the day, the torchbearer who lights the way for radiant Aurora on her triumphal progress through the skies. Hence he was called Eosphorus, or Phosphorus, the bearer of the dawn, translated into Latin as Lucifer, the Light-bearer. The son of Eos, or Aurora, and the Titan Astraeus, he was of the same parentage as the other multitude of the starry host, to whom a similar origin was ascribed, and from whom in Greek mythology he was evidently believed to differ only in the superior order of his brightness. Homer, who mentions the planet in the following passage:
"But when the star of Lucifer appeared, The harbinger of light, whom following close, Spreads o'er the sea the saffron-robed morn."
(LORD DERBY'S "Iliad.")
recognizes no distinction between those celestial nomads, the planets, "wandering stars," as the Arabs call them, which visibly change their position relatively to the other stars, and the latter, whose places on the sphere are apparently fixed and immutable. In this he and his compatriots were far behind the ancient Egyptians, who probably derived their knowledge from still earlier speculators in Asia, for they not only observed the movements of some at least of the planets, but believed that Mercury and Venus revolved as satellites round the sun, which in its turn circled round our lesser world. Pythagoras is said to have been the first to identify Hesperus with Phosphor, as the
"Silver planet both of eve and morn,"
and by Plato the same fact is recognized. The other planets, all of which had, according to him, been originally named in Egypt and Syria, have each its descriptive title in his nomenclature. Thus the innermost, "the Star of Mercury," is called Stilbon, "the Sparkler," Mars, Pyroeis, "the Fiery One," while Jupiter, the planet of the slowest course but one, is designated as Phaeton, and Saturn, the tardiest of all, Phaenon. These names were in later times abandoned in favor of those of the divinities to whom they were respectively dedicated, unalterably associated now with the days of the week, over which they have been selected to preside.
The Copernican theory, which once and forever "brushed the cobwebs out of the sky," by clearing away the mists of pre-existing error, first completely explained the varying positions of the Shepherd's star, irradiating the first or last watch of night, according to her alternate function as the follower or precursor of the sun. As she travels on a path nearer to him by more than twenty-five and a half million miles than that of the earth, she is seen by us on each side of him in turn after passing behind or in front of him. The points at which her orbit expands most widely to our eyes—an effect of course entirely due to perspective, as her distance from the sun is not then actually increased—are called her eastern and western elongations; that at which she passes by the sun on the hither side her inferior, and on the farther side her superior conjunction. At both conjunctions she is lost to our view, since she accompanies the sun so closely as to be lost in his beams, rising and setting at the same time, and travelling with him in his path through the heavens during the day. When at inferior conjunction, or between us and the sun, she turns her dark hemisphere to us like the new moon, and would consequently be invisible in any case, but when in the opposite position, shows us her illuminated face, and is literally a day star, invisible only because effaced by the solar splendor. It is as she gradually separates from him, after leaving this latter position, circling over that half of her orbit which lies to the east of him, that she begins to come into view as an evening star, following him at a greater and greater distance, and consequently setting later, until she attains her greatest eastern elongation, divided from the sun about 45 deg. of his visible circuit through the heavens, and consequently remaining above the horizon for some four hours after him. From this point she again appears to draw nearer to him until she passes on his hither side in inferior conjunction, from which she emerges on the opposite side to the westward, and begins to shine as a morning star, preceding him on his track, at a gradually increasing distance, until attaining her greatest westward elongation, and finally completing her cycle by returning to superior conjunction once more in a period of about five hundred and eighty-four days.
Venus is thus Hesperus or Vesper, the evening star, when following the sun as she passes from beyond him in superior conjunction to inferior conjunction where she is nearest to the earth. As she again leaves him behind in her course from this point to the opposite one of superior conjunction, she appears in her second aspect as Phosphorus or Lucifer, "the sun of morning," and herald of the day, shining as
"The fair star That gems the glittering coronet of morn."
THE PHASES OF VENUS.
But the changes in the aspect of Venus due to her varying positions in her orbit are not confined to those which cause her to oscillate with a pendulum movement eastward and westward from the sun. The discovery that she undergoes phases exactly like those of the moon, followed that of the existence of Jupiter's satellites as the second great result achieved by the use of the telescope in the hands of Galileo. The fact that the planets were intrinsically dark bodies revolving round the sun, and reflecting its light, as he and Copernicus had maintained, thus received a further ocular demonstration. The Florentine astronomer describes in a letter to a friend how the planet, after emerging from superior conjunction as a morning star, gradually loses her rotundity on the side remote from the luminary, changing first to a half sphere and then to a waning crescent; until, after passing through the stage of absolute extinction when intervening between us and the sun, she re-appears as a morning star, and undergoes the same series of transformations in inverse order. The revelation was indeed so novel and unexpected, that when the slight deformation of the planet's shape was first detected by him, he did not venture to announce it in plain terms but veiled it, according to the prevailing fashion of the time, under a Latin anagram. His celebrated sentence—
"Haec immatura a me jam frustra leguntur."
("Those incomplete observations are as yet read by me in vain.")
forms, by transposing the letters, the more definite statement,
"Cynthiae figuras aemulatur Mater Amorum."
("The mother of the loves imitates the aspects of Diana.")
that is to say, Venus vies with the phases of the moon. The discovery was an important one from its bearing on popular superstition ascribing to the planets special influences on human affairs, for since they were thus shown to transmit to us only borrowed light, belief in their beneficent or malefic powers over man's destinies received a rude shock.
Galileo's announcement, published in September, 1610, when only a slight flattening of the planet's disk was visible, received absolute confirmation in the ensuing months, as she completed her full half-circle of change on February 24th of the following year, and consequently exhibited herself to him in all her varying aspects. It was the first time they had been looked upon by a human eye, since its unaided powers do not enable it to discern them, although one exception to this rule is said to have existed. This was the case of the Swiss mathematician Gauss, who, when a child, on being shown the crescent star through the telescope, exclaimed to his mother that it "was turned wrong"; the inference being that he recognized the reversal of the image in the field of the glass. If it were indeed so, he deserves to rank with the Siberian savage, who described the eclipses, or Jupiter's satellites; or the shoemaker of Breslau, who could see and declare the positions of those minute orbs.
The phases exhibited to us by Venus are due to her moving in an orbit within that of the earth, at one side of which she is between us and the sun, while at the other this position is exactly reversed. We may compare her to a performer in a great celestial circus, lit by a central chandelier, and ourselves to spectators in an external ring, from which we see her at one time facing us with the light full on her, at the opposite point in complete shadow, and at the intermediate ones in varying degrees of illumination according to our changing views of her. The same illustration may serve to show why Venus is brightest, not when full, since she is then beyond the sun, and at the farthest possible point from us, but when she approaches us at inferior conjunction, more nearly by over one hundred and thirty million miles, and still shows us a crescent of her illuminated surface, before passing into the last phase of total obscuration. When actually nearest to us she is absolutely invisible, being then, like the new moon, between us and the sun. Her varying degrees of brilliancy, even when in the same phase, are thus accounted for by her alternate retreat from and advance towards us as she circles round the sun. Completing, as she does, her revolution in about seven months and a half, she would of course go through the whole series of her metamorphoses in that time, were the earth, from which we observe them, a fixed point. Their protraction instead, over a term of five hundred and eighty-four days, or more than nineteen months, is due to the simultaneous motion of the earth in the same direction, over her larger orbit in a longer period, causing the same relative position of the sister planet to recur only as often as she overtakes her in her career. Thus the hour and minute hands of a watch, moving at different rates of speed after meeting on the dial plate at twelve o'clock, will not again come together until five minutes past one, when the swifter paced of the two will have completed a revolution and a twelfth. But were we to retard the motion of the latter, reducing it to only twice that of its companion, they would always meet at the figure twelve, as it would exactly complete two circuits while the hour hand was performing one. Venus thus overtakes and passes the earth once in five hundred and eighty-four days, or nearly two and a half of her own years, constituting what is called her synodic period of apparent revolution as seen from this globe. She thus presents to us all the phases undergone by our own satellite during a lunar month, passing from new to full, and vice versa, through the various intervening gradations of form.
The phases of Venus are amongst the most beautiful subjects for observation in a moderate telescope, as her silver bow, gradually brightening in the evening dusk, or fading in the dawn,
"On a bed of daffodil sky,"
is, after the two greater luminaries that rule the day and night, the most brilliant object in the heavens.
THE SILVER CROWN.
The parallel between Venus and
"That orbed maiden with fire laden, Whom mortals call the moon,"
is carried a stage further. Most of us are familiar with the spectacle in which the Ancient Egyptians saw symbolized Horus on the lap of Isis, but which we more prosaically term "the old moon in the new moon's arms." The strongly illuminated half circle next the sun is then seen embracing with its horns a dusky sphere, contrasting with it as tarnished silver does with the newly burnished metal. The same phenomenon is occasionally, though very rarely, exhibited by Venus, while close to the sun at inferior injunction, when the shadowy form of the full orb is seen to shine dimly within her crescent with what is termed "the ashen light." More wonderful still, this "glimmering sphere" is then crowned, as with a silver halo, by a thin luminous arch, forming a secondary sickle facing the one nearest the sun, and doubtless due to the refraction of his rays round the globe of the planet, through the upper regions of her twilight atmosphere. This spectacle was first observed by the Jesuit Ricciolo, an opponent of the Copernican theory, on January 9th, 1643. He describes the planet as ruddy near the sun, yellowish in the middle, and of greenish blue on the side remote from the sun; while he also noted the bow of light limiting the dark hemisphere. Scarcely daring to trust his own eyesight, he ascribed these appearances, although he recorded them, to illusory reflection in the telescope.
They were again seen, however, by Derham about 1715, and six years later by Kirch, in Berlin, who has the following entry in his diary for Saturday, June 29, 1721:—"I found Venus in a region where the sky was not very clear. The planet was narrow, and I seemed to see its dark side, though this is almost incredible. The diameter of Venus was 65", and its sickle seemed to tremble in the atmospheric vapors." Again, on March 8th, 1726, he records a similar observation. "We observed Venus with the twenty-six foot telescope. I perceived her dark side, and its edge seemed to describe a smaller circle than that of the light side, as is the case of the moon." This effect is due to irradiation, that is to say, to the glare from a bright surface, giving a deceptive enlargement to its apparent area. He again saw the dark side of the planet in October, 1759, as did Harding at Goettingen, with Herschel's ten-foot reflector, on January 24th, 1806. This latter observer saw it on this occasion stand out against the background of the sky as of a pale ashen green, while on February 28th following, it seemed to him of a pale reddish gray, like the color of the eclipsed moon.
That the latter body should send to us from her nocturnal shadows sufficient light to be visible is easily explicable, since she is then flooded with earth-light reflected on her from a surface thirteen and one-half times greater than her own, and probably casting on her an illumination transcending our full moonlight in the same proportion. But the secondary light of Venus admits of no such explanation, since earth-light on her surface, diminished by 1/12000th part compared to what it is on that of the moon, would be quite insufficient to render her visible to our eyes. The phenomenon was therefore adduced as an argument for the habitability of the planets by Gruithuisen, of the Munich Observatory, who, writing early in this century, suggested that the ashen light of Venus might be due to general illuminations in celebration by her inhabitants of some periodically recurring festivity, The materials for a flare-up on so grand a scale would, he thought, exist in abundance, as he conjectured the vegetation of our planetary neighbor to be more luxuriant than that of our Brazilian forests. The phosphorescence of the Aphroditean oceans, warm and teeming with life, as they are held to be by Zollner, was advanced as an explanatory hypothesis, with scarcely more plausibility, by Professor Safarik, while others have resorted to the supposition of atmospheric or electrical luminosity producing on a large scale some such display as that of the aurora borealis.
Professor Vogel, of Berlin, who himself saw part of the night-side of Venus, in its semi-obscurity in November, 1871, ascribed its visibility to a twilight effect caused by a very extensive atmosphere. The light thus transmitted to us by aerial diffusion and giving the ashen light, is reflected sunlight, while that sent by the luminous arc on its edge is direct sunlight, refracted, or bent round to us, from behind the planet. The silver selvedge of the dawn edging the dark limb may consequently be the brightest part of the broken nimbus that then seems to surround her.
A similar appearance is observed during transits of Venus, when she passes directly between us and the actual solar disk. A silver thread is then seen encircling that side of the planet which has not yet entered on the face of the sun or "a shadowy nebulous ring," as it was described by Mr. Macdonnell at Eden, surrounds the whole planetary disk when two-thirds of it have passed the solar edge. As it moves off it, the same aureole again becomes visible, testifying to the existence of an atmosphere of considerable extent exterior to the sharply outlined surface ordinarily visible. The shimmering haze of reflected sunlight which perpetually enfolds her is only made apparent to us under exceptional circumstances which cut off some portion of her more immediate light, just as we see the motes in the air illuminated by a candle if we hide the actual flame from our eyes. The perennial twilight which seems to reign over the nocturnal hemisphere of Venus may compensate, perhaps, for the want of a satellite to modify its darkness.
The great prolongation at other times of the horns of her crescent, so as to embrace almost her entire circumference with a tenuous ring of light, is doubtless due to the same cause, as their visibility should otherwise be limited to a half segment of a circle. The regions thus shining to us are obviously those on which the sun has not yet set, his appearance above the horizon being prolonged, as in our own case, by refraction, though to a much larger extent. The magnitude of the sun's disk as seen from Venus, a third larger than it appears to us, is also adducted by Mr. Proctor in his posthumous work, "The Old and the New Astronomy," edited and completed by Mr. A.C. Ranyard, as an element in extending the illumination of Venus to more than a hemisphere of her surface. As his diameter there is 44-1/4 deg., a zone of more than 22 deg. wide outside the sunward hemisphere is he thinks illuminated by direct though partial sunlight, the orb being throughout this tract still partially above the horizon.
BY SIR ROBERT S. BALL.
The group of bodies which cluster around our sun forms a little island, so to speak, in the extent of infinite space. We may illustrate this by a map in which we shall endeavor to show the stars placed at their proper relative distances. We first open the compasses one inch, and thus draw a little circle to represent the path of the earth. We are not going to put in all the planets. We take Neptune, the outermost, at once. To draw its path I open the compasses to thirty inches, and draw a circle with that radius. That will do for our solar system, though the comets no doubt will roam beyond these limits. To complete our map we ought of course to put in some stars. There are a hundred million to choose from, and we shall begin with the brightest. It is often called the Dog Star, but astronomers know it better as Sirius. Let us see where it is to be placed on our map. Sirius is beyond Neptune, so it must be outside somewhere. Indeed, it is a good deal further off than Neptune; so I try at the edge of the drawing-board; I have got a method of making a little calculation that I do not intend to trouble you with, but I can assure you that the results it leads me to are quite correct; they show me that this board is not big enough. But could a board which was big enough fit into this lecture theatre? Here, again, I make my little calculations, and I find that there would not be room for a board sufficiently great; in fact, if I put the sun here at one end, with its planets around it? Sirius would be too near on the same scale if it were at the further corner. The board would have to go out through the wall of the theatre, out through London. Indeed, big as London is, it would not be large enough to contain the drawing-board that I should require. It would have to stretch about twenty miles from where we are now assembled. We may therefore dismiss any hope of making a practical map of our system on this scale if Sirius is to have its proper place. Let us, then, take some other star. We shall naturally try with the nearest of all. It is one that we do not know in this part of the world, but those that live in the southern hemisphere are well acquainted with it. The name of this star is Alpha Centauri. Even for this star we should require a drawing three or four miles long if the distance from the earth to the sun is to be taken as one inch. You see what an isolated position our sun and his planets occupy. The members of the family are all close together, and the nearest neighbors are situated at enormous distances. There is a good reason for this separation. The stars are very pretty and perfectly harmless to us where they are at present situated. They might be very troublesome neighbors if they were very much closer to our system. It is therefore well they are so far off; they would be constantly making disturbances in the sun's family if they were near at hand. Sometimes they would be dragging us into unpleasantly great heat by bringing us too close to the sun, or producing a coolness by pulling us away from the sun, which would be quite as disagreeable.
The Stars are Suns.
We are about to discuss one of the grandest truths in the whole of nature. We have had occasion to see that this sun of ours is a magnificent globe immensely larger than the greatest of his planets, while the greatest of these planets is immensely larger than this earth; but now we are to learn that our sun is, indeed, only a star not nearly so bright as many of those which shine over our heads every night. We are comparatively close to the sun, so that we are able to enjoy his beautiful light and cheering heat. Each of those other myriads of stars is a sun, and the splendor of those distant suns is often far greater than that of our own. We are, however, so enormously far from them that they appear dwindled down to insignificance. To judge impartially between our sun or star and such a sun or star as Sirius we should stand halfway between the two; it is impossible to make a fair estimate when we find ourselves situated close to one star and a million times as far from the other. After allowance is made for the imperfections of our point of view, we are enabled to realize the majestic truth that the sun is no more than a star, and that the other stars are no less than suns. This gives us an imposing idea of the extent and magnificence of the universe in which we are situated. Look lip at the sky at night—you will see a host of stars; try to think that every one of them is itself a sun. It may probably be that those suns have planets circling round them, but it is hopeless for us to expect to see such planets. Were you standing on one of those stars and looking towards our system, you would not perceive the sun to be the brilliant and gorgeous object that we know so well. If you could see him at all, he would merely seem like a star, not nearly as bright as many of those you can see at night. Even if you had the biggest of telescopes to aid your vision, you could never discern from one of these bodies the planets which surround the sun, no astronomer in the stars could see Jupiter, even if his sight were a thousand times as powerful as any sight or telescope that we know. So minute an object as our earth would, of course, be still more hopelessly beyond the possibility of vision.
The Number of the Stars.
To count the stars involves a task which lies beyond the power of man to accomplish. Even without the aid of any telescope, we can see a great multitude of stars from this part of the world. There are also many constellations in the southern hemisphere which never appear above our horizon. If, however, we were to go to the equator, then, by waiting there for a twelve-month, all the stars in the heavens would have been successively exposed to view. An astronomer, Houzeau, with the patience to count them, enumerated about six thousand. This is the naked-eye estimate of the star-population of the heavens; but if instead of relying on unaided vision, you get the assistance of a little telescope, you will be astounded at the enormous multitude of stars which are disclosed.
An ordinary opera-glass or binocular is a very useful instrument for looking at the stars in the heavens. If you employ an instrument of this sort, you will be amazed to find that the heavens teem with additional hosts of stars that your unaided vision would never have given you knowledge of. Any part of the sky may be observed; but, just to give an illustration, I shall take one special region, namely, that of the Great Bear (Fig. 1). The seven well-known stars are here shown, four of which form a sort of oblong, while the other three represent the tail. I would like you to make this little experiment. On a fine clear night, count how many stars there are within this oblong; they are all very faint, but you will be able to see a few, and, with good sight, and on a clear night, you may see perhaps ten. Next take your opera-glass and sweep it over the same region; if you will carefully count the stars it shows, you will find fully two hundred; so that the opera-glass has, in this part of the sky, revealed nearly twenty times as many stars as could be seen without its aid. As six thousand stars can be seen by the eye all over the heavens, we may fairly expect that twenty times that number—that is to say, one hundred and twenty thousand stars—could be shown by the opera-glass over the entire sky. Let us go a step further, and employ a telescope, the object-glass of which is three inches across. This is a useful telescope to have, and, if a good one, will show multitudes of pleasing objects, though an astronomer would not consider it very powerful. An instrument like this, small enough to be carried in the hand, has been applied to the task of enumerating the stars in the northern half of the sky, and three hundred and twenty thousand stars were counted. Indeed, the actual number that might have been seen with it is considerably greater, for when the astronomer Argelander made this memorable investigation he was unable to reckon many of the stars in localities where they lay very close together. This grand count only extended to half the sky, and, assuming that the other half is as richly inlaid with stars, we see that a little telescope like that we have supposed will, when swept over the heavens, reveal a number of stars which exceeds that of the population of any city in England except London. It exhibits more than one hundred times as many stars as our eyes could possibly reveal. Still, we are only at the beginning of the count; the very great telescopes add largely to the number. There are multitudes of stars which in small instruments we cannot see, but which are distinctly visible from our great observatories. That telescope would be still but a comparatively small one which would show as many stars in the sky as there are people living in the mighty city of London; and with the greatest instruments, the tale of stars has risen to a number far greater than that of the entire population of Great Britain.
In addition to those stars which the largest telescopes show us, there are myriads which make their presence evident in a wholly different way. It is only in quite recent times that an attempt has been made to develop fully the powers of photography in representing the celestial objects. On a photographic plate which has been exposed to the sky in a great telescope the stars are recorded by thousands. Many of these may, of course, be observed with a good telescope, but there are not a few others which no one ever saw in a telescope, which apparently no one ever could see, though the photograph is able to show them. We do not, however, employ a camera like that which the photographer uses who is going to take your portrait. The astronomer's plate is put into his telescope, and then the telescope is turned towards the sky. On that plate the stars produce their images, each by its own light. Some of these images are excessively faint, but we give a very long exposure of an hour or two hours; sometimes as much as four hours' exposure is given to a plate so sensitive that a mere fraction of a second would sufficiently expose it during the ordinary practice of taking a photograph in daylight. We thus afford sufficient time to enable the fainter objects to indicate their presence upon the sensitive film. Even with an exposure of a single hour a picture exhibiting sixteen thousand stars has been taken by Mr. Isaac Roberts, of Liverpool. Yet the portion of the sky which it represents is only one ten-thousandth part of the entire heavens. It should be added that the region which Mr. Roberts has photographed is furnished with stars in rather exceptional profusion.
Here, at last, we have obtained some conception of the sublime scale on which the stellar universe is constructed. Yet even these plates cannot represent all the stars that the heavens contain. We have every reason for knowing that with larger telescopes, with more sensitive plates, with more prolonged exposures, ever fresh myriads of stars will be brought within our view.
You must remember that every one of these stars is truly a sun, a lamp, as it were, which doubtless gives light to other objects in its neighborhood as our sun sheds light upon this earth and the other planets. In fact, to realize the glories of the heavens you should try to think that the brilliant points you see are merely the luminous points of the otherwise invisible universe.
Standing one fine night on the deck of a Cunarder we passed in open ocean another great Atlantic steamer. The vessel was near enough for us to see not only the light from the mast-head but also the little beams from the several cabin ports; and we could see nothing of the ship herself. Her very existence was only known to us by the twinkle of these lights. Doubtless her passengers could see, and did see, the similar lights from our own vessel, and they probably drew the correct inference that these lights indicated a great ship.
Consider the multiplicity of beings and objects in a ship: the captain and the crew, the passengers, the cabins, the engines, the boats, the rigging, and the stores. Think of all the varied interests there collected and then reflect that out on the ocean, at night, the sole indication of the existence of this elaborate structure was given by the few beams of light that happened to radiate from it. Now raise your eyes to the stars; there are the twinkling lights. We cannot see what those lights illuminate, we can only conjecture what untold wealth of non-luminous bodies may also lie in their vicinity; we may, however, feel certain that just as the few gleaming lights from a ship are utterly inadequate to give a notion of the nature and the contents of an Atlantic steamer, so are the twinkling stars utterly inadequate to give even the faintest conception of the extent and the interest of the universe. We merely see self-luminous bodies, but of the multitudes of objects and the elaborate systems of which these bodies are only the conspicuous points we see nothing and we know very little. We are, however, entitled to infer from an examination of our own star—the sun—and of the beautiful system by which it is surrounded, that these other suns may be also splendidly attended. This is quite as reasonable a supposition as that a set of lights seen at night on the Atlantic Ocean indicates the existence of a fine ship.
The Clusters of Stars.
On a clear night you can often see, stretching across the sky, a track of faint light, which is known to astronomers as the "Milky Way." It extends below the horizon, and then round the earth to form a girdle about the heavens. When we examine the Milky Way with a telescope we find, to our amazement, that it consists of myriads of stars, so small and so faint that we are not able to distinguish them individually; we merely see the glow produced from their collective rays. Remembering that our sun is a star, and that the Milky Way surrounds us, it would almost seem as if our sun were but one of the host of stars which form this cluster.
There are also other clusters of stars, some of which are exquisitely beautiful telescopic spectacles. I may mention a celebrated pair of these objects which lies in the constellation of Perseus. The sight of them in a great telescope is so imposing that no one who is fit to look through a telescope could resist a shout of wonder and admiration when first they burst on his view. But there are other clusters. Here is a picture of one which is known as the "Globular Cluster in the Centaur" (Fig. 2). It consists of a ball of stars, so far off that, however large these several suns may actually be, they have dwindled down to extremely small points of light. A homely illustration may serve to show the appearance which a globular cluster presents in a good telescope. I take a pepper-caster, and on a sheet of white paper I begin to shake out the pepper until there is a little heap at the centre and other grains are scattered loosely about. Imagine that every one of those grains of pepper was to be transformed into a tiny electric light, and then you have some idea of what a cluster of stars would look like when viewed through a telescope of sufficient power. There are multitudes of such groups scattered through the depths of space. They require our biggest telescopes to show them adequately. We have seen that our sun is a star, being only one of a magnificent cluster that forms the Milky Way. We have also seen that there are other groups scattered through the length and depth of space. It is thus we obtain a notion of the rank which our earth holds in the scheme of things celestial.
The Rank of the Earth as a Globe in Space.
Let me give an illustration with the view of explaining more fully the nature of the relation which the earth bears to the other globes which abound through space, and you must allow me to draw a little upon my imagination. I shall suppose that the mails of our country extend not only over this globe, but that they also communicate with other worlds; that postal arrangements exist between Mars and the earth, between the sun and Orion—in fact, everywhere throughout the whole extent of the universe. We shall consider how our letters are to be addressed. Let us take the case of Mr. John Smith, merchant, who lives at 1001, Piccadilly; and let us suppose that Mr. John Smith's business transactions are of such an extensive nature that they reach not only all over this globe, but away throughout space. I shall suppose that the firm has a correspondent residing—let us say in the constellation of the Great Bear; and when this man of business wants to write to Mr. Smith from these remote regions, what address must he put upon the letter, so that the Postmaster-General of the universe shall make no mistake about its delivery? He will write as follows:—
MR. JOHN SMITH, 1001 Piccadilly, London, England, Europe, Earth, Near the Sun, Milky Way, The Universe.
Let us now see what the several lines of this address mean. Of course we put down the name of Mr. John Smith in the first line, and then we will add "1001 Piccadilly" for the second; but as the people in the Great Bear are not likely to know where Piccadilly is, we shall add "London" underneath. As even London itself cannot be well known everywhere, it is better to write "England." This would surely find Mr. John Smith from any post-office on this globe. From other globes, however, the supreme importance of England may not be so immediately recognized, and therefore it is as well to add another line, "Europe." This ought to be sufficient, I think, for any post-office in the solar system. Europe is big enough to be visible from Mars or Venus, and should be known to the post-office people there, just as we know and have names for the continents on Mars. But further away there might be a little difficulty; from Uranus and Neptune the different regions on our earth can never have been distinguished, and therefore we must add another line to indicate the particular globe of the solar system which contains Europe. Mark Twain tells us that there was always one thing in astronomy which specially puzzled him, and that was to know how we found out the names of the stars. We are, of course, in hopeless ignorance of the name by which this earth is called among other intelligent beings elsewhere who can see it. I can only adopt the title of "Earth," and therefore I add this line. Now our address is so complete that from anywhere in the solar system—from Mercury, from Jupiter, or Neptune—there ought to be no mistake about the letter finding its way to Mr. John Smith. But from his correspondent in the Great Bear this address would be still incomplete; they cannot see our earth from there, and even the sun himself only looks like a small star—like one, in fact, of thousands of stars elsewhere. However, each star can be distinguished, and our sun may, for instance, be recognized from the Great Bear by some designation. We shall add the line "Near the Sun," and then I think that from this constellation, or from any of the other stars around us, the address of Mr. John Smith may be regarded as complete. But Mr. Smith's correspondence may be still wider. He may have an agent living in the cluster of Perseus or on some other objects still fainter and more distant; then "Near the Sun" is utterly inadequate as a concluding line to the address, for the sun, if it can be seen at all from thence, will be only of the significance of an excessively minute star, no more to be designated by a special name than are each of the several leaves on the trees of a forest. What this distant correspondent will be acquainted with is not the earth or the sun but only the cluster of stars among which the sun is but a unit. Again we use our own name to denote the cluster, and we call it the "Milky Way." When we add this line, we have made the address of Mr. John Smith as complete as circumstances will permit. I think a letter posted to him anywhere ought to reach its destination. To perfect it, however, we will finish up with one line more—"The Universe."
The Distances of the Stars.
I must now tell you something about the distances of the stars. I shall not make the attempt to explain fully how astronomers make such measurements, but I will give you some notion of how it is done. You may remember I showed you how we found the distance of a globe that was hung from the ceiling. The principle of the method for finding the distance of a star is somewhat similar, except that we make the two observations not from the two ends of a table, not even from opposite sides of the earth, but from two opposite points on the earth's orbit, which are therefore at a distance of one hundred and eighty-six million miles. Imagine that on Midsummer Day, when standing on the earth here, I measure with a piece of card the angle between the star and the sun. Six months later, on Midwinter Day, when the earth is at the opposite point of its orbit, I again measure the angle between the same star and the sun, and we can now determine the star's distance by making a triangle. I draw a line a foot long, and we will take this foot to represent one hundred and eighty-six million miles, the distance between the two stations; then placing the cards at the corners, I rule the two sides and complete the triangle, and the star must be at the remaining corner; then I measure the sides of the triangle, and how many feet they contain, and recollecting that each foot corresponds to one hundred and eighty-six million miles, we discover the distance of the star. If the stars were comparatively near us, the process would be a very simple one; but, unfortunately, the stars are so extremely far off that this triangle, even with a base of only one foot, must have its sides many miles long. Indeed, astronomers will tell you that there is no more delicate or troublesome work in the whole of their science than that of discovering the distance of a star.
In all such measurements we take the distance from the earth to the sun as a conveniently long measuring-rod, whereby to express the results. The nearest stars are still hundreds of thousands of times as far off as the sun. Let us ponder for a little on the vastness of these distances. We shall first express them in miles. Taking the sun's distance to be ninety-three million miles, then the distance of the nearest fixed star is about twenty millions of millions of miles—that is to say, we express this by putting down a 2 first, and then writing thirteen ciphers after it. It is, no doubt, easy to speak of such figures, but it is a very different matter when we endeavor to imagine the awful magnitude which such a number indicates. I must try to give some illustrations which will enable you to form a notion of it. At first I was going to ask you to try and count this number, but when I found it would require at least three hundred thousand years, counting day and night without stopping, before the task was over, it became necessary to adopt some other method.
When on a visit in Lancashire I was once kindly permitted to visit a cotton mill, and I learned that the cotton yarn there produced in a single day would be long enough to wind round this earth twenty-seven times at the equator. It appears that the total production of cotton yarn each day in all the mills together would be on the average about one hundred and fifty-five million miles. In fact, if they would only spin about one-fifth more, we could assert that Great Britain produced enough cotton yarn every day to stretch from the earth to the sun and back again! It is not hard to find from these figures how long it would take for all the mills in Lancashire to produce a piece of yarn long enough to reach from our earth to the nearest of the stars. If the spinners worked as hard as ever they could for a year, and if all the pieces were then tied together, they would extend to only a small fraction of the distance; nor if they worked for ten years, or for twenty years, would the task be fully accomplished. Indeed, upwards of four hundred years would be necessary before enough cotton could be grown in America and spun in this country to stretch over a distance so enormous. All the spinning that has ever yet been done in the world has not formed a long enough thread!
There is another way in which we can form some notion of the immensity of these sidereal distances. You will recollect that, when we were speaking of Jupiter's moons, I told you of the beautiful discovery which their eclipses enabled astronomers to make. It was thus found that light travels at the enormous speed of about one hundred and eighty-five thousand miles per second. It moves so quickly that within a single second a ray would flash two hundred times from London to Edinburgh and back again.
We said that a meteor travels one hundred times as swiftly as a rifle-bullet; but even this great speed seems almost nothing when compared with the speed of light, which is ten thousand times as great. Suppose some brilliant outbreak of light were to take place in a distant star—an outbreak which would be of such intensity that the flash from it would extend far and wide throughout the universe. The light would start forth on its voyage with terrific speed. Any neighboring star which was at a distance of less than one hundred and eighty-five thousand miles would, of course, see the flash within a second after it had been produced. More distant bodies would receive the intimation after intervals of time proportioned to their distances. Thus, if a body were one million miles away, the light would reach it in from five to six seconds, while over a distance as great as that which separates the earth from the sun the news would be carried in about eight minutes. We can calculate how long a time must elapse ere the light shall travel over a distance so great as that between the star and our earth. You will find that from the nearest of the stars the time required for the journey will be over three years. Ponder on all that this involves. That outbreak in the star might be great enough to be visible here, but we could never become aware of it till three years after it had happened. When we are looking at such a star to-night we do not see it as it is at present, for the light that is at this moment entering our eyes has travelled so far that it has been three years on the way. Therefore, when we look at the star now we see it as it was three years previously. In fact, if the star were to go out altogether, we might still continue to see it twinkling for a period of three years longer, because a certain amount of light was on its way to us at the moment of extinction, and so long as that light keeps arriving here, so long shall we see the star showing as brightly as ever. When, therefore, you look at the thousands of stars in the sky to-night, there is not one that you see as it is now, but as it was years ago.
I have been speaking of the stars that are nearest to us, but there are others much farther off. It is true we cannot find the distances of these more remote objects with any degree of accuracy, but we can convince ourselves how great that distance is by the following reasoning. Look at one of the brightest stars. Try to conceive that the object was carried away further into the depths of space, until it was ten times as far from us as it is at present, it would still remain bright enough to be recognized in quite a small telescope; even if it were taken to one hundred times its original distance it would not have withdrawn from the view of a good telescope; while if it retreated one thousand times as far as it was at first it would still be a recognizable point in our mightiest instruments. Among the stars which we can see with our telescopes, we feel confident there must be many from which the light has expended hundreds of years, or even thousands of years, on the journey. When, therefore, we look at such objects, we see them, not as they are now, but as they were ages ago; in fact, a star might have ceased to exist for thousands of years, and still be seen by us every night as a twinkling point in our great telescopes.
Remembering these facts, you will, I think, look at the heavens with a new interest. There is a bright star, Vega, or Alpha Lyrae, a beautiful gem, so far off that the light from it which now reaches our eyes started before many of my audience were born. Suppose that there are astronomers residing on worlds amid the stars, and that they have sufficiently powerful telescopes to view this globe, what do you think they would observe? They will not see our earth as it is at present; they will see it as it was years (and sometimes many years) ago. There are stars from which if England could now be seen, the whole of the country would be observed at this present moment to be in a great state of excitement at a very auspicious event. Distant astronomers might notice a great procession in London, and they could watch the coronation of a youthful queen amid the enthusiasm of a nation. There are other stars still further, from which, if the inhabitants had good enough telescopes, they would now see a mighty battle in progress not far from Brussels. One splendid army could be beheld hurling itself time after time against the immovable ranks of the other. They would not, indeed, be able to hear the ever-memorable "Up, Guards, and at them!" but there can be no doubt that there are stars so far away that the rays of light which started from the earth on the day of the battle of Waterloo are only just arriving there. Further off still, there are stars from which a bird's-eye view could be taken at this very moment of the signing of Magna Charta. There are even stars from which England, if it could be seen at all, would now appear, not as the great England we know, but as a country covered by dense forests, and inhabited by painted savages, who waged incessant war with wild beasts that roamed through the island. The geological problems that now puzzle us would be quickly solved could we only go far enough into space and had we only powerful enough telescopes. We should then be able to view our earth through the successive epochs of past geological time; we should be actually able to see those great animals whose fossil remains are treasured in our museums tramping about over the earth's surface, splashing across its swamps, or swimming with broad flippers through its oceans. Indeed, if we could view our own earth reflected from mirrors in the stars, we might still see Moses crossing the Red Sea, or Adam and Eve being expelled from Eden.
So important is the subject of star distance that I am tempted to give one more illustration in order to bring before you some conception of how vast such distances are. I shall take, as before, the nearest of the stars so far as known to us, and I hope to be forgiven for taking an illustration of a practical and a commercial kind instead of one more purely scientific. I shall suppose that a railway is about to be made from London to Alpha Centauri. The length of that railway, of course, we have already stated: it is twenty billions of miles. So I am now going to ask your attention to the simple question as to the fare which it would be reasonable to charge for the journey. We shall choose a very cheap scale on which to compute the price of a ticket. The parliamentary rate here is, I believe, a penny for every mile. We will make our interstellar railway fares much less even than this; we shall arrange to travel at the rate of one hundred miles for every penny. That, surely, is moderate enough. If the charges were so low that the journey from London to Edinburgh only cost fourpence, then even the most unreasonable passenger would be surely contented. On these terms how much do you think the fare from London to this star ought to be? I know of one way in which to make our answer intelligible. There is a National Debt with which your fathers are, unhappily, only too well acquainted; you will know quite enough about it yourselves in those days when you have to pay income tax. This debt is so vast that the interest upon it is about sixty thousand pounds a day, the whole amount of the National Debt being six hundred and thirty-eight millions of pounds.
If you went to the booking-office with the whole of this mighty sum in your pocket—but stop a moment; could you carry it in your pocket? Certainly not, if it were in sovereigns. You would find that after you had as many sovereigns as you could conveniently carry there would still be some left—so many, indeed, that it would be necessary to get a cart to help you on with the rest. When the cart had as great a load of sovereigns as the horse could draw there would be still some more, and you would have to get another cart; but ten carts, twenty carts, fifty carts, would not be enough. You would want five thousand of these before you would be able to move off towards the station with your money. When you did get there and asked for a ticket at the rate of one hundred miles for a penny, do you think you would get any change? No doubt some little time would be required to count the money, but when it was counted the clerk would tell you that there was not enough—that he must have nearly two hundred millions of pounds more.
That will give some notion of the distance of the nearest star, and we may multiply it by ten, by one hundred, and even by one thousand, and still not attain to the distance of some of the more remote stars that the telescope shows us.
On account of the immense distances of the stars we can only perceive them to be mere points of light. We can never see a star to be a globe with marks on it like the moon, or like one of the planets—in fact, the better the telescope the smaller does the star seem, though, of course, its brightness is increased with every addition to the light-grasping power of the instrument.
The Brightness and Color of Stars.
Another point to be noticed is the arrangement of stars in classes, according to their lustre. The brightest stars, of which there are about twenty, are said to be of the first magnitude. Those just inferior to the first magnitude are ranked as the second; and those just lower than the second are estimated as the third; and so on. The smallest points that your unaided eyes will show you are of about the sixth magnitude. Then the telescope will reveal stars still fainter and fainter, down to what we term the seventeenth or eighteenth magnitudes, or even lower still. The number of stars of each magnitude increases very much in the classes of small ones.
Most of the stars are white, but many are of a somewhat ruddy hue. There are a few telescopic points which are intensely red, some exhibit beautiful golden tints, while others are blue or green.
There are some curious stars which regularly change their brilliancy. Let me try to illustrate the nature of these variables. Suppose that you were looking at a street gas-lamp from a very long distance, so that it seemed a little twinkling light; and suppose that some one was preparing to turn the gas-cock up and down. Or, better still, imagine a little machine which would act regularly so as to keep the light first of all at its full brightness for two days and a half, and then gradually turn it down until in three or four hours it declines to a feeble glimmer. In this low state the light remains for twenty minutes; then during three or four hours the gas is to be slowly turned on again until it is full. In this condition the light will remain for two days and a half, and then the same series of changes is to recommence. This would be a very odd form of gas-lamp. There would be periods of two days and a half during which it would remain at its full; these would be separated by intervals of about seven hours, when the gradual turning down and turning up again would be in progress.
The imaginary gas-lamp is exactly paralleled by a star Algol, in the constellation of Perseus (Fig. 3), which goes through the series of changes I have indicated. Ordinarily speaking, it is a bright star of the second magnitude, and, whatever be the cause, the star performs its variations with marvellous uniformity. In fact, Algol has always arrested the attention of those who observed the heavens, and in early times was looked on as the eye of a demon. There are many other stars which also change their brilliancy. Most of them require much longer periods than Algol, and sometimes a new star which nobody has ever seen before will suddenly kindle into brilliancy. It is now known that the bright star Algol is attended by a dark companion. This dark star sometimes comes between Algol and the observer and cuts off the light. Thus it is that the diminution of brightness is produced.
Whenever you have a chance of looking at the heavens through a telescope, you should ask to be shown what is called a double star. There are many stars in the heavens which present no remarkable appearance to the unaided eye, but which a good telescope at once shows to be of quite a complex nature. These are what we call double stars, in which two quite distinct stars are placed so close together that the unaided eye is unable to separate them. Under the magnifying power of the telescope, however, they are seen to be distinct. In order to give some notion of what these objects are like, I shall briefly describe three of them. The first lies in that best known constellation, the Great Bear. If you look at his tail, which consists of three stars, you will see that near the middle one of the three a small star is situated; we call this little star Alcor, but it is the brighter one near Alcor to which I specially call your attention. The sharpest eye would never suspect that it was composed of two stars placed close together. Even a small telescope will, however, show this to be the case, and this is the easiest and the first observation that a young astronomer should make when beginning to turn a telescope to the heavens. Of course you will not imagine that I mean Alcor to be the second component of the double star; it is the bright star near Alcor which is the double. Here are two marbles, and these marbles are fastened an inch apart. You can see them, of course, to be separate; but if the pair were moved further and further away, then you would soon not be able to distinguish between them, though the actual distance between the marbles had not altered. Look at these two wax tapers which are now lighted; the little flames are an inch apart. You would have to view them from a station a third of a mile away if the distance between the two flames were to appear the same as that between the two components of this double star. Your eye would never be able to discriminate between two lights only an inch apart at so great a distance; a telescope would, however, enable you to do so, and this is the reason why we have to use telescopes to show us double stars.
You might look at that double star year after year throughout the course of a long life without finding any appreciable change in the relative positions of its components. But we know that there is no such thing as rest in the universe; even if you could balance a body so as to leave it for a moment at rest, it would not stay there, for the simple reason that all the bodies round it in every direction are pulling at it, and it is certain that the pull in one direction will preponderate, so that move it must. Especially is this true in the case of two suns like those forming a double star. Placed comparatively near each other they could not remain permanently in that position; they must gradually draw together and come into collision with an awful crash. There is only one way by which such a disaster could be averted. That is by making one of these stars revolve around the other just as the earth revolves around the sun, or the moon revolves around the earth. Some motion must, therefore, be going on in every genuine double star, whether we have been able to see that motion or not.
Let us now look at another double star of a different kind. This time it is in the constellation of Gemini. The heavenly twins are called Castor and Pollux. Of these, Castor is a very beautiful double star, consisting of two bright points, a great deal closer together than were those in the Great Bear; consequently a better telescope is required for the purpose of showing them separately. Castor has been watched for many years, and it can be seen that one of these stars is slowly revolving around the other; but it takes a very long time, amounting to hundreds of years, for a complete circuit to be accomplished. This seems very astonishing, but when you remember how exceedingly far Castor is, you will perceive that that pair of stars which appear so close together that it requires a telescope to show them apart must indeed be separated by hundreds of millions of miles. Let us try to conceive our own system transformed into a double star. If we took our outermost planet—Neptune—and enlarged him a good deal, and then heated him sufficiently to make him glow like a sun, he would still continue to revolve round our sun at the same distance, and thus a double star would be produced. An inhabitant of Castor who turned his telescope towards us would be able to see the sun as a star. He would not, of course, be able to see the earth, but he might see Neptune like another small star close to the sun. If generations of astronomers in Castor continued their observations of our system, they would find a binary star, of which one component took a century and a half to go round the other. Need we then be surprised that when we look at Castor we observe movements that seem very slow?