Great Sun-Spot Group, August 8, 1917.The disk in the lower left corner represents the comparative size of the earth. (Photo, Mt. Wilson Solar Observatory.)
Great Sun-Spot Group, August 8, 1917.The disk in the lower left corner represents the comparative size of the earth. (Photo, Mt. Wilson Solar Observatory.)
The Sun's Disk.The view shows the "rice grain" structure of the photosphere and brilliant calcium flocculi. (Photo, Yerkes Observatory.)
The Sun's Disk.The view shows the "rice grain" structure of the photosphere and brilliant calcium flocculi. (Photo, Yerkes Observatory.)
The Lunar Surface Visible During a Total Eclipse of the Moon, February 8, 1906.(Photo, Yerkes Observatory.)
The Lunar Surface Visible During a Total Eclipse of the Moon, February 8, 1906.(Photo, Yerkes Observatory.)
Another research of exceptional promise will be undertaken, which is of great importance in a general study of stellar evolution; and that is the determination of the spectral-energy curves of stars of various classes, for the purpose of measuring their surface temperatures. A very few of the nebulæ are found to be variable, and their peculiarities need investigation, also special problems of variable stars and temporary stars, and the spectra of the components of close double stars which are beyond the power of all other instruments to photograph.
Such a program of research conveys an excellent idea of many of the great problems that are under investigation by astronomers to-day, and gives some notion of the instrumental means requisite in executing comprehensive plans of this character. It will not escape notice that the climax of instrumental development attained at Mount Wilson has only been made possible by an unbroken chain of progress, link by link, each antecedent link being necessary to the successful forging of its following one. In very large part, and certainly indispensable to these instrumental advances, has the art of working in glass and metals been the mainstay of research. As we review the history of astronomical progress, from Galileo's time to our own, the consummate genius of the artisan and his deft handiwork compel our admiration almost equally with the keen intelligence of the astronomer who uses these powerful engines of his own devising to wrest the secrets of nature from the heavens.
Now let us go upward in imagination, far, far beyond the tops of the highest mountains, beyond the moon and sun, and outward in space until we reach a point in the northern heavens millions and millions of miles away, directly above and equally distant from all points in the ecliptic, or path in which our earth travels yearly round the sun. Then we should have that sort of comprehensive view of the solar system which is necessary if we are to visualize as a whole the working of the vast machine, and the motions, sizes, and distances of all the bodies that comprise it. Of such stupendous mechanism our earth is part.
Or in lieu of this, let us attempt to get in mind a picture of the solar system by means of Sir William Herschel's apt illustration: "Choose any well-leveled field. On it place a globe two feet in diameter. This will represent the sun; Mercury will be represented by a grain of mustard seed on the circumference of a circle 164 feet in diameter for its orbit; Venus, a pea on a circle of 284 feet in diameter; the Earth also a pea, on a circle of 430 feet; Mars a rather larger pin's head on a circle of 654 feet; the asteroids, grains of sand in orbits of 1,000 to 1,200 feet; Jupiter, a moderate sized orange in a circle of nearly half a mile across; Saturn, a small orange on a circle of four-fifths of a mile; Uranus, a full-sizedcherry or small plum upon the circumference of a circle more than a mile and a half; and finally Neptune, a good-sized plum on a circle about two miles and a half in diameter…. To imitate the motions of the planets in the above mentioned orbits, Mercury must describe its own diameter in 41 seconds; Venus in 4 minutes, 14 seconds; the Earth in 7 minutes; Mars in 4 minutes 48 seconds; Jupiter in 2 minutes 56 seconds; Saturn in 3 minutes 13 seconds; Uranus in 2 minutes 16 seconds; and Neptune in 3 minutes 30 seconds."
Now, let us look earthward from our imaginary station near the north pole of the ecliptic. All these planetary bodies would be seen to be traveling eastward round the sun, that is, in a counter-clockwise direction, or contrary to the motions of the hands of a timepiece. Their orbits or paths of motion are very nearly circular, and the sun is practically at the center of all of them except Mercury and Mars; of Venus and Neptune, almost at the absolute center. The planes of all their orbits are very nearly the same as that of the ecliptic, or plane in which the earth moves. These and many other resemblances and characteristics suggest a uniformity of origin which comports with the idea of a family, and so the whole is spoken of as the solar system, or the sun and his family of planets.
In addition to the nine bodies already specified, the solar system comprises a great variety of other and lesser bodies; no less than twenty-six moons or satellites tributary to the planets and traveling round them in various periods as the moon does round our earth. Then between the orbits of Mars and Jupiter are many thousands of asteroids, so called, or minor planets (about 1,000 of them haveactually been discovered, and their paths accurately calculated). And at all sorts of angles with the planetary orbits are the paths of hundreds of comets, delicate filmy bodies of a wholly different constitution from the planets, and which now and then blaze forth in the sky, their tails appearing much like the beam of a searchlight, and compelling for the time the attention of everybody. Connected with the comets and doubtless originally parts of them are uncounted millions of millions of meteors, which for the time become a part of the solar system, their minute masses being attracted to the planets, upon which they fall, those hitting the earth being visible to us as familiar shooting stars.
We next follow the story of astronomy through the solar system, beginning with the sun itself and proceeding outward through his family of planets, now much more numerous and vastly more extended than it was to the ancient world, or indeed till within a century and a half of our own day.
As lord of day, king of the heavens, mankind in the ancient world adored the sun. By their researches into the epoch of the Assyrians, Hittites, Phœnicians and other early peoples now passed from earth, archæologists have unearthed many monuments that evidence the veneration in which the early peoples who inhabited Egypt and Asia Minor many thousand years ago held the sun. A striking example is found in the architecture of early Egyptian temples, on the lintels of which are carved representations of the winged globe or the winged solar disk, and there is a bare possibility that the wings of the globe were suggested by a type of the solar corona as glimpsed by the ancients.
Little knew they about the distance and size of the sun; but the effects of his light and heat upon all vegetal and animal life were obvious to them. Doubtless this formed the basis for their worship of the sun. Occasional huge spots must have been visible to the naked eye, and the sun's corona was seen at rare intervals. Plutarch and Philostratus describe it very much as we see it to-day.
How completely dependent mankind is upon the sun and its powerful radiations, only the science of the present day can tell us. By means of the sun's heat the forests of early geologic ages were enabled to wrest carbon from the atmosphere and store it informs later converted by nature's chemistry into peat and coal. Through processes but imperfectly understood, the varying forms of vegetable life are empowered to conserve, from air and soil, nitrogen and other substances suitable for and essential to the life maintenance of animal creatures. Breezes that bring rain and purify the air; the energy of water held under storage in stream and dam and fall; trade winds facilitating commerce between the continents; oceanic currents modifying coastal climates; the violence of tornado, typhoon and water-spout, together with other manifestations of natural forces—all can be traced back to their origin in the tremendous heating power of the solar rays. In everything material the sun is our constant and bountiful benefactor. If his light and heat were withdrawn, practically every form of human activity on this planet would come to an early end.
How far away is the sun? What is the size of the sun? These are questions that astronomers of the present day can answer with accuracy.
So closely do they know the sun's distance that it is employed as their yardstick of the sky, or unit of celestial measurement. Many methods have been utilized in ascertaining the distance of the sun, and the remarkable agreement among them all is very extraordinary. Some of them depend upon pure geometry, and the basic measure which we make from the earth is not the distance of the sun directly; but we find out how far away Venus is during a transit of Venus, for example, or how far away Mars is or some of the asteroids are at their closer oppositions. Then it is possible to calculate how far away the sun is, because one measurement of distance in the solar system affords us the scaleon which the whole structure is built. But perhaps the simplest method of getting the sun's distance is by the velocity of light, 186,300 miles a second. From eclipses of Jupiter's moons we know that light takes 8 minutes 20 seconds to pass from sun to earth. So that the sun's distance is the simple product of the two, or 93 millions of miles.
Once this fundamental unit is established, we have a firm basis on which to build up our knowledge of the distances, the sizes and motions of the heavenly bodies, especially those that comprise the solar system. We can at once ascertain the size of the sun, which we do by measuring the angle which it fills, that is, the sun's apparent diameter. Finding this to be something over a half a degree in arc, the processes of elementary trigonometry tell us that the sun's globe is 865,000 miles in diameter. For nearly a century this has been accurately measured with the greatest care, and diameters taken in every direction are found to be equal and invariably the same. So we conclude that the sun is a perfect sphere, and so far as our instruments can inform us, its actual diameter is not subject to appreciable change.
The vastness of the sun's volume commands our attention. As his diameter is 110 times that of the earth, his mere size or volume is 110×110×110 or 1,300 thousand times that of the earth, because the volumes of spheres are in proportion as the cubes of their diameters. If the materials that compose the sun were as heavy as those that make up the earth, it would take 1,300 thousand earths to weigh as much as the sun does. But by a method which we need not detail here, the sun's actual weight or mass is found to be only 300 thousand (more nearly330,000), times greater than the earth's. So we must infer that, bulk for bulk, the component materials of the sun are about one-fourth lighter than those of the earth, that is, about one and one-half times as dense as water.
To look at this in another way: it is known that a body falling freely toward the earth from outer space would acquire a speed of seven miles a second, whereas if it were to fall toward the sun instead, the velocity would be 383 miles a second on reaching his surface. If all the other bodies of the solar system, that is, the earth and moon, all the planets and their satellites, the comets and all were to be fused together in a single globe, it would weigh only one-seven hundred and fiftieth as much as the sun does.
At the surface, however, the disproportion of gravity is not so great, because of the sun's vast size: it is only about twenty-eight times greater on the sun than on the earth; and instead of a body falling 16 feet the first second as here, it would fall 444 feet there. Pendulums of clocks on the sun would swing five times for every tick here, and an athlete's running high jump would be scaled down to three inches.
Let us next inquire into the amount of the sun's light and heat, and the enormously high temperature of a body whose heat is so intense even at the vast distance at which we are from it. The intensity of its brightness is such that we have no artificial source of light that we can readily compare it with. In the sky the next object in brightness is the full moon, but that gives less than the half-millionth part as much light as the sun. The standard candle used in physics gives so little light in comparisonthat we have to use an enormous number to express the quantity of light that the sun gives.
A sperm candle burning 120 grains hourly is the standard, and if we compare this with the sun when overhead, and allow for the light absorbed by the atmosphere, we get the number 1575 with twenty-four ciphers following it, to express the candlepower of the sun's light. If we interpose the intense calcium light or an electric arc light between the eye and the sun, these artificial sources will look like black spots on the disk. Indeed, the sun is nearly four times brighter than the "crater," or brightest part of the electric arc. The late Professor Langley at a steel works in Pennsylvania once compared direct sunlight with the dazzling stream of molten metal from a Bessemer converter; but bright as it was, sunlight was found to be five thousand times brighter.
Equally enormous is the heat of the sun. Our intensest sources of artificial heat do not exceed 4,000 degrees Fahrenheit, but the temperature at the sun's surface is probably not less than 16,000 degrees F. One square meter of his surface radiates enough heat to generate 100,000 horsepower continuously. At our vast distance of 93 millions of miles, the sun's heat received by the earth is still powerful enough to melt annually a layer of ice on the earth more than a hundred feet in thickness. If the solar heat that strikes the deck of a tropical steamship could be fully utilized in propelling it, the speed would reach at least ten knots.
Many attempts have been made in tropical and sub-tropical climates to utilize the sun's heat directly for power, and Ericsson in Sweden, Mouchot in France, and Shuman in Egypt have built successful and efficient solar engines. Necessary intermissionof their power at night, as well as on cloudy days, will preclude their industrial introduction until present fuels have advanced very greatly in cost. All regions of the sun's disk radiate heat uniformly, and the sun's own atmosphere absorbs so much that we should receive 1.7 times more heat if it were removed. So far as is known, solar light and heat are radiated equally in all directions, so that only a very minute fraction of the total amount ever reaches the earth, that is, 1 2200 millionth part of the whole. Indeed all the planets and other bodies of the solar system together receive only one one hundred millionth part; the vast remainder is, so far as we know, effectively wasted. It is transformed, but what becomes of it, and whether it ever reappears in any other form, we cannot say.
How is this inconceivably vast output of energy maintained practically invariable throughout the centuries? Many theories have been advanced, but only one has received nearly universal assent, that of secular contraction of the sun's huge mass upon itself. Shrinkage means evolution of heat; and it is found by calculation that if the sun were to contract its diameter by shrinking only two-hundred and fifty feet per year, the entire output of solar heat might thus be accounted for. So distant is the sun and so slow this rate of contraction that centuries must elapse before we could verify the theory by actual measurements. Meanwhile, the progress of physical research on the structure and elemental properties of matter has brought to light the existence of highly active internal forces which are doubtless intimately concerned in the enormous output of radiant energy, though the mechanism of its maintenance is as yet known only in part.
Abbot, from many years' observations of the solar constant, at Washington, on Mount Wilson, and in Algeria, finds certain evidence of fluctuation in the solar heat received by the earth. It cannot be a local phenomenon due to disturbances in our atmosphere, but must originate in causes entirely extraneous to the earth. Interposition of meteoric dust might conceivably account for it, but there is sufficient evidence to show that the changes must be attributed to the sun itself. The sun, then, is a variable star; and it has not only a period connected with the periodicity of the sun spots, but also an irregular, nonperiodic variation during a cycle of a week or ten days, though sometimes longer, and occasioning irregular fluctuations of two to ten per cent of the total radiation. Radiation is found to increase with the spottedness.
Attempts have been made on the basis of the contraction theory to find out the past history of the sun and to predict its future. Probably 20 to 50 millions of years in the past represents the life of the sun much as it is at present; and if solar radiation in the future is maintained substantially as now, the sun will have shrunk to one-half its present diameter in the next five million years.
So far then as heat and light from the sun are concerned, the sun may continue to support life on the earth not to exceed ten million years in the future. But the sun's own existence, independently of the orbs of the system dependent upon it, might continue for indefinite millions of aeons before it would ever become a cold dead globe; indeed, in the present state of science, we cannot be sure that it is destined to reach that condition within calculable time.
A few words on observing the sun, an object much neglected by amateurs. On account of the intense light, a very slight degree of optical power is sufficient. Indeed a piece of window glass, smoked in a candle flame with uniform graduation from end to end, will be found worth while in a beginner's daily observation of the sun. The glass should be smoked densely enough at one end so that the sunlight as seen through it will not dazzle the eye on the clearest days. At the other end of the glass, the degree of smoke film should not be quite so dense, so that the sun can be examined on hazy, foggy or partly cloudy days. An occasional naked-eye spot will reward the patient observer.
If a small spyglass, opera glass or field glass is at hand, excellent views of the sun may be had by mounting the glass so that it can be held steadily pointed on the sun, and then viewing the disk by projection on a white card or sheet of paper. Care must be taken to get a good focus on the projected image, and then the faculæ, or whitish spots, or mottling nearer the sun's edge will usually be well seen. By moving the card farther away from the eyepiece, a larger disk may be obtained, in effect a higher degree of magnification. But care must be used not to increase it too much. Keep direct sunlight outside the tube from falling on the card where the image is being examined. This is conveniently done by cutting a large hole, the size of the brass cell of the object glass, through a sheet of corrugated strawboard, and slipping this on over the cell. In this way the spots on the sun can be examined with ease and safety to the eye.
For large instruments a special type of eyepiece is provided known as a helioscope, which disposes ofthe intense heat rays that are harmful to the eye. Frequent examination of the eyepiece should be made and the eyepiece cooled if necessary. That part of the sun's surface under observation is known as the photosphere, that is, the part which radiates light. If the atmosphere admits the use of high magnifying powers, the structure of the photosphere will be found more and more interesting the higher the power employed. It is an irregularly mottled surface showing a species of rice-grain structure under fairly high magnification. These grains are grouped irregularly and are about 500 miles across. Under fine conditions of vision they may be subdivided into granules. The faculæ, or white spots, are sometimes elevations above the general solar level; they have occasionally been seen projecting outside the limb, or edge of the disk.
Dark spots of a deep bluish black will often be seen on the photosphere of the sun. Sometimes single, though generally in groups, the larger ones will have a dark center, called the umbra, surrounded by the very irregular penumbra which is darker near its outer edge and much brighter apparently on its inner edge where it joins on the umbra. The penumbra often shows a species of thatch-work structure, and systematic sketches of sun spots by observers skilled in drawing are greatly to be desired, because photography has not yet reached the stage where it is possible to compete with visual observation in the matter of fine detail. The spots themselves nearly always appear like depressions in the photosphere, and on repeated occasions they have been seen as actual notches when on the edge of the sun.
Many spots, however, are not depressions: some appear to be actual elevations, with the umbra perhaps a central depression, like the crater in the general elevation of a volcano. Spots are sometimes of enormous size. The largest on record was seen in 1858; it was nearly 150,000 miles in breadth, and covered a considerable proportion of the whole visible hemisphere of the sun. A spot must be nearly 30,000 miles across in order to be seen with the naked eye.
In their beginning, development, and end, each spot or group of spots appears to be a law unto itself. Sometimes in a few hours they will form, though generally it is a question of days and even weeks. Very soon after their formation is complete, tonguelike encroachments of the penumbra appear to force their way across the umbra, and this splitting up of the central spot usually goes on quite rapidly. Sun spots in violent disturbance are rarely observed. As the sun turns round on his axis, the spots will often be carried across the disk from the center to the edge, when they become very much foreshortened. The sun's period of rotation is 28 days, so that if a spot lasts more than two weeks without breaking up, it may reappear on the eastern limb of the sun after having disappeared at the western edge. Two or three months is an average duration for a spot; the longest on record lasted through 18 months in 1840-41.
The position of the sun's axis is well known, its equator being tilted about 7 degrees to the ecliptic, and the spots are distributed in zones north and south of the equator, extending as far as 30 degrees of solar latitude. In very high latitudes spots are never seen; they are most abundant in about latitude 15 degrees both north and south, and rather more numerous in the northern than in the southern hemisphere of the sun. Recent research at Mount Wilson makes the sun a great magnet; and its magnetic axis is inclined at an angle of 6 degrees to the axis of rotation, around which it revolves in 32 days.
There is a most interesting periodicity of the spots on the sun, for months will sometimes elapse with spots in abundance and visible every day, whileat other periods, days and even weeks will elapse without a single spot being seen. There is a well recognized period of eleven and one-tenth years, the reason underlying which is not, however, known. After passing through the minimum of spottedness, they begin to break out again first in latitudes of 25 degrees-30 degrees, rather suddenly, and on both sides of the equator, and they move toward the equator as their number and individual size decrease.
The last observed epoch of maximum spot activity on the sun was passed in 1917.
Many attempts have been made to ascertain the cause of the periodicity of sun spots, but the real cause is not yet known. If the spots are eruptional in character, the forces held in check during seasons of few spots may well break out in period. The brighter streaks and mottlings known as faculæ are probably elevations above the general photosphere, and seem to be crusts of luminous matter, often incandescent calcium, protruding through from the lower levels. Generally the faculæ are numerous around the dark spots, and absorption of the sun's light by his own atmosphere affords a darker background for them, with better visibility nearer the rim of the solar disk. The spectroheliograph reveals vast zones of faculæ otherwise invisible, related to the sun-spot zones proper on both sides of the equator.
In some intimate way the magnetism of sun and earth are so related that outbreaks of solar spots are accompanied with disturbances of electrical and other instruments on the earth; also the aurora borealis is seen with greater frequency during periods when many spots are visible.
Within very recent years the discovery of a magnetic field in sun spots has been made by Hale with powerful instruments of his own design. Sun spots had never been investigated before with adequate instrumental means. He recognized the necessity of having a spectroscope that would record the widened lines of sun-spot spectra, and the strengthened and weakened lines on a large scale. Certain changes in relative intensity were traced to a reduced temperature of the spot vapors by comparison with photographs of the spectrum of iron and other metallic vapors in an electric arc at different temperatures. Here the work of the laboratory was essential. Sun spots were thus found to be regions of reduced temperature in the solar atmosphere. Chemical unions were thus possible, and thousands of faint lines in spot-spectra were measured and identified as band lines due to chemical compounds. Thus the chemical changes at work in sun-spot vapors were recognized.
Then followed the highly significant investigations of solar vortices and magnetic fields. Improvements in photographic methods had revealed immense vortices surrounding sun spots in the higher part of the hydrogen atmosphere; and this led to the hypothesis that a sun spot is a solar storm, resembling a terrestrial tornado, and in which the hot vapors whirling at high velocity are cooled by expansion. This would account for the observed intensity changes of the spectrum lines and the presence of chemical compounds. The vortex hypothesis suggested an explanation of the widening of many spot lines, and the doubling or trebling of some of them. As it is known that electrons are emitted by hot bodies, they must be present in vast numbers in thesun; and positive or negative electrons, if caught and whirled in a vortex, would produce a magnetic field.
Zeeman in 1896 had discovered that the lines in the spectrum of a luminous vapor in a magnetic field are widened, or even split into several components if the field is strong enough. Characteristic effects of polarization appear also. The new apparatus of the observatory in conjunction with experiments in the laboratory immediately provided evidence that proved the existence of magnetic fields in sun spots, and strengthened the view that the spots are caused by electric vortices.
Extended investigations have led Hale to the conclusion that the sun itself is a magnet, with its poles situated at or near the poles of rotation. In this respect the sun resembles the earth, which has long been known to be a magnet. The sun's axial rotation permits investigation of the magnetic phenomena of all parts of its surface, so that ultimately the exact position of the sun's magnetic poles and the intensity of the field at different levels in the solar atmosphere will be ascertained. Schuster is of the opinion that not only the sun and earth, but every star, and perhaps every rotating body, becomes a magnet by virtue of its rotation. Hale is confident that the 100-inch reflector will permit the test for magnetism to be applied to a few of the stars.
The sun can be observed at Mount Wilson on at least nine-tenths of all the days in the year, and a daily record of the polarities of all spots with the 150-foot tower telescope is a part of the routine. A method has been devised for classifying sun spots on the basis of their magnetic properties, and more than a thousand spots have already been so classified.About 60 per cent of all sun spots are found to be binary groups, the single or multiple members of which are of opposite magnetic polarity. Unipolar spots are very seldom observed without some indication of the characteristics of bipolar groups. These are usually exhibited in the form of flocculi following the spot. The bipolar spot seems to be the dominant type, and the unipolar type a variant of it.
Although devised for quite another purpose, that of photographing the hydrogen prominences on the limb of the sun, the spectroheliograph has contributed very effectively to many departments of solar research. The prominences are dull reddish cloudlets that were first seen during total eclipses of the sun. Probably Vassenius, a Swedish astronomer, during the total eclipse of 1733, made the earliest record of them, as pinkish clouds quite detached from the edge of the moon; and in that day, when it had not yet been proved that the moon was without atmosphere, he naturally thought they belonged to the moon, not the sun. Undoubtedly Ulloa, a Spanish admiral, also saw the prominences in observing the total eclipse of 1778; but they seem to have attracted little attention till 1842, when a very important total eclipse was central throughout Europe, and observed with great care by many of the eminent astronomers of all countries.
So different did the prominences appear to different eyes, and so many were the theories as to what they were, that no general consensus of opinion was reached, and some thought them no part of either sun or moon, but a mere mirage or optical illusion. But at the return of this eclipse in 1860, photography was employed so as to demonstrate beyonda shadow of doubt the real existence and true solar character of the prominences. By the slow progress of the moon across the sun and the prominences on the edge, a unique series of photographs by De la Rue showed the moon's edge gradually cutting off the prominences piecemeal on one side of the sun, and equally gradually uncovering them on the opposite side.
The prominences, then, were known to be real phenomena of the sun, some of them disconnectedly floating in his atmosphere, as if clouds. Their forms did not vary rapidly, they were very abundant, and their light was so rich in rays of great photographic intensity that many were caught on the plate which the eye failed to see; they appeared at every part of the sun's limb and their height above it indicated that they must be many thousand miles in actual dimension. What they were, however, remained an entire mystery, and no one even thought it possible to find out what their chemical constitution might be or to measure the speed with which they moved.
A few years later came the great Indian eclipse (August 28, 1868), at that date the longest total eclipse ever observed. Janssen of France and many others went out to India to witness it. Fortunately the prominences were very brilliant and this led Janssen to believe it would be possible for him to see them the day after the eclipse was over. By modifying the adjustment of his apparatus suitably and changing its relation to the sun's edge, he found that hydrogen is the main constituent in the light of the prominences. In addition to this he was able to trace out the shapes of the prominences, and even measure their dimensions. His station in India was at Guntoor, many weeks by post from home; so thathis account of this important discovery reached the Paris Academy of Sciences for communication with another from the late Sir Norman Lockyer of England, announcing a like discovery, wholly independently.
The principle is simply this, and admirably stated by Young: "Under ordinary circumstances the prominences are invisible, for the same reason as the stars in the daytime: they are hidden by the intense light reflected from the particles of our own atmosphere near the sun's place in the sky; and if we could only sufficiently weaken this aerial illumination, without at the same time weakeningtheirlight, the end would be gained. And the spectroscope accomplishes this very thing. Since the air-light is reflected sunshine, it of course presents the same spectrum as sunlight, a continuous band of color crossed by dark lines. Now, this sort of spectrum is greatly weakened by every increase of dispersive power, because the light is spread out into a longer ribbon and made to cover a more extended area. On the other hand, a spectrum of bright lines undergoes no such weakening by an increase in the dispersive power of the spectroscope. The bright lines are only more widely separated—not in the least diffused or shorn of their brightness."
Simultaneous announcement of this great discovery, by astronomers of different nations, working in widely separate regions of the earth, led to the striking of a gold medal by the French Government in honor of both astronomers and bearing their united effigies. Ever since the famous Indian eclipse of 1868, it has not been necessary to wait for a total eclipse in order to observe the solar prominences, but every observer provided with suitable apparatushas been able to observe them in full sunlight whenever desired, and the charting of them is part of the daily routine at several observatories in different parts of the world. So vast has been the accumulation of data about them that we know their numbers to fluctuate with the spots on the sun; and their distribution over the sun's surface resembles in a way that of the spots.
While the spots and protuberances are most numerous around solar latitude 20 degrees both north and south, the prominences do not disappear above latitude 35 to 40 degrees, as the spots do, but from latitude 60 degrees they increase in number to about 75 degrees, and are occasionally observed even at the sun's poles. Faculæ and prominences are more closely related than the sun spots and prominences. There are wide variations in both magnitude and type of the prominences. Heights above the sun's limb of a few thousand miles are very common, and they rarely reach elevations as great as 100,000 miles, though a very occasional one reaches even greater heights.
Classification of the prominences divides them into two broad types, the quiescent and the eruptive. The former are for the most part hydrogen, and the latter metallic. The quiescent prominences resemble closely the stratus and cirrus type of terrestrial clouds, and are frequently of enormous extent along the sun's edge. They are relatively long-lived, persisting sometimes for days without much change. The eruptive prominences are more brilliant, changing their form and brightness rapidly. Often they appear as brilliant spikes or jets, reaching altitudes that average about 25,000 miles. Rarely seen near the sun's poles, they are much more numerous nearerthe sun spots. Speed of motion of their filaments sometimes exceeds one hundred miles a second, and the changing variety of shapes of the eruptive prominences is most interesting. Oftentimes they change so rapidly that only photography can do them justice.
Prominence photography began with Young a half century ago, who obtained the first successful impression on a microscope slide with a sensitized film of collodion; as was necessary in the earlier wet-plate process of photography, which required exposures so long that little progress was effected for about twenty years. Then it was taken up by Deslandres of Paris and Hale of Chicago independently, both of whom succeeded in devising a complex type of apparatus known as the spectroheliograph, by which all the prominences surrounding the entire limb of the sun can be photographed at any time by light of a single wave-length, together with the disk of the sun on the same negative.
The prominences appear to be intimately connected with a gaseous envelope surrounding the solar photosphere, in which sodium and magnesium are present as well as hydrogen. The depth of the chromosphere is usually between 5,000 and 10,000 miles, and its existence was first made out during the total solar eclipses of 1605 and 1706, when it appeared as an irregular rose-tinted fringe, though not at the time recognized as belonging to the sun.
The constitution of the sun and its envelopes are still under discussion, and no complete theory of the sun has yet been advanced which commands the widest acceptance. Of the interior of the sun we can only surmise that it is composed of gases which, because of intense heat and compression, are in a state unfamiliar on earth and impossible to reproduce inour laboratories. Their consistency may be that of melted pitch or tar.
Surrounding the main body of the sun are a series of layers, shells, or atmospheres. Outside of all and very irregular in structure, indeed probably not a solar atmosphere at all, is the solar corona, parts of which behave much as if it were an atmosphere, but it appears to be bound up in some way with the sun's radiation. It has streamers that vary with the sun-spot period, but its constitution and function are very imperfectly known, because it has never been seen or photographed except at rare intervals on occasion of total eclipses of the sun.
Beneath the corona we meet the projecting prominences, to which parts of the corona are certainly related, and beneath them the first true layer or atmosphere of the sun known as the chromosphere, its average depth being about one-hundredth part of the sun's diameter. Beneath the chromosphere is the layer of the sun from which emanates the light by which we see it, called the photosphere. It appears to be composed of filaments due to the condensation of metallic vapors, and it is the outer extremities of these filaments which are seen as the granular structures everywhere covering the disk of the sun. Their light shines through the chromosphere and the spots are ruptures in this envelope.
Between photosphere and chromosphere is a very thin envelope, probably not over 700 miles in thickness, called the reversing layer. It is this relatively thin shell that is responsible for the absorption which produces the dark lines in the spectrum of the sun. Under normal conditions the filaments of the photosphere are radial, that is vertical on thesun; but whenever eruptions take place, as during the occurrence of spots, the adjacent filaments are violently swept out of their normal vertical lines and these displaced columns then form what we view as the spot's penumbra. From the outer surface of the sun's chromosphere rise in eruptive columns vapors of hydrogen and the various metals of which the sun is composed. These and the spots would naturally occur in periods just as we see them.
We have said that the sun is composed of a mass of highly heated or incandescent vapors or gases, whose compression on account of gravity must render their physical condition quite different from any gaseous forms known on the earth or which we can reproduce here. As the result of more than half a century of studious observation of the sun and mapping of its spectrum in every part, and diligent comparison with the spectra of all known chemical elements on the earth, we find that the sun contains no elements not already found here, but that a great preponderance of elements known to earth are found in the sun.
The intensity of their spectral lines is one prominent indication of the presence of elements in the sun, and the number of coincidences of spectral lines is another. Iron, nickel, calcium, manganese, sodium, cobalt, and carbon are among the elements most strongly identified. A few of the rarer terrestrial elements are of doubtful existence in the sun, and a very few, as gold, bismuth, antimony, and sulphur are not found there, and the existence of oxygen in the sun is regarded by some experts as doubtful. But if the whole earth were vaporized by heat, probably its spectrum would resemble that of the sun very closely.
What are the effects of the sun, and sun spots in particular, on our weather? Is the influence of their periodicity potent or negligible? If we investigate conditions pertaining to terrestrial magnetism, as fluctuations of the magnetic needle, and the frequency of auroræ, there is no occasion for doubt of the sun's direct influence, although we are not able to say just how that influence becomes potent. If, however, we look into questions of temperature, barometric pressure, rainfall, cyclones, crops, and consequent financial conditions, we find fully as much evidence against solar influence as for it. The slight variations of the sun's light and heat due to the presence or absence of sun spots can scarcely be sensible, and much longer periods of closer observation are necessary before such questions can be finally decided. The slighter such influences are, if they actually exist, and the more veiled they are by other influences more or less powerful, the more difficult it is to discover their effects with certainty.
The importance of solar radiation in the prediction of terrestrial weather has long been recognized, but until very recently no practical application has been made. The Smithsonian Astrophysical Observatory at Washington, under the direction of Dr. Abbot, has for many years carried on at a number of stations a series of determinations of the constant of solar radiation by the spectro-bolometric method originated by Langley. A new station in Calama, Chile, has recently been inaugurated, at which the solar constant is worked out each day, and telegraphed to the Argentine weather service, where it is employed in forecasting for the day.
Abbot's new method of solar constant determination is based on the fact that atmospheric transparencyvaries oppositely to the variations of brightness of the sky. Increase of haziness presents more reflecting surface to scatter the solar rays indirectly to the earth. Of course it presents also additional surface to obstruct the direct rays from the sun. By measuring the brightness of the sky near the sun, it becomes possible to infer the coefficients of atmospheric transmission at all wave lengths. The direct observations and the complete deduction of the solar constant for the day can all be completed within two or three hours.
Clayton of Buenos Aires has now employed these results in the Argentine weather predictions for two years, and the introduction of this new element in forecasting has brought about a pronounced gain in the value of the predictions. Its adoption by the weather bureaus of other nations will doubtless come in due time, and the new method take a firmly established rank in practical meteorology.
Abbot's observations many years ago first called attention to the variability of the solar constant through a range of several per cent both from year to year, and in irregular short periods of weeks or even days. Abbot considers this the more likely explanation than that atmospheric changes should take place simultaneously all over the earth. The sun is but a star, the stars that are irregularly variable in light and heat are numerous, and the sun itself appears to be one of these.
Especially important to the agricultural and vineyard interests of Argentina is the question of precipitation, and Clayton finds this very dependent on solar radiation. At epochs of practically stationary solar intensity, there is little or no precipitation; but quite generally he finds that great decrease ofsolar radiation is followed in from three to five days by heavy precipitation. Direct temperature effects are also traced in Buenos Aires and other South American cities, lagging from two to three days behind the observed solar fluctuations.
The station at Calama yields about 250 determinations of the solar constant each year, and the Mount Wilson station about half that number. They are the only stations of this character at present in existence, and others should be established in widely separated and cloudless regions, as Egypt, southern California and Australia. Uniformity in the methods of observing would be highly desirable, and the Smithsonian Institution has perfected the details of common control of such stations which it is expected may be established at an early day.
About the middle of the last century, Le Verrier, a great French astronomer, having added the planet Neptune beyond the outside confines of the solar system, sought evidence of a lesser planet traveling round the sun within the orbit of Mercury. For many years close watch was kept on the sun in the hope of discovering such a body in the act of passing across the disk, or in transit, as it is technically termed. Lescarbault, a French physician, announced that he had actually seen such a planet, Vulcan it was called, passing over the sun in 1859. Total eclipses of the sun would afford the best opportunity for seeing such a body, and on several such occasions astronomers thought they had found it. But the signal advantages of photography have been applied so often to this search, and always unsuccessfully, that the existence of Vulcan, or the intramercurian planet, is now regarded as mythical.
This planet is an elusive body that very few, even astronomers, have ever seen. It is not very bright, has a rapid motion and never retreats far from the sun, so that it was a puzzle to the ancients who saw it, sometimes in the twilight after sunset and again in the twilight of dawn. When following the sundown in the west, in March or April, Mercury is likely to be best seen; twinkling rather violently and nearly as bright as a star of the first magnitude.
Very little is to be seen on the minute disk of this planet, except that it goes through all the phases of the moon—crescent, gibbous, full, gibbous, crescent. Whether Mercury turns round on its axis or not, cannot be said to be known, because the markings that are suspected on its surface are too indefinite to permit exact observation. More than likely the planet presents always the same side or face to the sun, so that it turns round on its axis once, while traveling once around the sun in its orbit. Mercury's day and year would therefore be equal in length. Nor have we much evidence on the question of an atmosphere surrounding Mercury; probably it is very thin, if indeed there is any at all. When Mercury comes directly between us and the sun, crossing in transit, the edge of the planet as projected against the sun is very sharply defined, and this would indicate an absence of atmosphere on Mercury.
Transits of Mercury can occur in May and November only: there was one on November 7, 1914, and there will be one on May 7, 1924. The latter will be nearly eight hours in length, which is almost the limit. Mercury's distance from the sun averages 36 million miles, the diameter of the planet is 3,000 miles, and his orbital speed is 30 miles per second, the swiftest of all the planets. No moon of Mercury is known to exist, although many times diligently searched for, especially during transits of the planet.
Brightest of all the planets, and the most beautiful of all is Venus. Its path is next outside the orbitof Mercury, but within that of the earth, so that it partakes of all the phases of the moon. Like Mercury it sometimes passes exactly between us and the sun, a rare phenomenon which is known as a transit of Venus.
Being without telescopes, the ancients knew nothing about these occurrences, but they were puzzled for centuries over the appearance of the planet in the west after sunset, when they called it Hesperus, and in early dawn in the east when they gave it the name Phosphorus.
Venus is known to be girdled with an atmosphere denser than ours, and it seems to be always filled with dense clouds. It is the reflection of sunlight from this perpetually cloudy exterior which gives Venus her singular radiance. So brilliant is she that even full daylight is not strong enough to overpower her rays; and she may often be seen glistening in the clear blue daytime sky, if one knows pretty nearly in what direction to look for her.
Venus is 67 million miles from the sun, and as our own distance is 93 million miles, this planet can come within 26 million miles of the earth. It is therefore at times our nearest known neighbor in space, excepting only the Moon and Eros, one of the erratic little planets that travel round the sun between Mars and Jupiter. Also possibly a comet might come much nearer.
Astronomers always take advantage of this nearness of Venus to us, if a transit across the sun takes place; because it affords an excellent method of finding out what the distance of the sun is from the Earth. A pair of these transits happens about once a century, there were transits in 1874 and 1882, and the next pair occur in 2004 and 2012. In actual size,Venus is almost as large a planet as our own, being 7,700 miles in diameter, as compared with 7,920 for the earth. Her velocity in her orbit is twenty-two miles per second, and she travels all the way round the sun in seven and one half months or 225 days.
Venus from her striking brilliancy always leads the novice to expect to see great things on applying the telescope. But aside from a brilliant disk, now a slender crescent, now half full like the moon at quarter, and again gibbous as the moon is between quarter and full, the telescope reveals but little. There is pretty good evidence that the markings thought to have been seen on the planet's surface are illusory, and so it is wholly uncertain in what direction the planet's axis lies; also there is great uncertainty about the length of the day on Venus, or the period of turning round on its axis. Probably it is the same in length as the planet's year.
Once when Venus passed very close to the sun, just barely escaping a transit, Lyman of Yale University caught sight of it by hiding the sun behind a tall building or church spire. The dark side of Venus was turned toward us and he could not of course see that. But the planet was clearly there, completely encircled by a narrow delicate luminous ring, which was due to sunlight shining through the atmosphere that surrounds the planet. Similar ring effects were seen by observers of the transits of Venus in 1874 and 1882; and from all their observations it is concluded that Venus has an atmosphere probably at least twice as dense and extensive as that which encircles the earth. Spurious satellites of Venus are many, but no real moon is known to attend this planet.