Popular Science Monthly/Volume 1/June 1872/The Spots on the Sun
SHALL the Eye of the Universe suffer from ophthalmia? Shall the orb of celestial fire in the incorruptible heavens type of purity and perfection—noblest symbol of God—be stained and blackened with impurities? Such were the questions indignantly asked when it began to be whispered about that there are spots of darkness on the face of the sun.
Yet again what was thought impious and impossible proved true: and again it was demonstrated that the cherished opinions of thousands of years were not only erroneous, but were in flat contradiction to the truth. Not only were spots, which it was thought profane even to suspect, shown to be realities, but they have turned out to be the principal means of letting us into a knowledge—such knowledge as we have—of the solar constitution.
It is now upward of 250 years since the invention of the spy-glass led to the revelation of the solar spots, which has been variously attributed to Fabricius, Galileo, and Scheiner, the Jesuit astronomer. It is said of Scheiner, and it well illustrates the spirit of his age, that he dared not publish his discovery, and at first confided it to only a few of his most intimate pupils. After repeated observations that removed all doubt as to their existence, he consulted the provincial father of his order, who refused to believe in any thing of the kind; "For," said he, "has not Aristotle said that the sun is all over shining with light? I have several times," he sagely observed, "read my Aristotle all through from beginning to end, and I can assure you that he mentions not a syllable about it. Go, my son, make yourself easy, and take it for certain that what you suppose to be spots on the sun, are nothing but specks in your eyes or flaws in your glasses." Scheiner obeyed, admitting that his eyes must be in the wrong, and Aristotle in the right, for he lived in an age of credulous faith and blind authority. A doubting pupil of his, however, wrote to Galileo, who replied: "Scheiner's eyes are as good as need be; I have myself watched those spots for some time past."Scheiner at first considered these dark specks to be minute planets travelling round the sun close to his surface, but Galileo more shrewdly concluded that they were part of the sun itself, perhaps floating scum or scoria, and he saw their importance in astronomy. For not only do they prove the revolution of the sun on its axis, but they afford the only means of ascertaining the time of that revolution, the position of the solar axis and its inclination to the earth's orbit. Galileo therefore watched the spots with an interest and assiduity so great that it finally cost him his eyesight.
When we look at the sun through clouds, or through a darkened glass, it appears as a uniform disk of light. But, when we look at it through a telescope of moderate magnifying power, the uniformity of
surface disappears, and we see it irregularly sprinkled with dark points known as solar spots. These are, however, not always seen; sometimes the solar disk is perfectly clear. It has been found that for a period of ten years, from 1840 to 1850, out' of a total number of 1,982 days when the sun was observed, there were only 372 days when spots were not seen.
The spots vary in size and number: sometimes but a single one appears, and as many as 80 have been visible at one time. Nor are they equally distributed over the solar surface. The region of the spots is generally confined to the part of the sun within about 30 of its equator, beyond which they are rarely seen.
The photosphere is the name given to the silver sea of light by which the spots are surrounded, and in which they seem to swim; it is the immediate source of illumination, and, viewed through a glass of low power, it appears flat and smooth, and of a uniform whiteness.
But, when a telescope of high magnifying power is directed to the sun, its aspect is greatly changed: the spots lose their simplicity, and the photosphere its uniformity, and in both there are a revelation of structure, a diversity of parts, and a variety of changes, which at once provoke questions in the mind of the observer, as to the causes of this diversified appearance, and the constitution of the body which presents
them. The hypotheses put forth are ingenious; but, while the facts of observation are rapidly increasing, and there is a growing agreement on many points, there is still profound uncertainty as to the interpretation to be given to the leading phenomena.
Spots upon the sun's surface have been frequently seen with the naked eye. Galileo saw one at sunrise. It was the observation of a solar spot with unassisted vision that so impressed Sir William Herschel as to determine his attention to the physical nature of the sun. The eminent observer Schwabe has also seen many without the aid of the telescope. Solar spots, therefore, are discernible without glasses at a distance of 91,000,000 of miles; but Lockyer says that the finest telescope enables us to see the sun as we should do with the naked eye at a distance of 186,000 miles, that is, 52,000 miles nearer than the moon. The sun is about 62,000,000 times larger than the moon, and Brande states that the smallest space that can be discovered in the sun's disk must subtend an angle of one second, which is equal to about 460 miles. These facts give us some idea of the mighty scale of solar phenomena.
The solar spots have a general movement with the surface of the revolving sun, and they have also minor movements of displacement. They always make their first appearance on the same side of it; they travel across it in from twelve to fourteen days, and then disappear on the other side. The form of a spot in its first appearance is that of a small, dark streak, the length of which is much greater than the breadth. Its motion appears slow at first; it afterward increases, then slowly diminishes, until again assuming the form of a narrow streak. Fig. 1, from Dr. Schellen's finely-illustrated work on Spectrum Analysis (as are all the illustrations of the present article), represents this change of appearance of a spot as it emerges, passes across the field of view, and disappears. Of course, its relative size, as here shown, is enormously exaggerated.
The solar spot consists principally of a dark, almost black, central portion, generally irregular in form, called the umbra. Mr. Dawes has shown that, within this part of a spot, one or more still blacker spots may generally be observed, to which he gives the name of nucleus. The term black, however, as applied to an object on the sun, is to be taken with caution, and means merely a diminished intensity of solar light, which, by contrast, appears black. Zollner states that the black umbra of a spot emits four thousand times as much light as that derived from an equal area of the full moon; and Sir John Herschel says that the Drummond light, which is so bright that the eye can hardly endure it, when projected on the sun, appears as a black spot!
Surrounding the umbra is a tract less dark, usually more regular in form, and of a fringe-like aspect, called the penumbra. Its appearance is illustrated in Fig. 2, and is represented by the half-tints around the darker umbra in all the accompanying delineations. But, before we can understand the structure of the penumbra, it will be necessary to refer to what is known of the surrounding bright solar surface.
Viewed by a telescope of moderate power, the photosphere loses its uniform aspect, and exhibits a coarsely mottled appearance. But, if an instrument of the highest grade is used, the photosphere is seen to have a far more definite structure, and exhibits appearances which are variously described by different observers. Sir William Herschel calls these appearances "corrugations." They are small points of unequal light, imperfectly separated from each other by rows of minute dark spots called pores; the intervals between them being extremely small, and occupied by a substance decidedly less luminous than the general surface. Some call them "rice-grains;" Mr. Huggins names them "granules," and his representation of them is given in Fig. 3. The photosphere is again described as resembling a net of bright meshes interwoven with dark threads.
Certain portions of the photosphere are much brighter than others; these are termed faculæ, and are, in fact, the opposites of the solar spots. They are devious, undulating, shining ridges, like irregular ranges of snowy mountains, and are represented as from 1,000 to 40,000 miles long, and from 1,000 to 4,000 miles in breadth. They are frequent near the sun's edge, and often accompany the coming and going of the spots. But they are generally the attendants of the spots, and often appear at points where spots are about to break out. Their bright, concentrated, tortuous appearance in the neighborhood of a spot is represented in Fig. 4.
Another remarkable appearance of the sun's surface consists of
streaks or blades of light—the "slashed straws" of Dawes, or the "willow-leaves" of Nasmyth. These, says Sir John Herschel, cover the whole disk of the sun (except the spaces occupied by the spots) in countless millions, and lie crossing each other in every imaginable direction. Mr. Dawes denies that they are so general, but they are universally recognized in the vicinity of t he solar spots, taking a radial direction around the penumbra, and giving it a jagged or coarsely-thatched appearance. Father Secchi says: "I would compare them to elongated masses of cotton of every possible form, sometimes entangled with one another, sometimes neatly truncated, or again spread out with no distinct termination. Their head is generally turned toward the centre of the nucleus. They are not unlike strokes of color laid on with a painting-brush, very white near the head, and gradually less brilliant toward the tail." Fig. 5 represents a group of solar spots, in which this closely-meshed appearance of the ovoid bodies is impressively presented; and in Fig. 6 we see the willow-leaf structure stretching completely across the umbra and forming bridges of light.
The dimensions of solar spots are as variable as their forms. Some are very small, appearing under the highest magnifying power like mere specks; others are of enormous magnitude. Many have been observed which measure from 30,000 to 100,000 miles in diameter, so that, if they are truly chasms and gulfs in the luminous envelope of the sun, our entire globe, as Guillemin remarks, would appear in their depths no larger than a fragment of a rock rolled into the crater of a volcano!
The changes which these bodies undergo are extraordinary. They seem to have an order of development. Proctor says the formation of a spot is usually preceded by the appearance of faculæ. Then a dark point makes its appearance, which increases in size, the penumbral fringe being presently recognized around it, and the distinction between the umbra and the penumbra being well defined. But, when the spot is about to diminish, the edges lose their sharpness as if screened by a luminous veil; capes and promontories jut out, and bridges of light are formed. Lockyer says sometimes changes are noticed even within an hour. Their periods are most variable; the smallest merely appear and disappear, lasting a mere fraction of the time of solar rotation. The larger live frequently during two rotations, and Schwabe saw one which returned eight times, continuing 200 days.
The remarkable changes that sometimes occur are well illustrated in Fig. 7, which represents four drawings of the large spot that appeared October 7, 1865, and which was more than 46,000 square miles in area. The drawings are numbered in the order of date. No. 1 exhibits the oblong, fore-shortened view presented when it first appeared on the edge of the sun. No. 2 represents its aspect three days afterward. No. 3 shows its appearance four days later, and No. 4 is a view of it on October 16th.
Sometimes spots exhibit a rotatory or whirlpool appearance, as if the solar envelope were subject to tremendous tornadoes. Fig. 8 represents a spot seen and drawn by Secchi at Rome, May 5, 1857, which represents a vortex, into which the substance of the photosphere is rushing with an eddying motion. De La Rue took two photographic pictures of the same spot at an interval of two days, and, when these are placed together and looked at through the stereoscope, the spot exhibits a funnel-like concavity with remarkable exactness. There is other evidence that the sun-spots are of the nature of depressions or openings in the photosphere. Both Herschels claim to have observed a notch in the sun's edge as a spot is disappearing; and the order in which the parts of a spot appear and disappear at the solar border is not what it would be if the spot were even with the surface, but is just what it would be if it were a rent or opening.
The question before its is one of solar meteorology. Spectrum analysis proves that matter exists upon the sun in a gaseous form. The great mobility of its envelope, and the rapidity and extent of its changes, imply an atmosphere of extreme complexity, and probably of great depth. The appearances of the solar atmosphere are believed to
be due to masses of the nature of clouds. But, while our clouds are watery, or are formed by precipitated aqueous vapor, the solar clouds are inferred to be more or less metallic, or to be composed of particles of various metals and other substances in a state of intense heat. Metallic vapors, at any rate, are proved to exist on the surface of the sun.
The first hypothesis of the solar constitution, which professed to account for the spots, was put forth in the last century by a Scotch astronomer, Alexander Wilson, and was accepted with modifications by Bode and Herschel, and continued to prevail until within a few years, being accepted by Arago. It was formed under the old anthropocentric bias; that is, the notion that man is the central object of the universe, which has been built around him and for him, and with reference to him as the supreme object. This led to the belief in the inhabitability of worlds; for, if stars and planets were not for man to live on, what can they be good for! And, hence, solar theories were so framed that it might be possible to conceive of the sun as the dwelling-place of man. The nucleus or body of the sun was assumed to be opaque and solid, or, at all events, that it had a solid shell corresponding to the earth's crust. Surrounding the spherical nucleus at a certain distance above it, there was supposed to be a first atmosphere which may be compared to the earth's atmosphere when the latter is occupied by a continuous layer of opaque, reflecting clouds. Above this first layer, and more or less distant from it, there was held to be a second atmosphere which is luminous, and answers to the photosphere, or the visible periphery of the solar orb.
On this view a solar spot results from a rent in the atmospheric layers, by which the dark nucleus becomes visible, forming the umbra. The rupture of the two atmospheres, it was supposed, might be caused by volcanic action, or by "gaseous matter formed from time to time at the surface of the dark nucleus, the high temperature of which causes its deflagration." Again it was said: "It may happen that the opening is wider in the cloudy atmosphere than in the luminous envelope or photosphere, in which case the dark nucleus alone would be seen, and we should have a spot without a penumbra. Or the rupture of the first gray envelope becoming closed before that of the photosphere, would have for effect to shut out the view of the dark globe, and we should have a penumbra without a nucleus."
The later tendency is to abandon the notion of a dark nucleus. Indeed, the explanations of the spots now most in favor recall that of Galileo: he suggested a floating scum, while all the late physicists hold that the spots are due to the agency of precipitated clouds.
Kirchhoff, whose honor it is to have first applied spectrum analysis to the study of the sun, and discovered its chemical elements, maintains that the visible portion of the sun, the surface which constitutes the photosphere, is a solid or liquid sphere in a state of incandescence. Its temperature is very high, and it is surrounded by a dense atmosphere formed of the elements which constitute the incandescent globe itself, whose extremely high temperature maintains them in a state of vapor or gas. The lines of the solar spectrum, instead of being bright and variously colored, are dark, which proves that the light has passed through a medium of absorption. Kirchhoff's view is, that the light emanates from the solid or liquid photosphere, and is filtered of its colors or has its lines reversed in passing through the sun's atmosphere.
He explains the spots by supposing that, from some unknown cause, certain parts of the sun's surface undergo a temporary cooling, by which clouds are condensed above that intercept the rays and then appear as an opaque layer—the umbras of spots. A cloud once formed in this manner acts as a screen toward the higher regions; hence a secondary cooling effect in those regions also, and the formation of another cloudy layer, less dense—the penumbra.
The speculations of M. Faye upon this subject have also attracted some attention. He assumes the sun to be still in a gaseous state. The photosphere he regards as consisting of clouds which are the simple consequence of cooling, and looks upon it, in fact, as the limit which separates the intense heat of the interior portions of the sun from the cold surrounding space. He holds to powerful ascending and descending currents, which result here and there in breaks and dispersions by which openings are made to the gaseous interior, giving the appearance of spots; to which Kirchhoff replies that an incandescent interior, at so high a temperature, would certainly be luminous.
In 1858, before the views of either Kirchhoff or Faye were announced, or spectrum analysis had been applied to the subject, Mr. Herbert Spencer published an article on "Recent Astronomy and the Nebular Hypothesis," in which he anticipated some of the most important results that have been arrived at since by others. He took the ground that the sun is still passing through that incandescent stage which all the planets have long ago passed through, the lateness of his cooling being due to the immensely greater ratio of his mass to his surface. He supposes the sun to have now reached the state of a molten shell with a gaseous nucleus; and that this shell is ever radiating its heat, but is sustained at its high temperature by the progressive condensation of the sun's total mass.
As respects the solar atmosphere, Mr. Spencer said in 1858:
This view was sustained in the most remarkable manner, by the subsequent discoveries, through spectrum analysis, of the metals iron, calcium, magnesium, sodium, chromium, and nickel, in a gaseous state in the atmosphere of the sun.
As respects the solar spots, in the article above quoted, Mr. Spencer suggested that they were due to cyclonic action. He has subsequently developed this view, which is now regarded as the most rational explanation we have of the cause of solar spots. In the latest edition of "The Heavens," by Guillemin, published last year, translated by Lockyer, and edited by Proctor, after a review of the subject, and an examination of all the theories that have been proposed, the view of Mr. Spencer is presented last, as more in conformity to the facts than any other. After showing how the hypothesis of M. Faye is discredited by the spectroscopic observations of Lockyer, Huggins, and Secchi, they say: "It follows conclusively that the spots are regions of increased absorption. This accords with Mr. Herbert Spencer's theory, with which also the observations of Mr. De La Rue and Dr. Balfour Stewart are in satisfactory agreement. Let us now," say they, "present the theory of Mr. Spencer, whose suggestions as to the possible causes of solar spots are very valuable.
"Mr. Spencer, basing his reasoning on terrestrial analogies, thus accounts for the spots: 'The central region of a cyclone must be a region of rarefaction, and consequently a region of refrigeration. In an atmosphere of metallic gases rising from a molten surface, and reaching a limit at which condensation takes place, the molecular
state, especially toward its upper part, must be such that a moderate diminution of density and fall of temperature will cause precipitation; that is to say, the rarefied interior of a solar cyclone will be filled with cloud; condensation, instead of taking place only at the level of the photosphere, will here extend to a great depth below it? It will be seen that Mr. Spencer, as opposed to Kirchhoff, not only accounts for the formation of a cloud, but places it where the objections made to Kirchhoff's clouds do not hold good. He next shows that a cloud thus occupying the interior of a cyclone will have a rotary motion, and this accords with observation. Being funnel-shaped, as analogy warrants us in assuming, its central parts will be much deeper than its peripheral parts, and therefore more opaque. This, too, corresponds with observation. Nor are we, on this hypothesis, without some interpretation of the penumbræ. If we may suppose the so-called 'willow-leaves'—the 'things' on the sun—to be the tops of the currents ascending from the sun's body, what changes of appearance are they likely to undergo in the neighborhood of a cyclone? For some distance round a cyclone there will be a drawing-in of the superficial gases toward the vortex. All the luminous spaces of more transparent clouds, forming the adjacent photosphere, will be changed in shape by these centripetal currents; they will be greatly elongated, and those peculiar aspects which the penumbra presents will so be produced."
Mr. Spencer, however, in his article in the former number of The Popular Science Monthly, says that at present none of the interpretations of the solar spots can be regarded as established.
"All this is, no doubt, very curious, but it is very remote and unpractical. What have the solar spots to do with me, or I with the solar spots?" exclaims some impatient reader. But suppose it turns out that the spots on the sun do have a very close connection with earthly affairs—what then? The story of these strange appearances is incomplete till this point also is noticed.
The view which connects the spots with cyclonic action is confirmed by the now demonstrated fact that they are mighty solar agitations, whose influence is felt in distant planets. Schwabe has discovered that the spots, instead of being uniform in number and intensity from year to year, have a periodicity increasing to a maximum, and then declining to a minimum in a course of years. The variations consisted in the number of days in the year in which spots were visible, and in the number of groups observed. The indomitable perseverance of this astronomer is something wonderful. The president of the English Astronomical Society, in awarding him its gold medal, said: "For thirty years, never has the sun exhibited his disk above the horizon of Dessau, without being confronted by Schwabe's imperturbable telescope, and that appears to have happened about 300 days in a year So that, supposing he observed but once a day, he has made 9,000 observations, in the course of which he has discovered 4,700 groups." In these observations, he traced three complete oscillations from maximum frequency, through minimum back to maximum again. The period assigned by Schwabe was about ten years, although Prof. Wolf, of Zurich, has showed that 11.11 years (or the ninth part of a century) is indicated rather than a ten-yearly period.
It has been established that there is a coincidence between this sun-spot period and magnetic disturbances on the earth. In every part of the earth the magnetic needle has at any given time a certain definite position, about which, under normal conditions, it will oscillate during the day. Proctor says:
Lamont, of Munich, in 1850, was the first to announce that these magnetic disturbances attain a maximum of frequency in about ten years. Two years later, Sabine, Wolf, and Gautier, noticed the coincidence of this period with that of the solar spots. And this coincidence was more than general. There was a coincidence of maximum spot-frequency with maximum of magnetic disturbance, and of minimum with minimum, which compelled them to assert a causal connection between the two periods. Wolf subsequently proved that the period of magnetic disturbance has the length of 11.11 years.
But there is still more striking evidence of this connection. On September 1, 1859, Mr. Carrington, an eminent English astronomer, happened to be intently engaged in observing and mapping a group of spots, when a sudden and most extraordinary outburst occurred, of so startling a nature that, having noted the exact moment by his chronometer, he hastily ran to call some one to see the exhibition with him;
when upon returning, within 60 seconds, the whole spectacle had disappeared. "The spots had travelled considerably from their first position, and vanished as two rapidly-fading dots of white light. In the interval of five minutes, the two spots transvered a space of about 35,000 miles." He likens the appearance to that of a sudden conflagration. This was also independently seen at the same moment by Mr. Hodgson, who says: "I was suddenly surprised at the appearance of a very brilliant star of light, much brighter than the sun's surface, and most dazzling to the protected eye. It lasted some five minutes, and disappeared instantaneously."
At the moment when the sun was thus disturbed, the magnetic instruments at Kew exhibited the signs of great magnetic disturbance. "It was found," says Dr. Balfour Stewart, "that a magnetic storm had broken out at the very moment when this singular appearance had been observed." Nor was this all. A magnetic storm never rages without various signs of electrical disturbance. On this occasion vivid auroras were seen, not only in both hemispheres, but in latitudes where auroras are very seldom witnessed. They were conspicuous, on the nights following this observation, in Cuba, Rome, South America, and Australia—at Melbourne greater than was ever witnessed there before. Sir John Herschel says:
The establishment of so important a fact as the periodicity of the phenomenon of solar spots, and a corresponding periodicity in the action of one of the most subtle of the terrestrial forces, was an event of great moment in the scientific world. For, as the physical forces are correlated and convertible, if any one of them be implicated there arises the presumption that others also may be involved, and a new branch of inquiry is thus opened. Wolf, of Zurich, impressed by the import of the case, addressed himself to the Herculean task of exploring the whole history of past observations of the sun-spots. He overhauled the unknown and forgotten records of old observations, collating the results found in some eleven hundred volumes of print and manuscript, and consulting double that number, which did not pay for the trouble of unearthing them from the dust in which they were buried. His data enable him to give the annual course of the phenomena of spots from 1750 to 1860, that is, for more than a hundred years. For one-half of that time he can make a monthly statement; and is able to trace the maxima and minima with sufficient exactness during the past 140 years. The data procured by Prof. Wolf in this protracted investigation comprehended observations in the seventeenth century on 2,113 days; in the eighteenth century on 5,490 days; in the nineteenth century on 14,860 days, or a total of 22,463 days. The old observers little suspected the ultimate meanings that were to be drawn from what they were doing. But in science nothing is lost; observations at first thought trivial, become at length significant, and serve to establish the most comprehensive views.It follows from these discoveries that, in the system of the universe, there is reason to rank the sun with the variable stars. How far the phenomena of his spots are linked with planetary influences, is an interesting question to which astronomers are directing their attention. They are also investigating the relations of the sun's spots to the temperature of the earth, and other terrestrial conditions, with results which we have no space left to consider.
- Edward Livingston Youmans (Wikisource contributor note)