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, while at 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 the sun; 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 beyond  a 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 that  his 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 weakening their light, 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 apparatus  has 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 nearer  the 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 in  our 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 the  sun; 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 transparency  varies 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 of solar 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.