Fig. 194.
Fig. 194 represents a group of sun-spots observed by Professor Langley, and drawn on the same scale as the small circle in the upper left-hand corner, which represents the surface of half of our globe.
Fig. 195.
181. The Penumbral Filaments.—Not unfrequently the penumbral filaments are curved spirally, indicating a cyclonic action, as shown in Fig. 195. In such cases the whole spot usually turns slowly around, sometimes completing an entire revolution in a few days. More frequently, however, the spiral motion lasts but a short time; and occasionally, after continuing for a while in one direction, the motion is reversed. Very often in large spots we observe opposite spiral movements in different portions of the umbra, as shown in Figs. 196 and 197.
Fig. 196.
Neighboring spots show no tendency to rotate in the same direction. The number of spots in which a decided cyclonic motion (like that shown in Fig. 198) appears is comparatively small, not exceeding two or three per cent of the whole.
Fig. 197.
Fig. 198.
Plate 2.
Plate II. represents a typical sun-spot as delineated by Professor Langley. At the left-hand and upper portions of this great spot the filaments present the ordinary appearance, while at the lower edge, and upon the great overhanging branch, they are arranged very differently. The feathery brush below the branch, closely resembling a frost-crystal on a window-pane, is as rare as it is curious, and has not been satisfactorily explained.
Fig. 199.
182. Birth and Decay of Sun-Spots.—The formation of a spot is sometimes gradual, requiring days or even weeks for its full development; and sometimes a single day suffices. Generally, for some time before its appearance, there is an evident disturbance of the solar surface, indicated especially by the presence of many brilliant faculæ, among which pores, or minute black dots, are scattered. These enlarge, and between them appear grayish patches, in which the photospheric structure is unusually evident, as if they were caused by a dark mass lying below a thin veil of luminous filaments. This veil seems to grow gradually thinner, and finally breaks open, giving us at last the complete spot with its penumbra. Some of the pores coalesce with the principal spot, some disappear, and others form the attendant train before described (179). The spot when once formed usually assumes a circular form, and remains without striking change until it disappears. As its end approaches, the surrounding photosphere seems to crowd in, and overwhelm the penumbra. Bridges of light (Fig. 199), often much brighter than the average of the solar surface, push across the umbra; the arrangement of the penumbra filaments becomes confused; and, as Secchi expresses it, the luminous matter of the photosphere seems to tumble pell-mell into the chasm, which disappears, and leaves a disturbed surface marked with faculæ, which, in their turn, gradually subside.
Fig. 200.
183. Motion of Sun-Spots.—The spots have a regular motion across the disk of the sun from east to west, occupying about twelve days in the transit. A spot generally appears first on or near the east limb, and, after twelve or fourteen days, disappears at the west limb. At the end of another fourteen days, or more, it re-appears at the east limb, unless, in the mean time, it has vanished from sight entirely. This motion of the spots is indicated by the arrow in Fig. 200. The interval between two successive appearances of the same spot on the eastern edge of the sun is about twenty-seven days.
Fig. 201.
184. The Rotation of the Sun.—The spots are evidently carried around by the rotation of the sun on its axis. It is evident, from Fig. 201, that the sun will need to make more than a complete rotation in order to bring a spot again upon the same part of the disk as seen from the earth. S represents the sun, and E the earth. The arrows indicate the direction of the sun's rotation. When the earth is at E, a spot at a would be seen at the centre of the solar disk. While the sun is turning on its axis, the earth moves in its orbit from E to E': hence the sun must make a complete rotation, and turn from a to a' in addition, in order to bring the spot again to the centre of the disk. To carry the spot entirely around, and then on to a', requires about twenty-seven days. From this synodical period of the spot, as it might be called, it has been calculated that the sun must rotate on its axis in about twenty-five days.
Fig. 202.
185. The Inclination of the Sun's Axis.—The paths described by sun-spots across the solar disk vary with the position of the earth in its orbit, as shown in Fig. 202. We therefore conclude that the sun's axis is not perpendicular to the plane of the earth's orbit. The sun rotates on its axis from west to east, and the axis leans about seven degrees from the perpendicular to the earth's orbit.
186. The Proper Motion of the Spots.—When the period of the sun's rotation is deduced from the motion of spots in different solar latitudes, there is found to be considerable variation in the results obtained. Thus spots near the equator indicate that the sun rotates in about twenty-five days; while those in latitude 20° indicate a period about eighteen hours longer; and those in latitude 30° a period of twenty-seven days and a half. Strictly speaking, the sun, as a whole, has no single period of rotation; but different portions of its surface perform their revolutions in different times. The equatorial regions not only move more rapidly in miles per hour than the rest of the solar surface, but they complete the entire rotation in shorter time.
Fig. 203.
There appears to be a peculiar surface-drift in the equatorial regions of the sun, the cause of which is unknown, but which gives the spots a proper motion; that is, a motion of their own, independent of the rotation of the sun.
Fig. 204.
187. Distribution of the Sun-Spots.—The sun-spots are not distributed uniformly over the sun's surface, but occur mainly in two zones on each side of the equator, and between the latitudes of 10° and 30°, as shown in Fig. 203. On and near the equator itself they are comparatively rare. There are still fewer beyond 35° of latitude, and only a single spot has ever been recorded more than 45° from the solar equator.
Fig. 204 shows the distribution of the sun-spots observed by Carrington during a period of eight years. The irregular line on the left-hand side of the figure indicates by its height the comparative frequency with which the spots occurred in different latitudes. In Fig. 205 the same thing is indicated by different degrees of darkness in the shading of the belts.
Fig. 205.
188. The Periodicity of the Spots.—Careful observations of the solar spots indicate a period of about eleven years in the spot-producing activity of the sun. During two or three years the spots increase in number and in size; then they begin to diminish, and reach a minimum five or six years after the maximum. Another period of about six years brings the return of the maximum. The intervals are, however, somewhat irregular.
Fig. 206.
Fig. 206 gives a graphic representation of the periodicity of the sun-spots. The height of the curve shows the frequency of the sun-spots in the years given at the bottom of the figure. It appears, from an examination of this sun-spot curve, that the average interval from a minimum to the next following maximum is only about four years and a half, while that from a maximum to the next following minimum is six years and six-tenths. The disturbance which produces the sun-spots is developed suddenly, but dies away gradually.
189. Connection between Sun-Spots and Terrestrial Magnetism.—The magnetic needle does not point steadily in the same direction, but is subject to various disturbances, some of which are regular, and others irregular.
(1) One of the most noticeable of the regular magnetic changes is the so-called diurnal oscillation. During the early part of the day the north pole of the needle moves toward the west in our latitude, returning to its mean position about ten P.M., and remaining nearly stationary during the night. The extent of this oscillation in the United States is about fifteen minutes of arc in summer, and not quite half as much in winter; but it differs very much in different localities and at different times, and the average diurnal oscillation in any locality increases and decreases pretty regularly during a period of about eleven years. The maximum and minimum of this period of magnetic disturbance are found to coincide with the maximum and minimum of the sun-spot period. This is shown in Fig. 206, in which the dotted lines indicate the variations in the intensity of the magnetic disturbance.
(2) Occasionally so-called magnetic storms occur, during which the compass-needle is sometimes violently disturbed, oscillating five degrees, or even ten degrees, within an hour or two. These storms are generally accompanied by an aurora, and an aurora is always accompanied by magnetic disturbance. A careful comparison of aurora observations with those of sun-spots shows an almost perfect parallelism between the curves of auroral and sun-spot frequency.
(3) A number of observations render it very probable that every intense disturbance of the solar surface is propagated to our terrestrial magnetism with the speed of light.
Fig. 207.
Fig. 207 shows certain of the solar lines as they were observed by Professor Young on Aug. 3, 1872. The contortions of the F line indicated an intense disturbance in the atmosphere of the sun. There were three especially notable paroxysms in this distortion, occurring at a quarter of nine, half-past ten, and ten minutes of twelve, A.M.
Fig. 208.
Fig. 208 shows the curve of magnetic disturbance as traced at Greenwich on the same day. It will be seen from the curve that it was a day of general magnetic disturbance. At the times of the three paroxysms, which are given at the bottom of the figure, it will be observed that there is a peculiar shivering of the magnetic curve.
190. The Spots are Depressions in the Photosphere.—This fact was first clearly brought out by Dr. Wilson of Glasgow, in 1769, from observations upon the penumbra of a spot in November of that year. He found, that when the spot appeared at the eastern limb, or edge of the sun, just moving into sight, the penumbra was well marked on the side of the spot nearest to the edge of the disk; while on the other edge of the spot, towards the centre of the sun, there was no penumbra visible at all, and the umbra itself was almost hidden, as if behind a bank. When the spot had moved a day's journey toward the centre of the disk, the whole of the umbra came into sight, and the penumbra on the inner edge of the spot began to be visible as a narrow line. After the spot was well advanced upon the disk, the penumbra was of the same width all around the spot. When the spot approached the sun's western limb, the same phenomena were repeated, but in the inverse order. The penumbra on the inner edge of the spot narrowed much faster than that on the outer, disappeared entirely, and finally seemed to hide from sight much of the umbra nearly a whole day before the spot passed from view around the limb. This is precisely what would occur (as Fig. 209 clearly shows) if the spot were a saucer-shaped depression in the solar surface, the bottom of the saucer corresponding to the umbra, and the sloping sides to the penumbra.
Fig. 209.
191. Sun-Spot Spectrum.—When the image of a sun-spot is thrown upon the slit of the spectroscope, the spectrum is seen to be crossed longitudinally by a continuous dark band, showing an increased general absorption in the region of the sun-spot. Many of the spectral lines are greatly thickened, as shown in Fig. 210. This thickening of the lines shows that the absorption is taking place at a greater depth. New lines and shadings often appear, which indicate, that, in the cooler nucleus of the spot, certain compound vapors exist, which are dissociated elsewhere on the sun's surface. These lines and shadings are shown in Fig. 211.
Fig. 211.
It often happens that certain of the spectral lines are reversed in the spectrum of the spot, a thin bright line appearing over the centre of a thick dark one, as shown in Fig. 212. These reversals are due to very bright vapors floating over the spot.
Fig. 212.
At times, also, the spectrum of a spot indicates violent motion in the overlying gases by distortion and displacement of the lines. This phenomenon occurs oftener at points near the outer edge of the penumbra than over the centre of the spot; but occasionally the whole neighborhood is violently agitated. In such cases, lines in the spectrum side by side are often affected in entirely different ways, one being greatly displaced while its neighbor is not disturbed in the least, showing that the vapors which produce the lines are at different levels in the solar atmosphere, and moving independently of each other.
Fig. 213.
192. The Cause and Nature of Sun-Spots.—According to Professor Young, the arrangement and relations of the photospheric clouds in the neighborhood of a spot are such as are represented in Fig. 213. "Over the sun's surface generally, these clouds probably have the form of vertical columns, as at aa. Just outside the spot, the level of the photosphere is the most part, overtopped by eruptions of hydrogen and usually raised into faculæ, as at bb. These faculæ are, for metallic vapors, as indicated by the shaded clouds.... While the great clouds of hydrogen are found everywhere upon the sun, these spiky, vivid outbursts of metallic vapors seldom occur except just in the neighborhood of a spot, and then only during its season of rapid change. In the penumbra of the spot the photospheric filaments become more or less nearly horizontal, as at pp; in the umbra at u it is quite uncertain what the true state of affairs may be. We have conjecturally represented the filaments there as vertical also, but depressed and carried down by a descending current. Of course, the cavity is filled by the gases which overlie the photosphere; and it is easy to see, that, looked at from above, such a cavity and arrangement of the luminous filaments would present the appearances actually observed."
Professor Young also suggests that the spots may be depressions in the photosphere caused "by the diminution of upward pressure from below, in consequence of eruptions in the neighborhood; the spots thus being, so to speak, sinks in the photosphere. Undoubtedly the photosphere is not a strictly continuous shell or crust; but it is heavy as compared with the uncondensed vapors in which it lies, just as a rain-cloud in our terrestrial atmosphere is heavier than the air; and it is probably continuous enough to have its upper level affected by any diminution of pressure below. The gaseous mass below the photosphere supports its weight and the weight of the products of condensation, which must always be descending in an inconceivable rain and snow of molten and crystallized material. To all intents and purposes, though nothing but a layer of clouds, the photosphere thus forms a constricting shell, and the gases beneath are imprisoned and compressed. Moreover, at a high temperature the viscosity of gases is vastly increased, so that quite probably the matter of the solar nucleus resembles pitch or tar in its consistency more than what we usually think of as a gas. Consequently, any sudden diminution of pressure would propagate itself slowly from the point where it occurred. Putting these things together, it would seem, that, whenever a free outlet is obtained through the photosphere at any point, thus decreasing the inward pressure, the result would be the sinking of a portion of the photosphere somewhere in the immediate neighborhood, to restore the equilibrium; and, if the eruption were kept up for any length of time, the depression in the photosphere would continue till the eruption ceased. This depression, filled with the overlying gases, would constitute a spot. Moreover, the line of fracture (if we may call it so) at the edges of the sink would be a region of weakness in the photosphere, so that we should expect a series of eruptions all around the spot. For a time the disturbance, therefore, would grow, and the spot would enlarge and deepen, until, in spite of the viscosity of the internal gases, the equilibrium of pressure was gradually restored beneath. So far as we know the spectroscopic and visual phenomena, none of them contradict this hypothesis. There is nothing in it, however, to account for the distribution of the spots in solar latitudes, nor for their periodicity."
IV. THE CHROMOSPHERE AND PROMINENCES.
193. The Sun's Outer Atmosphere.—What we see of the sun under ordinary circumstances is but a fraction of his total bulk. While by far the greater portion of the solar mass is included within the photosphere, the larger portion of his volume lies without, and constitutes a gaseous envelope whose diameter is at least double, and its bulk therefore sevenfold, that of the central globe.
This outer envelope, though mainly gaseous, is not spherical, but has an exceedingly irregular and variable outline. It seems to be made up, not of regular strata of different density, like our atmosphere, but rather of flames, beams, and streamers, as transient and unstable as those of the aurora borealis. It is divided into two portions by a boundary as definite, though not so regular, as that which separates them both from the photosphere. The outer and far more extensive portion, which in texture and rarity seems to resemble the tails of comets, is known as the coronal atmosphere, since to it is chiefly due the corona, or glory, which surrounds the darkened sun during an eclipse.
194. The Chromosphere.—At the base of the coronal atmosphere, and in contact with the photosphere, is what resembles a sheet of scarlet fire. It appears as if countless jets of heated gas were issuing through vents over the whole surface, clothing it with flame, which heaves and tosses like the blaze of a conflagration. This is the chromosphere, or color-sphere. It owes its vivid redness to the predominance of hydrogen in the flames. The average depth of the chromosphere is not far from ten or twelve seconds, or five thousand or six thousand miles.
195. The Prominences.—Here and there masses of this hydrogen, mixed with other substances, rise far above the general level into the coronal regions, where they float like clouds, or are torn to pieces by conflicting currents. These cloud-masses are known as solar prominences, or protuberances.
196. Magnitude and Distribution of the Prominences.—The prominences differ greatly in magnitude. Of the 2,767 observed by Secchi, 1,964 attained an altitude of eighteen thousand miles; 751, or nearly a fourth of the whole, reached a height of twenty-eight thousand miles; several exceeded eighty-four thousand miles. In rare instances they reach elevations as great as a hundred thousand miles. A few have been seen which exceeded a hundred and fifty thousand miles; and Secchi has recorded one of three hundred thousand miles.
Fig. 214.
The irregular lines on the right-hand side of Fig. 214 show the proportion of the prominences observed by Secchi, that were seen in different parts of the sun's surface. The outer line shows the distribution of the smaller prominences, and the inner dotted line that of the larger prominences. By comparing these lines with those on the opposite side of the circle, which show the distribution of the spots, it will be seen, that, while the spots are confined mainly to two belts, the prominences are seen in all latitudes.
197. The Spectrum of the Chromosphere.—The spectrum of the chromosphere is comparatively simple. There are eleven lines only which are always present; and six of these are lines of hydrogen, and the others, with a single exception, are of unknown elements. There are sixteen other lines which make their appearance very frequently. Among these latter are lines of sodium, magnesium, and iron.
Where some special disturbance is going on, the spectrum at the base of the chromosphere is very complicated, consisting of hundreds of bright lines. "The majority of the lines, however, are seen only occasionally, for a few minutes at a time, when the gases and vapors, which generally lie low (mainly in the interstices of the clouds which constitute the photosphere), and below its upper surface, are elevated for the time being by some eruptive action. For the most part, the lines which appear only at such times are simply reversals of the more prominent dark lines of the ordinary solar spectrum. But the selection of the lines seems most capricious: one is taken, and another left, though belonging to the same element, of equal intensity, and close beside the first." Some of the main lines of the chromosphere and prominences are shown in Fig. 215.
Fig. 215.
198. Method of Studying the Chromosphere and Prominences.—Until recently, the solar atmosphere could be seen only during a total eclipse of the sun; but now the spectroscope enables us to study the chromosphere and the prominences with nearly the same facility as the spots and faculæ.
The protuberances are ordinarily invisible, for the same reason that the stars cannot be seen in the daytime; they are hidden by the intense light reflected from our own atmosphere. If we could only get rid of this aerial illumination, without at the same time weakening the light of the prominences, the latter would become visible. This the spectroscope enables us to accomplish. 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 weakened by increase of dispersive power (159), because the light is spread out into a longer ribbon, and made to cover a greater area. On the other hand, the spectrum of the prominences, being composed of bright lines, undergoes no such diminution by increased dispersion.
Fig. 216.
When the spectroscope is used as a means of examining the prominences, the slit is more or less widened. The telescope is directed so that the image of that portion of the solar limb which is to be examined shall be tangent to the opened slit, as in Fig. 216, which represents the slit-plate of the spectroscope of its actual size, with the image of the sun in the proper position for observation.
Fig. 217.
If, now, a prominence exists at this part of the solar limb, and if the spectroscope itself is so adjusted that the C line falls in the centre of the field of view, then one will see something like Fig. 217. "The red portion of the spectrum will stretch athwart the field of view like a scarlet ribbon with a darkish band across it; and in that band will appear the prominences, like scarlet clouds, so like our own terrestrial clouds, indeed, in form and texture, that the resemblance is quite startling. One might almost think he was looking out through a partly-opened door upon a sunset sky, except that there is no variety or contrast of color; all the cloudlets are of the same pure scarlet hue. Along the edge of the opening is seen the chromosphere, more brilliant than the clouds which rise from it or float above it, and, for the most part, made up of minute tongues and filaments."
199. Quiescent Prominences.—The prominences differ as widely in form and structure as in magnitude. The two principal classes are the quiescent, cloud-formed, or hydrogenous, and the eruptive, or metallic.
Plate 3.
The quiescent prominences resemble almost exactly our terrestrial clouds, and differ among themselves in the same manner. They are often of enormous dimensions, especially in horizontal extent, and are comparatively permanent, often undergoing little change for hours and days. Near the poles they sometimes remain during a whole solar revolution of twenty-seven days. Sometimes they appear to lie upon the limb of the sun, like a bank of clouds in the terrestrial horizon, probably because they are so far from the edge that only their upper portions are in sight. When fully seen, they are usually connected to the chromosphere by slender columns, generally smallest at the base, and often apparently made up of separate filaments closely intertwined, and expanding upward. Sometimes the whole under surface is fringed with pendent filaments. Sometimes they float entirely free from the chromosphere; and in most cases the larger clouds are attended by detached cloudlets. Various forms of quiescent prominences are shown in Plate III. Other forms are given in Figs. 218 and 219.
Fig. 218.
Their spectrum is usually very simple, consisting of the four lines of hydrogen and the orange D3: hence the appellation hydrogenous. Occasionally the sodium and magnesium lines also appear, even near the tops of the clouds.
Fig. 219.
200. Eruptive Prominences.—The eruptive prominences ordinarily consist of brilliant spikes or jets, which change very rapidly in form and brightness. As a rule, their altitude is not more than twenty thousand or thirty thousand miles; but occasionally they rise far higher than even the largest of the quiescent protuberances. Their spectrum is very complicated, especially near their base, and often filled with bright lines. The most conspicuous lines are those of sodium, magnesium, barium, iron, and titanium: hence Secchi calls them metallic prominences.
Fig. 220.
They usually appear in the immediate vicinity of a spot, never very near the solar poles. They change with such rapidity, that the motion can almost be seen with the eye. Sometimes, in the course of fifteen or twenty minutes, a mass of these flames, fifty thousand miles high, will undergo a total transformation; and in some instances their complete development or disappearance takes no longer time. Sometimes they consist of pointed rays, diverging in all directions, as represented in Fig. 220. "Sometimes they look like flames, sometimes like sheaves of grain, sometimes like whirling water-spouts capped with a great cloud; occasionally they present most exactly the appearance of jets of liquid fire, rising and falling in graceful parabolas; frequently they carry on their edges spirals like the volutes of an Ionic column; and continually they detach filaments, which rise to a great elevation, gradually expanding and growing fainter as they ascend, until the eye loses them."
Fig. 221.
201. Change of Form in Prominences.—Fig. 221 represents a prominence as seen by Professor Young, Sept. 7, 1871. It was an immense quiescent cloud, a hundred thousand miles long and fifty-four thousand miles high. At a there was a brilliant lump, somewhat in the form of a thunder-head. On returning to the spectroscope less than half an hour afterwards, he found that the cloud had been literally blown into shreds by some inconceivable uprush from beneath. The prominence then presented the form shown in Fig. 222. The débris of the cloud had already attained a height of a hundred thousand miles. While he was watching them for the next ten minutes, they rose, with a motion almost perceptible to the eye, till the uppermost reached an altitude of two hundred thousand miles. As the filaments rose, they gradually faded away like a dissolving cloud.
Fig. 222.
Meanwhile the little thunder-head had grown and developed into what appeared to be a mass of rolling and ever-changing flame. Figs. 223 and 224 give the appearance of this portion of the prominence at intervals of fifteen minutes. Other similar eruptions have been observed.
Fig. 223.
Fig. 224.
V. THE CORONA.
202. General Appearance of the Corona.—At the time of a total eclipse of the sun, if the sky is clear, the moon appears as a huge black ball, the illumination at the edge of the disk being just sufficient to bring out its rotundity. "From behind it," to borrow Professor Young's vivid description, "stream out on all sides radiant filaments, beams, and sheets of pearly light, which reach to a distance sometimes of several degrees from the solar surface, forming an irregular stellate halo, with the black globe of the moon in its apparent centre. The portion nearest the sun is of dazzling brightness, but still less brilliant than the prominences which blaze through it like carbuncles. Generally this inner corona has a pretty uniform height, forming a ring three or four minutes of arc in width, separated by a somewhat definite outline from the outer corona, which reaches to a much greater distance, and is far more irregular in form. Usually there are several rifts, as they have been called, like narrow beams of darkness, extending from the very edge of the sun to the outer night, and much resembling the cloud-shadows which radiate from the sun before a thunder-shower; but the edges of these rifts are frequently curved, showing them to be something else than real shadows. Sometimes there are narrow bright streamers, as long as the rifts, or longer. These are often inclined, occasionally are even nearly tangential to the solar surface, and frequently are curved. On the whole, the corona is usually less extensive and brilliant over the solar poles, and there is a recognizable tendency to accumulations above the middle latitudes, or spot-zones; so that, speaking roughly, the corona shows a disposition to assume the form of a quadrilateral or four-rayed star, though in almost every individual case this form is greatly modified by abnormal streamers at some point or other."
Fig. 225.
203. The Corona as seen at Recent Eclipses.—The corona can be seen only at the time of a total eclipse of the sun, and then for only a few minutes. Its form varies considerably from one eclipse to another, and apparently also during the same eclipse. At least, different observers at different stations depict the same corona under very different forms. Fig. 225 represents the corona of 1857 as observed by Liais. In this view the petal-like forms, which have been noticed in the corona at other times, are especially prominent.
Fig. 226.
Fig. 226 shows the corona of 1860 as it was observed by Temple.
Fig. 227.
Fig. 227 shows the corona of 1867. This is interesting as being a corona at the time of sun-spot minimum.
Fig. 228 represents the corona of 1868. This is a larger and more irregular corona than usual.
Fig. 229.
The corona of 1869 is shown in Fig. 229.
Fig. 230.
Fig. 230 is a view of the corona of 1871 as seen by Capt. Tupman.
Fig. 231.
Fig. 231 shows the same corona as seen by Foenander.
Fig. 232.
Fig. 232 shows the same corona as photographed by Davis.
Fig. 233.
Fig. 233 shows the corona of 1878 made up from several views as combined by Professor Young.
204. The Spectrum of the Corona.—The chief line in the spectrum of the corona is the one usually designated as 1474, and now known as the coronal line. It is seen as a dark line on the disk of the sun; and a spectroscope of great dispersive power shows this dark line to be closely double, the lower component being one of the iron lines, and the upper the coronal line. This dark line is shown at x, Fig. 234.
Fig. 234.
Besides this bright line, the hydrogen lines appear faintly in the spectrum of the corona. The 1474 line has been sometimes traced with the spectroscope to an elevation of nearly twenty minutes above the moon's limb, and the hydrogen lines nearly as far; and the lines were just as strong in the middle of a dark rift as anywhere else.
The substance which produces the 1474 line is unknown as yet. It seems to be something with a vapor-density far below that of hydrogen, which is the lightest substance of which we have any knowledge. It can hardly be an "allotropic" form of any terrestrial element, as some scientists have suggested; for in the most violent disturbances in prominences and near sun-spots, when the lines of hydrogen, magnesium, and other metals, are contorted and shattered by the rush of the contending elements, this line alone remains fine, sharp, and straight, a little brightened, but not otherwise affected. For the present it remains, like a few other lines in the spectrum, an unexplained mystery.
Besides bright lines, the corona shows also a faint continuous spectrum, in which have been observed a few of the more prominent dark lines of the solar spectrum.
This shows, that, while the corona may be in the main composed of glowing gas (as indicated by the bright lines of its spectrum), it also contains considerable matter in such a state as to reflect the sunlight, probably in the form of dust or fog.
V. ECLIPSES.
Fig. 235.
205. The Shadows of the Earth and Moon.—The shadows cast by the earth and moon are shown in Fig. 235. Each shadow is seen to be made up of a dark portion called the umbra, and of a lighter portion called the penumbra. The light of the sun is completely excluded from the umbra, but only partially from the penumbra. The umbra is in the form of a cone, with its apex away from the sun; though in the case of the earth's shadow it tapers very slowly. The penumbra surrounds the umbra, and increases in size as we recede from the sun. The axis of the earth's shadow lies in the plane of the ecliptic, which in the figure is the surface of the page. As the moon's orbit is inclined five degrees to the plane of the ecliptic, the axis of the moon's shadow will sometimes lie above, and sometimes below, the ecliptic. It will lie on the ecliptic only when the moon is at one of her nodes.
206. When there will be an Eclipse of the Moon.—The moon is eclipsed whenever it passes into the umbra of the earth's shadow. It will be seen from the figure that the moon can pass into the shadow of the earth only when she is in opposition, or at full. Owing to the inclination of the moon's orbit to the ecliptic, the moon will pass either above or below the earth's shadow when she is at full, unless she happens to be near her node at this time: hence there is not an eclipse of the moon every month.
When the moon simply passes into the penumbra of the earth's shadow, the light of the moon is somewhat dimmed, but not sufficiently to attract attention, or to be denominated an eclipse.
Fig. 236.
207. The Lunar Ecliptic Limits.—In Fig. 236 the line AB represents the plane of the ecliptic, and the line CD the moon's orbit. The large black circles on the line AB represent sections of the umbra of the earth's shadow, and the smaller circles on CD represent the moon at full. It will be seen, that, if the moon is full at E, she will just graze the umbra of the earth's shadow. In this case she will suffer no eclipse. Were the moon full at any point nearer her node, as at F, she would pass into the umbra of the earth's shadow, and would be partially eclipsed. Were the moon full at G, she would pass through the centre of the earth's shadow, and be totally eclipsed.
It will be seen from the figure that full moon must occur when the moon is within a certain distance from her node, in order that there may be a lunar eclipse; and this space is called the lunar ecliptic limits.
The farther the earth is from the sun, the less rapidly does its shadow taper, and therefore the greater its diameter at the distance of the moon; and, the nearer the moon to the earth, the greater the diameter of the earth's shadow at the distance of the moon. Of course, the greater the diameter of the earth's shadow, the greater the ecliptic limits: hence the lunar ecliptic limits vary somewhat from time to time, according to the distance from the earth to the sun and from the earth to the moon. The limits within which an eclipse is inevitable under all circumstances are called the minor ecliptic limits; and those within which an eclipse is possible under some circumstances, the major ecliptic limits.
Fig. 237.
208. Lunar Eclipses.—Fig. 237 shows the path of the moon through the earth's shadow in the case of a partial eclipse. The magnitude of such an eclipse depends upon the nearness of the moon to her nodes. The magnitude of an eclipse is usually denoted in digits, a digit being one-twelfth of the diameter of the moon.
Fig. 238.
Fig. 238 shows the path of the moon through the earth's shadow in the case of a total eclipse. It will be seen from the figure that it is not necessary for the moon to pass through the centre of the earth's shadow in order to have a total eclipse. When the moon passes through the centre of the earth's shadow, the eclipse is both total and central.
At the time of a total eclipse, the moon is not entirely invisible, but shines with a faint copper-colored light. This light is refracted into the shadow by the earth's atmosphere, and its amount varies with the quantity of clouds and vapor in that portion of the atmosphere which the sunlight must graze in order to reach the moon.
The duration of an eclipse varies between very wide limits, being, of course, greatest when the eclipse is central. A total eclipse of the moon may last nearly two hours, or, including the partial portions of the eclipse, three or four hours.
Every eclipse of the moon, whether total or partial, is visible at the same time to the whole hemisphere of the earth which is turned towards the moon; and the eclipse will have exactly the same magnitude at every point of observation.
209. When there will be an Eclipse of the Sun.—There will be an eclipse of the sun whenever any portion of the moon's shadow is thrown on the earth. It will be seen from Fig. 235 that this can occur only when the moon is in conjunction, or at new. It does not occur every month, because, owing to the inclination of the moon's orbit to the ecliptic, the moon's shadow is usually thrown either above or below the earth at the time of new moon. There can be an eclipse of the sun only when new moon occurs at or near one of the nodes of her orbit.
210. Solar Ecliptic Limits.—The distances from the moon's node within which a new moon would throw some portion of its shadow on the earth so as to produce an eclipse of the sun are called the solar ecliptic limits. As in the case of the moon, there are major and minor ecliptic limits; the former being the limits within which an eclipse of the sun is possible under some circumstances, and the latter those under which an eclipse is inevitable under all circumstances.
The limits within which a solar eclipse may occur are greater than those within which a lunar eclipse may occur. This will be evident from an examination of Fig. 235. Were the moon in that figure just outside of the lines AB and CD, it will be seen that the penumbra of her shadow would just graze the earth: hence the moon must be somewhere within the space bounded by these lines in order to cause an eclipse of the sun. Now, these lines mark the prolongation to the sun of the cone of the umbra of the earth's shadow: hence, in order to produce an eclipse of the sun, new moon must occur somewhere within this prolongation of the umbra of the earth's shadow. Now, it is evident that the diameter of this cone is greater on the side of the earth toward the sun than on the opposite side: hence the solar ecliptic limits are greater than the lunar ecliptic limits.
211. Solar Eclipses.—An observer in the umbra of the moon's shadow would see a total eclipse of the sun, while one in the penumbra would see only a partial eclipse. The magnitude of this partial eclipse would depend upon the distance of the observer from the umbra of the moon's shadow.