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.”
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 shows the corona of 1860 as it was observed by Temple.
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.
The corona of 1869 is shown in Fig. 229.
Fig. 230 is a view of the corona of 1871 as seen by Capt. Tupman.
Fig. 231 shows the same corona as seen by Foenander.
Fig. 232 shows the same corona as photographed by Davis.
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.
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
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.
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.
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 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.
The umbra of the moon’s shadow is just about long enough to reach the earth. Sometimes the point of this shadow falls short of the earth’s surface, as shown in Fig. 239, and sometimes it falls upon the earth, as shown in Fig. 240, according to the varying distance of the sun and moon from the earth. The diameter of the umbra at the surface of the earth is seldom more than a hundred miles: hence the belt of a total eclipse is, on the average, not more than a hundred miles wide; and a total eclipse seldom lasts more than five or six minutes, and sometimes only a few seconds. Owing, however, to the rotation of the earth, the umbra of the moon’s shadow may pass over a long reach of the earth’s surface. Fig. 241 shows the track of the umbra of the moon’s shadow over the earth in the total eclipse of 1860.
Fig. 242 shows the track of the total eclipse of 1871 across India and the adjacent seas.
In a partial eclipse of the sun, more or less of one side of the sun’s disk is usually concealed, as shown in Fig. 243. Occasionally, however, the centre of the sun’s disk is covered, leaving a bright ring around the margin, as shown in Fig. 244. Such an eclipse is called an annular eclipse. An eclipse can be annular only when the cone of the moon’s shadow is too short to reach the earth, and then only to observers who are in the central portion of the penumbra.
212. Comparative Frequency of Solar and Lunar Eclipses.—There are more eclipses of the sun in the year than of the moon; and yet, at any one place, eclipses of the moon are more frequent than those of the sun.
There are more lunar than solar eclipses, because, as we have seen, the limits within which a solar eclipse may occur are greater than those within which a lunar eclipse may occur. There are more eclipses of the moon visible at any one place than of the sun; because, as we have seen, an eclipse of the moon, whenever it does occur, is visible to a whole hemisphere at a time, while an eclipse of the sun is visible to only a portion of a hemisphere, and a total eclipse to only a very small portion of a hemisphere. A total eclipse of the sun is, therefore, a very rare occurrence at any one place.
The greatest number of eclipses that can occur in a year is seven, and the least number, two. In the former case, five may be of the sun and two of the moon, or four of the sun and three of the moon. In the latter case, both must be of the sun.