Popular Science Monthly/Volume 60/January 1902/Recent Total Eclipses of the Sun
|RECENT TOTAL ECLIPSES OF THE SUN.|
HARVARD COLLEGE OBSERVATORY.
NATURE, when in her sublimest moods, is seldom seen without fear and danger. The tornado furnishes an exhibition full of weird beauty and scientific interest; yet man, in his haste to reach a place of safety, has little time for their contemplation. In the total eclipse of the sun, however, nature provides one spectacle, unsurpassed in grandeur, which may be observed in perfect safety. There was a time, indeed, when the chief emotion caused by an eclipse was fear, that superstitious dread of impending evil, which the presence of the unknown causes. This has now passed away, with the increase of knowledge. Perhaps no better illustration of the changed thought of the world in regard to natural phenomena could be found than a comparison of the following extracts. The first is from the early English chroniclers; William of Malmesbury, writing of the eclipse of March 20, 1140, says:
The 'New York Herald' of January 2, 1889, announced the eclipse of the previous day with the following headlines: "The Sun Knocked Out. After about two minutes it comes up smiling. Viewing the Eclipse. Clear skies almost universal along the belt of totality. Fine photographs taken." etc., etc.
Scientific study is now the chief attraction of an eclipse, although its spectacular beauty is appreciated as never before. Many natural phenomena, which otherwise would attract the systematic attention of scientists, fail to do this in consequence of the irregularity with which they occur. An eclipse of the sun, however, can be computed many years in advance, so that careful plans can be made for its observance. Even here grave trouble is caused by the uncertainties of meteorological science. It is a striking and somewhat discouraging fact that, while one can compute with reasonable accuracy the place and time of an eclipse a hundred years in advance, he cannot safely predict a single day before the event whether the sky will be clear or clouded. Under these circumstances it is not surprising that many people do not travel to the scenes of total eclipses. Expeditions to eclipses were practically unknown until half a century ago. Before that time man received with varied emotions those which Providence sent him, but did not travel far to seek them. Now, expeditions half way round the earth are common. This is not due entirely to the greater scientific zeal of the present day; probably few living astronomers would care to journey to the antipodes for an eclipse, under the conditions of travel which prevailed one or two centuries ago.
About seventy total eclipses of the sun occur each century. The average duration is, perhaps, three minutes, which amounts to about three and a half hours per century. If some Wandering Jew, at the beginning of the Christian era, had started to observe total eclipses of the sun, and had visited every one possible since that time, he would have had less than three whole days for observation. The time, indeed, would have been much less, since many of these eclipses occurred on the ocean, or at inaccessible regions of the earth, and clouds undoubtedly obscured the sky during half the time of totality. During the last half century, since spectroscopic observations have been carried on, the time during which an individual could have obtained favorable observations has been little, if any, more than a single hour. Under these circumstances the wonder is that it has been possible to accomplish so much. Many men, however, have worked at different stations along the narrow but extended path of totality, and every device which ingenuity could suggest has been utilized in order to obtain as much as possible in the brief seconds of totality. Nothing has contributed so much to increase the amount and accuracy of the results as photography. There is hardly a line of investigation which cannot be done more quickly and better by photographic than by visual methods. Nevertheless it would be a mistake to abandon visual observations altogether.
It may hardly need to be stated that for the most part scientific observations of total eclipses have for their object the promotion of our knowledge about the sun. No one, who understands at all how intimate is our dependence upon that great body, will question the wisdom of such efforts. In order to understand why certain problems can be better studied when the sun's face is covered by the moon, it may be well to outline our knowledge on the subject.
The sun, the center of our system, is an exceedingly hot, intensely bright, highly condensed, gaseous body. Its distance is a little less than 93,000,000 miles. Its volume is more than a million times that of the earth. Its specific gravity is somewhat greater than that of water. A gaseous body, denser than water, is something very different from our ordinary conception of a gas. That which we see, which gives the sun its apparent size, which sends us our light, is known as the photosphere. This is probably a brilliant shell of metallic clouds floating in an atmosphere of vapors of the same materials. There are certain details in this photosphere with which we are familiar, such as bright patches, known by different names, and sun-spots. For convenience we may regard this photosphere and all that it contains as the Sun, and all that lies outside this shell as the solar atmosphere. With the sun itself we have little to do in this article, since it can be better observed on any clear day than at time of eclipse. It is, however, only at time of total eclipse that we clearly see all those strange and complex features which make up what we have called the solar atmosphere. In our study of it, however, we must not be governed too much by any analogy with our own atmosphere. Lying next to the body of the sun is a layer of crimson flame, known as the chromosphere, which has a thickness of perhaps 5,000 or 6,000 miles. This may seem like a great depth for such a sea of fire, but compared with the enormous size of the sun it is very small indeed, and forms but a thin rose-colored rim about the edge of the sun. At the bottom of this is probably the so-called reversing layer. The solar spectrum is crossed by dark lines due to the elements which there exist. By these dark absorption lines, which are seen in the ordinary solar spectrum, the presence is known of many familiar elements. The higher regions of the chromosphere are less complex and consist in large part of hydrogen. From these regions, by forces which there operate, great masses of brilliant colored gas are thrown upward to enormous distances, in general 10,000, or 20,000 miles, but often much higher, even to 200,000 or 300,000 miles. Resting also on the photosphere is the corona, which extends its pearly light outward from the sun to immense distances which must be reckoned in millions of miles.
The different parts of the solar atmosphere are brightly luminous, and stand forth in splendid beauty at the instant of totality. The only reason why we do not see them on any clear day is that they are lost in the blinding light of the central sun. The sun's face must be shut out. This service is rendered by the moon at an eclipse. At other times the chief trouble is not that the sun shines directly into our eyes, since a piece of cardboard could be so placed as to cut off the rays. The real difficulty arises from the presence of our atmosphere, which becomes so bright from the diffused light of the sun, that the solar appendages are lost to view. This will be apparent from the daily phenomenon of the appearance by night, and the disappearance by day, of the stars. They are shining just as brightly by day as by night, and could be seen perfectly well if the atmosphere were removed for a moment.One of the most successfully observed of recent eclipses was that of May 28, 1900. The duration of totality was only two minutes, but almost perfect weather prevailed everywhere. It was visited by a large number of skilled observers, and an examination of the work performed and attempted will give a good idea of what astronomers at the present day hope to learn about the sun at times of total eclipse. As stated above, the ordinary solar spectrum consists of a bright band crossed by dark absorption lines due to a reversing layer present in the chromosphere. At the eclipse of 1870, Professor C. A. Young, who was watching the spectrum of the fast disappearing sun, saw, at the instant when the last bit of the photosphere was covered by the moon, the solar spectrum with its dark lines replaced by a spectrum composed of bright lines. This phenomenon, from the suddenness of its appearance became known as the 'Flash.' The 'flash' spectrum is one of the most interesting features of a total eclipse. The depth of the flash layer is very small, and the duration of its greatest intensity very brief, since it is covered by the moon after two or three seconds. To obtain good photographs of this phenomenon is somewhat difficult. This has been accomplished, however, at the eclipses of 1896 and 1898, and, especially, by several observers, at the eclipse of 1900. Several kinds of spectroscopes are in use. Ordinarily an astronomical spectroscope consists of a telescope, a narrow slit, a train of prisms, and a small telescope which brings the spectrum to the eye or to the photographic plate. When the object which is to be examined has an area like the sun the use of a slit cannot be avoided. When the source of light is a point, or a narrow line of light, there is no such necessity and the more simple apparatus, known as the slitless spectroscope, or objective prism, may be used. This consists of a prism placed over the lens of the telescope and a photographic plate at the focus. Instead of the prism or prisms a diffraction grating may be used. Professor Pickering, the director of the Harvard Observatory, has obtained for many years fine spectra of the stars by this method, which is an adaptation of the original method of Fraunhofer. An apparatus of this sort used in eclipse work is known as a 'prismatic camera.' It is evident that this form of spectroscope could not be successfully used on the uneclipsed sun, since the resulting spectrum would be simply a confused mass of colored light. There must be a slit, but in the case of total eclipse, nature furnishes it. As the moon at such times has an apparent diameter greater than that of the sun, it is readily seen that at the instant before the moon's disc completely covers the sun there will remain a very narrow crescent of light. At the instant after totality has begun the photosphere will be entirely covered, but for two or three seconds the thin line of chromospheric light remains in view. The two spectra taken at these moments, the one an instant before
and the other an instant after, the beginning of totality have been called the 'cusp' and the 'flash' spectrum. A similar pair occur, of course, when totality ends, but in reverse order. Figure 1 shows an enlargment of a portion of the cusp spectrum at third contact. This photograph was made by Professor E. B. Frost, of the Yerkes Observatory, at the eclipse of 1900. It furnishes an opportunity to compare directly the dark lines of the ordinary solar spectrum with the bright lines of the chromosphere. It is of the greatest interest to learn whether the two series of lines are identical, in whole or in part, though reversed, and in any case to study the characteristics of these bright lines. This photograph was made about ten seconds after the end of totality. The thin line of the photosphere, which had then emerged from behind the moon, was drawn out by the prism into the bright band, which constitutes the larger portion of the picture. This is the ordinary solar spectrum. It will be noted that while in spectra as usually seen the lines are straight, since a straight slit is used, here the lines are arcs, since nature furnishes a crescent of light. An examination of these dark arcs shows that in nearly all cases they become bright lines at the upper edge of the spectrum. This 'reversal' is due to the fact that just beyond the point where the crescent of sunshine ceased, was a small extension of the chromosphere, which was not covered by the moon. The precise determination of all the facts, which this and other similar photographs teach, is one of the important problems of total eclipses. The problem is somewhat complicated, as pointed out by Professor Frost; for although few dark arcs can be seen which do not terminate in a bright tip the curvature and position appear to be slightly different in some cases for the bright lines. Figure 2 shows the 'flash' spectra made at the second contact, that is, at the beginning of totality. The sun is entirely hidden by the moon, and all the lines which appear are doubtless due to the chromosphere. Certain irregularities, or 'bunches,' in the arcs, however, are due to solar prominences. From an examination of these and other photographs Professor Frost has measured and identified several hundred lines, and has reached the following conclusions: "At least 60 per cent, (and probably many more) of the stronger dark lines of the solar spectrum are found to be bright in a stratum not exceeding (for the majority of the lines) 1″, or less than 500 miles in height above the solar photosphere. There is moreover no reason in general to suppose that this is not equally true of the fainter lines. Therefore we may regard the existence of a reversing layer at the base of the chromosphere as fully confirmed by the photographs." These results are especially important since they contradict to some extent those which have been previously obtained. While the elevation of the strata which produce the most of the lines is less than 500 miles, the height of other gases above the photosphere is as great as 4,000 miles. The bright lines are identified as belonging to iron, titanium, chromium, hydrogen and other elements. The origin of some of the lines is unknown.
Although no other time may be so favorable for the study of the reversing layer as at total eclipses, the chromosphere and prominences may nevertheless be well studied on any clear day.
In connection with the eclipse of 1868 Janssen and Lockyer each independently discovered that by spectroscopic means the light of the chromosphere and prominences may be so separated from that of the sky as to become visible without an eclipse. The light from the region just outside the sun's limb is composed of skylight and the light of the solar atmosphere. Each is about equally bright. When this combined light is passed through a prism, that due to the sky is spread out into a continuous surface, thus becoming much fainter, while that due to the chromosphere or prominence, from its gaseous nature, is collected into bright bands, which thus surpass the skylight in intensity and may be seen or photographed. This line of work has been
|Fig. 3. Great Eruptive Prominence. With Hale Spectroheliograph. Made March 25, 1895, 10h. 34m. A. M.||Fig. 4. Great Eruptive Prominence. With Hale Spectroheliograph. Made March 25, 1895, 10h. 58m., A. M.|
greatly extended by different scientists, notably by Hale, of this country, who, by a device known as the spectroheliograph, has succeeded in making, without an eclipse, photographs showing all the prominences surrounding the sun and the details of the solar surface at the same time. These photographs are made in monochromatic light. They represent what would be seen if the eye were sensitive to light of the wave-length of the K line only. Figures 3 and 4 show a great eruptive prominence photographed by Professor Hale, March 25, 1895. The interval between the two photographs was 24 minutes, during which time the prominence was thrown upward from a height of 135,000 miles to 281,000 miles. This implies a velocity of at least 100 miles per second.
At times of total eclipse it is perhaps possible to obtain better photographs showing finer details than can be made under other conditions. Figure 5 is an enlargement of a photograph made at the eclipse of 1900, by Professor E. E. Barnard, assisted by Mr. G. W. Ritchey. It shows a mass of prominences at the southwest quadrant of the sun. Along the irregular limb of the moon, which appears black, is seen the ragged storm-tossed surface of the chromosphere, of increasing depth toward the right owing to the moon's position at the instant of the exposure. Thrown up from this are the vast fantastic masses of the prominences or 'red flames.' They remind us of pictures which show the effects produced by the explosion of submarine torpedoes. The larger mass at the left rises to the height of 60,000
miles. This photograph was made with a telescope of only six inches aperture and six and a half feet focal length, a small instrument compared with some which have been used at recent eclipses. The writer has seen no other photograph of prominences, however, which, in delicacy of detail, surpasses the one here shown.
The single feature of a total eclipse which can be seen and studied only at such times is the corona. In early ages small mention was made of the corona. Apparently the dread of impending evil overwhelmed man, and prevented careful observations. As fear disappeared and scientific interest grew, attention was drawn to the 'red flames,' and at nearly the same time to the beautiful halo of light which has been fittingly named the 'corona.' Since that time the favorable moments of totality have been too few to clear up the mystery of its nature. Reasoning from the methods which have made the study of the chromosphere and prominences possible without an eclipse, various attempts have been also made to thus observe and photograph the corona. The simplest way would be by direct vision or photography. There is no doubt but that, if we could remove for a moment the earth's atmosphere, whose glare interferes with our vision, we should be able to see the chromosphere, prominences and corona without any artificial aid. The brightness of the inner corona is about the same as that of the ordinary sky near the sun. If then one could find
a locality where the sky was extraordinarily clear, he might hope, by placing a shield in front of the sun itself, to see these fainter features. The writer of this article made an attempt several years ago in this way on the summit of El Misti, Peru, at an elevation of 19,200 feet. At this altitude one-half the earth's atmosphere is below the observer and that which remains is of extraordinary clearness. Photographs were made of the region immediately about the sun, using an opaque disc to protect the plate from the sun's direct image. The true corona did not appear upon the plates. Other methods promised better results, such as the use of monochromatic light, presumably that of the line 'K 1474.' Experiments in this line have been carried on by Professor Hale with skill and enthusiasm on the summit of Pike's Peak, on Mount Etna and elsewhere, but without success. He has also attempted to solve the difficulty by a study of the heat, using the bolometer. Recent investigations given below explain the failure
of this method. The polarization of the coronal light also suggests a method which has not yet yielded successful results. Although the future may furnish the solution, none of the attempts yet made has been successful, and for the present our only knowledge of the corona must be obtained from what can be learned during the brief moments of total eclipses. Good photographs of the corona can be easily and rapidly made and if an abundance of these were alone necessary our knowledge would be well advanced. The general features of the corona have a certain permanence. Comparatively slight changes are known to take place during the three or four hours while an eclipse is passing over the surface of the earth. There may be, however, finer details than are shown on the best photographs yet obtained, which would give witness to more rapid changes. From year to year large changes in the form of the corona occur and these appear to be associated with the sun-spot period. This is a natural inference, especially since the solar prominences are thus associated. This is well shown by a comparison of the form of the corona in 1889 and 1900, which occurred near the sun-spot minimum, with the form in 1893, which was near sun-spot maximum. These are given in Figures 6, 7 and 8. The equatorial streamers and the divergent polar streamers are much more pronounced at the time of sun-spot minimum. At maximum the corona is more nearly circular. The polar streamers are beautifully shown in Figure 9, a photograph made by the eclipse party, which was under the direction of Secretary Langley, of the Smithsonian Institution. The true nature of the corona and the complex changes which it undergoes are unknown. The spectroscope is the magician's wand which science generally uses to reveal the constitution of unknown objects, but in this case the revelation is only partial. In 1869 Professor Young found the spectrum to be characterized by a bright line in the green, which he identified as Kirchhoff's line 1474. The unknown substance which produces this line has been given the name 'coronium.' There are also other less conspicuous bright lines. When the name 'helium' was assigned to the origin of certain lines in the solar spectrum, no such terrestrial substance was known. Later it was found by Ramsay. A similar issue for coronium would be very acceptable. The corona also yields a faint continuous spectrum, in which Janssen and others have reported certain dark lines of the solar spectrum. This signifies, that in addition to luminous gases, giving a spectrum of bright lines, the corona contains some substance, like a cloud, which is capable of reflecting ordinary sunlight. A part of the light appears to be polarized. It is thought by some observers that there is also a bright continuous spectrum free from dark lines. If true, this would imply a three-fold origin to the coronal light. For the explanation of the corona we have the diffraction theory of Hastings, the mechanical theory of Schaeberle, the magnetic theory of Bigelow, and others. The complete solution of the problem is of the greatest difficulty and of the greatest importance. At the eclipse of 1900 some experiments with that remarkable instrument, the bolometer, appear to throw new light on this subject. These experiments were made by Secretary Langley's chief assistant, Mr. C. I. Abbott, who reached the following conclusions:
These conclusions are of so great importance that it is very desirable that the observations upon which they depend, should be repeated at other eclipses. It is, therefore, very unfortunate that Mr. Abbott at the recent eclipse at Sumatra was prevented by clouds from carrying out the observations which he had traveled so far to obtain. Other observers, however, were no more fortunate. Professor E. E. Barnard was provided with a telescope of 611⁄2 feet focus, and with plates forty inches square, but was prevented by clouds from obtaining results of much value. This was the fate, also, of many other observers from different countries, who had taken stations in different parts of Sumatra. Results of value were obtained, however, by the party from the Massachusetts Institute of Technology, whose photographs of the corona are unsurpassed. At the Island of Mauritius, also, the English astronomers obtained valuable results. As a whole the eclipse of 1901 probably failed to add much to our knowledge of the sun.
Aside from the problems relating to the sun's constitution, there is still outstanding the question as to the existence of an intramercurial planet. This problem can be studied to much greater advantage at total eclipses than at other times. Photographic charts can be made of the whole region about the sun during totality. An examination of several sets of such photographs, taken at different eclipses, should confirm or refute the existence of such a planet. For greater certainty the sky should be photographed in duplicate at each eclipse. Although sufficient material for the decision of this question could apparently he accumulated rapidly, this has not yet been accomplished for a variety of reasons. At the eclipse of 1900, several parties were provided with apparatus especially planned for this work. The weather was everywhere perfect, but accidents of one kind or another affected the results. The Smithsonian party, however, obtained photographs, one of which showed stars fainter than the eighth magnitude. Several suspicious objects were found on these plates, which remain unconfirmed, owing to the failure of other attempts. This and other questions, which, it was hoped, would be decided by the eclipse of 1901, must await some later eclipse for their solution.
To-day, although much is known about the sun, its deeper secrets are yet unraveled. The foundations of physical science appear, indeed, to be somewhat shaken. It is hinted that molecules and atoms are, after all, but 'convenient fictions,' signifying, perhaps, that the human mind is not capable of grasping the ultimate conditions of matter. We hear of corpuscles, which are inconceivably small 'fragments of atoms.' These corpuscles are carriers of electricity. It may be that in this line lies the explanation of many terrestrial, solar, and even cosmical, phenomena.