Popular Science Monthly/Volume 65/June 1904/The Total Solar Eclipse of August 30 1905
|THE TOTAL SOLAR ECLIPSE OF AUGUST 30, 1905.|
By Professor W. W. CAMPBELL,
DIRECTOR OF THE LICK OBSERVATORY, UNIVERSITY OF CALIFORNIA.
THE last total eclipse of the sun observed was that of May 17, 1901, whose path crossed the islands of Mauritius, Sumatra, Borneo and New Guinea. Its durations, in Sumatra six and a half minutes, was the greatest of any observable eclipse of the last half century. The shadow touched the islands at very few accessible points, and the choice of observing stations was unusually limited. Nevertheless, observations were undertaken by a relatively large number of well-equipped expeditions from this country and Europe. At nearly all stations clouds of various degrees of thickness covered the eclipsed sun, and the work was seriously hampered by them. Fortunately, many valuable photographs were secured through thin clouds. For example, Professor Perrine, in charge of the William H. Crocker Expedition from the Lick Observatory, obtained results of great value with each of his ten instruments, though only five to twenty-five per cent, of the light passed through the clouds. In fact, it would be difficult to say wherein they could have been better, except that the intra-mercurial planet search was incomplete in one third of the area called for in the program.
A total eclipse, of short duration, occurred on September 20, 1903, in the southern Indian Ocean. The shadow did not pass over land, unless within the closed south polar continent, and no effort was made to secure observations.
A long eclipse will occur on September 9, 1904. It, too, will come and go practically unobserved, for its path passes eastward over the central Pacific Ocean without touching any known islands, and terminates on the coast of northern Chile about six minutes before sunset. With the sun at such a low altitude, the atmospheric disturbances and the almost complete absorption of actinic rays will preclude the possibility of securing satisfactory observations, except perhaps as to the general form of the corona. It is known that the Chilean astronomers are expecting to view the phenomenon. Further plans do not seem to be called for.
The next observable eclipse is that of August 30, 1905. It is well situated, and will be looked forward to with unusual interest. The shadow path begins at sunrise south of Hudson's Bay, enters the Atlantic Ocean a short distance north of Newfoundland, crosses northeastern Spain, northeastern Algiers and northern Tunis, passes centrally over Assuan on the Nile, and ends at sunset in southeastern Arabia. The durations on the coast of Labrador, in Spain and at Assuan, are two and a half, three and three fourths and two and three fifths minutes, respectively.
It is none too soon to form plans for observing this eclipse. In this connection, an account of the leading eclipse problems now pressing for solution may have interest for the general reader, and perhaps some usefulness to those who will plan programs of work, though the latter will prefer a more detailed article than would be justified here.
There is probably no phenomenon of nature more beautiful and impressive than a total eclipse of the sun. Every such event is of great human interest. Even the uncivilized tribes of the earth realize, crudely, the force of the scientific fact that the sun is the origin of the light, heat and other forms of energy which make life on this planet possible.
The absorbing interest taken in eclipses by astronomers is on a broader basis. Our sun is one of the ordinary stars. In size it is perhaps only an average star; or it may even be below the average. It is the only star near enough to us to show a disk. All other stars are as mathematical points, even when our greatest telescopes magnify them 3,000-fold. The point-image of a distant star includes all its details, and it must be studied as a whole, whereas the sun can be studied in geometrical detail. Our sun is likewise the only star bright enough to supply metrical standards demanded in the study of other stars. It is not too much to say that our physical knowledge of the stars would to-day be practically a blank if we had been unable to approach them through the study of our sun. If we would understand the other stars, we must first make a complete study of our own star. Several of the most interesting portions of our sun are invisible, except at times of solar eclipse. Our knowledge of the sun will be incomplete until these portions are thoroughly understood; and this is the reason why eclipse expeditions are despatched, at great expense of time and money, to occupy stations within the narrow shadow belts.
The difficulties of solar study, in spite of comparative nearness and intense brightness, are very great. It is not generally appreciated that we are unable to study the body of the sun except by indirect methods. The interior is invisible. The spherical body which we popularly speak of as the sun is bounded by the opaque photosphere—a cloud covering composed of condensed vapors of the metallic elements. The photospheric veil, including the larger interruptions in it which we call the sunspots; the brighter areas, closely connected with the photosphere, called the faculæ; the reversing layer, a few hundred miles in thickness, immediately overlying the photosphere; the chromosphere, a shell several thousand miles thick, associated with and overlying the reversing layer; the prominences, apparently ejected from the chromosphere; and the corona, extending outward from the sun in all directions to enormous distances; these superlatively interesting features of the sun, constituting the only portions accessible for direct observation by telescope and spectroscope, are an insignificant part of its mass. They are literally the sun's outcasts. Our knowledge of the sun is based almost entirely upon a study of these outcasts. We might hope to reach safe conclusions as to the characteristics of a hermit nation by making a careful study of its banished subjects, provided the observed types correspond with types produced by our own civilization; but if new types, new customs, new forms, presented themselves, and were observable only at long range, our conclusions as to the characteristics of the country from which they were expelled would come slowly and uncertainly. It is a difficult matter to comprehend the structure and condition of any one of the sun's outcasts; the chromosphere, for example. To determine what the conditions within the body of the sun must be in order to create and maintain such an outcast shell is far more difficult.
The influence of eclipse observations upon solar and astrophysical research has been most remarkable. The reversing layer, the chromosphere, the prominences and the corona were in fact discovered at eclipses. Many of our present every-day methods of studying them are also eclipse products. The richness of eclipse results, considering the remarkably short intervals available for observation, is unique in science. To realize this, we need only recall that the durations of observable total eclipses, clear and cloudy, have amounted altogether to about one hour since the spectroscope was applied to the problem, and about half an hour since photographic methods have prevailed.
Eclipse problems relate not only to the properties of the less massive portions of the sun—everything, apparently, outside of the photospheric layer—but to the question of possible planets between the sun and Mercury. It is well known that mathematical theory, based upon Newton's law of gravitation, has not yet fully accounted for the motion of Mercury. The perihelion of its orbit moves forward at least 40″ in a century more than theory calls for. The most plausible way of accounting for this progression has been the supposition that an undiscovered planet, or a group of small planets, exists within the orbit of Mercury. The search for such objects has been a well-defined eclipse problem; the sun-lit sky prevents effective search by every-day methods. Organized efforts to discover such bodies by visual means were made at the eclipses of the late seventies and early eighties, but they were unsuccessful. Photographic methods, though not planned for efficiency in that particular problem, were applied in the nineties. Early in the year 1900 it occurred independently to Professor W. H. Pickering, of Harvard College Observatory, and to Messrs. Perrine and Campbell, of the Lick Observatory, that efficiency in the photographic method requires the cameras to be of relatively long focus, in order to reduce the intensity of sky illumination on the photographic plate; and each of these astronomers, unknown to the other two, fixed upon the proportions which such instruments should have. Their results were in good general accordance. The first attempt to apply this method was made by Professor Pickering at the eclipse of May, 1900, with camera lenses three inches in aperture and 135 inches in focal length, but no evidence was secured. Mr. Abbot, of the Smithsonian Institution party, obtained one photograph with a similar lens, covering a limited area of the sun's surroundings, which recorded eighth magnitude stars. Four suspicious images on the plates were noticed; but whether they were ordinary photographic defects or images of real objects could not be determined, as the required second plate of the same region was not secured by this party or others.
The last word on the subject is by Perrine, who applied the method in Sumatra in May, 1901. His four telescopes, making three exposures each, secured negatives in duplicate of a region 6° wide and 38° long—19° on each side of the sun, in the direction of the sun's equator. Through thin clouds covering two thirds of this area, one hundred and sixty-two stars, including several as faint as the ninth magnitude, were photographed; and through thicker clouds covering the remaining third, eight stars, four of them between 6.0 and 6.5 visual magnitudes, were recorded. While these instruments were in use in the preceding February at the Lick Observatory, exposures were made on the region of the sky which would be occupied by the eclipsed sun in May. All objects on the Sumatra eclipse plates were recognized as known stars, by means of the February Mount Hamilton plates.
It is probable that any such planets would be well within the region covered, provided their orbit planes make a small angle with the sun's equator. The earth was very nearly in the plane of the sun's equator on May 18—exactly in it on June 3—which was a favorable circumstance. Again, there is little probability that such bodies would be as much as nineteen degrees from the sun, and a width of six degrees would therefore allow for a considerable departure of the orbit planes from the solar equator.
Professor Perrine has deduced the following interesting results from these observations:
|Asteroid.||Visual Magnitude.||Distance, Miles.|
Assuming that the distance of the 'mean asteroid' from the earth is 153 million miles, we find that such a body, if transported to a distance of twenty-eight million miles from the' sun (corresponding to an elongation distance of eighteen degrees), and seen from the earth at elongation, would be one hundred and ten times as bright. This corresponds to an increase in brightness of 5.1 magnitudes. Such a body would be relatively brighter near superior conjunction, and fainter near inferior conjunction. An intramercurial planet at the above mean distance from the sun would have to be only one tenth the diameter of the mean asteroid to appear of the same brightness.
From the dimensions and brightness of the four brighter asteroids we find that on the average one of these bodies, three hundred miles in diameter, seen at the opposition distance of the mean asteroid, would appear as of the eighth magnitude. Hence an intramercurial planet of similar constitution and thirty miles in diameter should appear as a star of eighth magnitude. If the hypothetical planet were closer to the sun, the difference of brightness and size would of course be correspondingly greater than that found above.
These observations indicate, therefore, with the exception to be noticed later, that there is no planetary body as bright as 5.0 visual magnitudes within eighteen degrees of the sun whose orbit is not inclined more than seven and one fourth degrees to the plane of the sun's equator. They further indicate that in two thirds of this region there was no such body as bright as seven and three fourths magnitude. The possible exception to be noted is that at the time of the eclipse such a body or bodies might be directly in line with the sun or with the brightest portion of the corona. The area covered by the moon's disk and corona was, however, less than one two-hundredth that of the region searched. Owing to the increased cloudiness at the end of totality, the search is not quite complete to the fainter magnitude, yet it seems altogether probable that were there any considerable number of bodies as bright as seven and three fourths magnitude, some of them would have been detected. A planetary body thirty-four miles in diameter would, under the conditions considered, appear as a star of seven and three fourths magnitude. The total mass required to produce the change observed in the orbit of Mercury is about one half the mass of the planet. It would require, therefore, no less than seven hundred thousand bodies thirty-four miles in diameter and as dense as Mercury to equal such a disturbing mass.
From the observations detailed above it does not seem possible that sufficient matter exists in the region close to the sun in the form of bodies of appreciable size to account for the observed perturbations.
Belief in the existence of intramercurial planets has been based upon anomalies in the orbital motion of Mercury, and Perrine's work has gone far to show that the discrepancies must seek some other explanation. Had the thicker clouds not reduced the minimum visible in one third the area observed in Sumatra from the ninth to the sixth magnitude, it is a question whether one could recommend that this search be continued at future eclipses. However, so long as we admit that it is a question, the effort to secure definite results, positive or negative, should be made. It is not impossible that existing bodies could have been in the region of thicker clouds, or in that occupied by the moon and inner corona, or in areas outside the limits of the strip six degrees wide.
The eclipse of August 30, 1905, will occur when the earth is seven degrees from the plane of the solar equator. The maximum distance occurs September 7. It will therefore be advisable to search over a region of considerably greater width than was the case in 1901. Inasmuch as increased area means increased instrumental equipment, expense, and difficulty, a corresponding shortening of strip to be observed would perhaps be justified. It is to be hoped that observing parties well equipped for the intramercurial search will be located in Labrador, Spain, Tunis and Egypt. If clear weather prevails at any of the four stations, very valuable results may be secured. Should a new planet be observed at three such stations, the enormous interest attaching to its discovery would be heightened by the fact that its approximate orbit could be determined at once. If no planets are revealed on first class plates, the negative result would be scarcely less valuable, though certainly less interesting, than positive results; and the intramercurial question would cease to be a pressing eclipse problem.
The sun's altitude will be only 26° in Labrador and 23° in Egypt. The altitude of the lower end of the area to be photographed will be small at these stations. The atmospheric disturbances and absorption at such low altitudes will require that the exposures be lengthened. Perhaps a better plan would be for the Labrador party to cover the entire critical region west of the sun, and only five or six degrees below it; and for the Egyptian party to cover the whole region east of the sun and only five or six degrees below it.
Eclipse observation of the sun itself concerns all that lies outside the photosphere and faculæ. While the main features of these outer volumes are for the most part quite irregular in form, yet in a general way they lie, going outward from the photosphere, in the order of reversing layer, chromosphere, prominences and corona.
The reversing layer was discovered at the eclipse of 1870 by Professor Young. It appears to consist of a thin stratum of incandescent gases, probably between five hundred and fifteen hundred miles in thickness, immediately overlying the photosphere. Its inner bounding surface seems to be quite definite and regular, but its outer surface is certainly not so. The depth of the stratum of vapor for each element composing it is probably a function of the properties and quantity of the element in question. The reversing layer is cooler than its substrata, yet abundantly hot, if isolated from its underlying strata, to produce a spectrum consisting of thousands of bright lines occupying the positions of the dark lines of the ordinary photospheric spectrum. When the moon, at the eclipse of 1870, gradually covered the photosphere, the dark-line spectrum lasted until the instant when the photosphere entirely vanished, whereupon the reversing layer was isolated, and Young observed the sudden flashing out of its bright-line spectrum. A bright line apparently replaced each dark line, and lasted perhaps two or three seconds, until the moon entirely covered the reversing stratum.
In so complex a spectrum, lasting but a few seconds, visual observations were difficult, and no records of any considerable consequence could be made. The bright-line (flash) spectrum was photographed for the first time by Shackleton at the eclipse of 1896; and several photographs of it were secured at the three succeeding eclipses, but many were defective on account of poor focusing or other cause. They confirm Young's discovery of the reversing layer, which, by the absorption of its cooler gases, introduces the dark lines in the solar spectrum. The lengths of the arcs not covered by the moon also tell us much concerning the thicknesses of the vapors of the various elements, and therefore much concerning the structure of the sun at those levels. Additional work, with more powerful instruments, in perfect adjustment, is demanded, with a view to securing better quantitative results.
Photographs of the reversing-layer spectrum, made with two, four, or more seconds' exposure, are integrated effects. Changes taking place during the exposure are lost. For this reason, it would be very valuable if a continuous record of the spectrum at one point on the limb could be secured on a plate moving in the direction of the length of the spectrum lines. The writer obtained such photographs in 1898 and 1900, but with small instruments, not designed especially for that work; and it is hoped that improved apparatus will be available for the eclipse of 1905. There is need that flash spectra with both fixed and moving plates should be secured, since each system has its advantages and disadvantages. On moving plates the faintest lines might not be recorded, but a continuous record of changes in the strengths of lines, as the moon gradually covers the reversing strata, should be obtained.
The chromospheric stratum, overlying the photosphere, is of irregular depth, varying from four thousand to ten thousand miles. The reversing layer, to the best of our knowledge, is included in its lower strata. The prominences seem to be flame-like or explosive projections extending outward from the chromosphere; the matter in them previously and subsequently forming a part of the chromosphere. Many of the salient facts known about chromosphere and prominences were learned at eclipses; and they are still studied with some profit on such occasions. However, the spectroscopic method of observing them, devised independently by Janssen and Lockyer in 1868, has made the prominences, and to some extent the chromosphere, available for everyday study. But it must not be overlooked that, while fairly satisfactory observations of one or both subjects can be secured without an eclipse, yet the eclipse negatives are still imperatively needed to show the mutual relations of the various structures—reversing layer, chromosphere and inner gaseous corona. It is known that the prominences are larger and more numerous at sunspot maxima than at other times. The question whether the chromospheric stratum is likewise thicker and more distorted at sunspot maxima than at minima is a question for eclipse observers to settle. Observations of the continuous spectrum of prominences or chromosphere can by present methods be made only at eclipses.
The corona, perhaps the most fascinating solar feature, is exclusively an eclipse phenomenon. Various attempts have been made to observe it visually, photographically and thermally, without an eclipse; but all failed, and there seems to be no hope of success by methods now known. Any chance for even moderate success would seem to be limited to the inner portion whose spectrum contains bright lines. A daily record of this would, no doubt, be extremely valuable, but the real problem of the corona would remain unsolved.
In many respects the corona is as enigmatical as ever. A coronal photograph is the result of a projection upon and into one plane, at right angles to the line of sight, of all that remains of the sun after subtracting the volume of matter hidden by the moon. The tops of some coronal streamers, the intermediate portions of others, the bases of those near the limb and the corresponding parts of prominences and chromosphere are all projected into one point. Whether every man who has gone forth to solve the riddle of the corona has fully realized the odds against success is doubtful.
Much has been written concerning a possible eruptive origin, or about magnetic influences in shaping the forms of its streamers. It has been shown that the details of the corona at one eclipse are totally different from those at another, and that the outline form of the corona is a function of the sun spot cycle. At sun spot maximum the general form is nearly circular, and the polar streamers are nearly as bright as the equatorial streamers. At minimum, the polar streamers are much fainter than the equatorial ones, and long wings seem to extend out approximately from the spot zones. It is a surprising fact that, with all the changes of form, we do not yet know whether the materials composing the streamers are moving in, or out, or both, or neither. The epoch-making, large-scale coronal photographs by Schaeberle in 1893 opened a promising way of determining such facts, but astronomers have been slow in taking advantage of the opportunity. Photographs of the corona should be secured for this purpose at widely separated stations—preferably at three or more stations—with essentially identical instruments, and with equivalent exposures, in order that results may be as nearly comparable as possible. This effort to determine motion in the corona, it seems to me, is the most important problem of the coming eclipse; and, fortunately, the circumstances of widely separated stations in Labrador, Spain, Tunis and Egypt, and promising weather conditions at the last three are favorable for the attack. Considering all elements of the question, including that of probable unsteadiness of the atmosphere at one or more stations, the five-inch aperture, forty-foot focus cameras, promise the most directly comparable, and therefore the best, results. The only case of motion on coronal plates thus far observed seems to be that detected by Schaeberle, on the Chile-Brazil-Africa plates of 1893; and in this instance the moving mass was decided to be a comet, and not a part of the real solar appendage.
One of the most intensely interesting features ever observed in the corona was the tremendous funnel-shaped disturbance recorded on the Sumatra plates of 1901. Perrine was able to show, with essentially no room for doubt, that the vertex of the disturbance was immediately over the large and only sun spot visible on the sun in the week preceding and the week following the eclipse. The circumstances were unusually favorable for reaching this conclusion: there was but one sun spot; it was very near the limb at the time of the eclipse; there was but one region of unusual disturbance visible in the corona; this was on an extraordinarily large scale, and its vertex was near the sun's limb; and the disturbance and the sun spot had identically the same position on the sun's limb. It was exceedingly unfortunate that three cameras of the forty-foot pattern could not have been working in harmony, at three stations widely separated, to determine what changes, if any, were taking place in the disturbed coronal area. Under excellent atmospheric conditions, cameras still larger than those referred to should record more minute details of coronal structure, and thus lead to valuable results; but such observations would reach their full value only in case comparisons could be instituted with photographs taken under similar conditions at distant stations. However, as already stated, cameras of the forty-foot pattern give greater promise of cooperative usefulness, taking into account the average atmospheric conditions which must be expected at some of the stations.
The spectra of recent coronas have led to most interesting results. They leave no doubt that, at those eclipses, the spectrum of the inner corona contained no perceptible dark lines. Perrine's Sumatra photographs seem to establish that the spectrum of the great outer portion is substantially a copy of the solar spectrum. The simplest interpretation of these observations is that the outer corona is largely composed of minute particles which reflect and diffract the sunlight falling upon them, whereas the portions near the hot solar surface are mostly incandescent, shining by their own light. Polarigraphic observations are in harmony with this theory. Opposed to the idea of the incandescence of the inner corona stands, alone, the thermographic observations by Abbot in 1900, of a corona less hot than the instrument with which he worked. While it is difficult to assign such a low temperature to particles near the solar surface, and one should perhaps look for other interpretations of the thermographic results, yet there is an urgent demand for a repetition of all the preceding observations bearing upon the nature of the corona.
The polarigraphic observations of recent coronas have been very interesting—leading to the knowledge that the light of the corona is strongly polarized, except, apparently, in close proximity to the sun's surface; and strengthening the view that the corona is very largely composed of minute particles of matter which receive their light from the photosphere. Unfortunately, the photographs do not permit the making of quantitative measurements of the amount of polarization in and across the solar radii; and future programs for eclipse observations in this line should make provision for securing comparable unpolarized coronal images for standards of reference.
Special interest will be taken in determining whether the comparatively shallow inner stratum of the corona which yields a brightline spectrum, is more extensive at the sunspot maximum of 1905 than it was at the minimum of 1898–1901. The chances are that it will be both thicker and more uniform in thickness. Should it be brighter than at recent eclipses, the opportunity to search for new coronal bright lines will be excellent.
The accurate wave-length of the principal coronal bright line, near λ 5303, should be determined. A modern spectrograph, holding three dense flint prisms, should make the problem easy. The accurate wavelengths of all truly coronal lines should be determined as rapidly as possible, partly in order that a serious effort may be made to represent them by a simple common law, as has been done for hydrogen and helium.
Of many other eclipse problems—the photometry, the shadow bands, etc.—it need only be said that accurate observations will prove very useful.
The tendency of recent eclipse work is toward a unification of the problem. The main divisions of the sun's structure are no longer to be studied separately. Close connection has been observed between spots and faculæ; between photosphere and reversing layer; between sun spot and coronal disturbances; between coronal streamers and prominences; between prominences and chromosphere; between the sunspot curve and the form of the corona; and in other ways the unity of the problem is emphasized. This is only what we should expect, for all these outward and visible features of the sun must be related products of the stupendous forces at work within its body. In reality, all observations of the sun, whether those made daily at fixed observatories or those secured at eclipses, bear upon the solution of one problem: the structure, composition and condition of the sun, from its center to the outermost limits of the coronal streamers.
It is well known to eclipse observers that a regrettably large proportion of observations of these phenomena are failures, or are but partially successful. Some of these unfortunate outcomes are due to nervousness at the critical moment; a psychological state of which some observers know nothing, and against which others are unable to contend. It is a mistake to invite nervousness by attempting to do too much in the limited duration of totality. If seven photographs can be secured with one instrument, working with moderate speed in changing plates, an attempt to secure eight by working under high nervous tension would be a serious error. However, the most prolific source of failure is that of new instruments and new methods used for the first time on eclipse day. It is not an uncommon practise to delay preparations until a few months or weeks before expeditions must depart for their stations; to order new instruments, or new parts of instruments, just in time to have them shipped from factory to station; to use new methods of focusing, etc., for the first time, at the station; and to leave insufficient time for the rehearsal of program after the instruments are in supposed adjustment. It is unnecessary to say that this is culpable management and all wrong. Every piece of apparatus should be set up, adjusted, tested and used at the home station; and time should be available thereafter for making modifications in apparatus, methods and program, and for retrial. With every possible preparation made before leaving home, the astronomer will find his time occupied at the eclipse station in solving the ninety and nine local problems whose coming is sure, but whose nature can not be foreseen. To install half a dozen instruments in a fixed observatory so that they will work satisfactorily, one at a time, and at the observer's leisure, is not a small problem. To construct a temporary observatory in an out of the way corner of the earth, to mount the eight or ten instruments, and to train the dozen or more assistants so that all the instruments and all the men will work together satisfactorily at the fixed instant of totality, is a problem of a very different order. The point which I wish to emphasize is that preparations for observing the eclipse of August, 1905, should begin early in 1904.