Popular Science Monthly/Volume 57/May 1900/The Coming Total Eclipse of the Sun
|THE COMING TOTAL ECLIPSE OF THE SUN.|
By FRANK H. BIGELOW,
PROFESSOR OF METEOROLOGY, UNITED STATES WEATHER BUREAU.
THE circumstance which renders the coming total eclipse of the sun, on May 28, 1900, of special significance to thousands of people who might otherwise entirely overlook the occasion is the fact that the path of the moon's shadow over the surface of the earth, or the track of the eclipse, is in such a convenient locality—namely, in our Southern States—as to render the places of visibility easily accessible. Instead of being obliged to go to the ends of the earth, at a heavy expenditure of time and money, all the while running the risk of not seeing the eclipsed sun on account of prevailing cloudiness, we are fortunate this time to have the show at home in our own country. While many foreigners will be induced to come to the United States to make observations, it is certain that more people will be in a position to see this eclipse with a minimum amount of trouble than has ever happened before in the history of eclipses, at least since the telescope was invented and careful records of the phenomenon preserved.
The track of May 28th enters the United States in southeastern Louisiana; passes over New Orleans, La., centrally; over Mobile, Ala., which is on its southern edge; over Montgomery, Ala., on the northern edge; over Columbus, Ga.; south of Atlanta, Ga., which lies about twenty-five miles to the north of it; near Macon, Milledgeville, and Augusta, Ga., Columbia, S. C, Charlotte, N. C.; over Raleigh, N. C, which is ten miles north of the central lino; and over Norfolk, Va., fifteen miles north of the center. The track is about fifty miles wide in all parts, and the duration of the eclipse varies from one minute and twelve seconds near New Orleans to one minute and forty-four seconds near Norfolk, on the central line. These durations diminish from the maximum at the middle of the track to zero at the northern and southern limits of it, so that an observer must be stationed as near the central line as possible in order to see much of the eclipse. The population of several of the above-mentioned cities is at present as follows: New Orleans, 242,000; Mobile, 31,000; Montgomery, 22,000; Columbus, 20,000; Atlanta, 66,000; Raleigh, 13,000; and Norfolk, 35,000. It is evident that with very little exertion more than 500,000 people can see this eclipse. It is most fortunate that the track passes near so many cities, because, with their facilities for the accommodation of visitors, many will be induced to undertake excursions with the purpose of taking in this rare sight, and a little enterprise on the part of railroads and transportation companies might easily increase the numbers. If people will go to a parade, yacht race, or an exposition, and consider themselves paid for their expenses, then surely they will find in this great spectacle of Nature not only an object of wonder and beauty, but also one of peculiar instruction in many important branches of science. All educators who can induce their pupils to make such an expedition will implant a love of astronomy in many impressionable minds which will become a source of pleasure to them for the rest of their lives.
Out of about seventy eclipses of the sun which have occurred somewhere in the world within the nineteenth century, there have been only eight total eclipses of more or less duration visible on the North American continent. The others happened in places often remote from civilization, and sometimes in entirely inaccessible localities, as over the ocean areas. The difficulty of transporting heavy baggage to the remote parts of Asia, Africa, or South America is such as to preclude all but a few scientists from any effort to observe eclipses. The writer was much impressed with the formidable nature of undertaking to establish eclipse stations in places which are distant from centers of population by his own experience on the West African Eclipse Expedition, sent out by the United States Government, for the eclipse of December 22, 1889, to Cape Ledo, on the west coast of Angola, about seventy miles south of St. Paul de Loanda. Nearly eight months were consumed in the course of the preparations at home and in the voyage out and back. The expedition, it should be said, however, went to Cape Town, South Africa, and halted also at St. Helena, Ascension Island, and Barbados for magnetic and gravity observations, so that all this time should not be charged to the eclipse proper. We sailed in the old frigate Pensacola, the companion
Chart 1.— Track in the United States of the Total Eclipse of May 28, 1900. (By permission of the United States Weather Bureau.)
to Farragut's flagship, the Hartford, with Captain Yates. In earlier days Admiral Dewey commanded this ship, and the expedition was fitted out while he was in charge of the Bureau of Equipment at Washington. The same fine courtesy that has become so well known to his countrymen was at that time extended to all the members of the expedition.
The cloudiness along the track of the eclipse in the Southern States on the 28th of May, 1900, is evidently a matter of much importance not only for all astronomers, but for non-professional spectators. If it could be foretold, with the same precision as the astronomical data give the time and the place of the occurrence of the eclipse, that the day itself will be fair or cloudy, or that certain portions of the track will be clear while others will be obscured, it would be of great benefit. The cost of these scientific expeditions is very great, since it is necessary to transport many heavy and delicate pieces of apparatus into the field, including telescopes, spectroscopes, polariscopes, and photographic cameras, and set them up in exact position for the day of observation. The expedition to Cape Ledo, West Africa, in 1889, carried out a large amount of material, prepared it for work during the totality, and then entirely lost the sun during the critical moments by a temporary obscuring of the sky through local cloud formations. There had been some clouds at the station during the forenoons for several days preceding the eclipse, but the sky was usually clear and very favorable during the middle of the afternoons. The totality came on at three o'clock, and photographs of the sun were taken at first contact about 1.30 p.m.; clouds thickened, however, and totality was entirely lost, while the sun came out again for the last contact at 4.30 p.m. This was a very trying experience, and of course could not have been avoided by any possible precautions. Some astronomers have thought that the advance of the moon's shadow is accompanied by a fall in temperature, and that cloudiness is more likely to be produced from this cause.
Soon after the West African eclipse Professor Todd, of Amherst College, proposed that more systematic observations be made for the probable state of the sky along eclipse tracks, with the view of at least selecting stations having the most favorable local conditions. The method was tried in Chili, April, 15, 1893, and in Japan, August 8, 1896, with some success. Heretofore the available meteorological records, which were originally taken for other general purposes, had been consulted, and some idea formed of the prevailing tendency to cloudy conditions. In accordance with the improved method, the United States Weather Bureau has been conducting special observations on the cloudiness occurring from May 15th to June 15th in each of the three years 1897, 1898, and 1899, for the morning hours of the eclipse—between 8 a. m. and 9 a. m. A tabular form was sent through the local offices to such observers as were willing to act as volunteers in making these records, and their reports have been studied to discover how the cloudiness behaves along the eclipse track at that season of the year. Each of the three years gives substantially the same conclusion—namely, that there is a maximum of cloudiness near the Atlantic coast in Virginia, extending back into North Carolina, and also near the Gulf coast in Louisiana and in southern Mississippi, while there is a minimum of cloudiness in eastern Alabama and central Georgia. The following table will serve to make this plain:
The Prevailing Cloudiness of the Sky along the Eclipse Track.
|State||General sky.||Sky near the sun.|
The significance of these figures is shown by transferring them to a diagram, given on Chart II, which indicates the average cloudiness prevailing over the several States where they are crossed by the track. The marked depression in the middle portions, especially over Alabama and Georgia, indicates that the stations in these districts make a much better showing than those nearer the coast line. The reasons for this difference are probably many in number, but the chief feature is that the interior of this region, especially over the higher lands of the southern reaches of the Appalachian Mountains, which are from six hundred to one thousand feet above the sea level, is somewhat freer from the moisture flowing inland from the ocean at that season of the year. The table shows also two divisions, one for the "general sky," wherein the relative cloudiness was noted in every portion of the visible sky, and for the "sky near the sun," where the observation was confined to the immediate vicinity of the sun. The two records agree almost exactly, except that the sky near the sun averages a little lower than the general sky. This indicates that although the sun will be seen in the morning hour of May 28th, when it is only from thirty to forty degrees above the horizon, yet this is not an unfavorable circumstance. The low altitude, on the other hand, makes it easier for those at the instruments to enjoy a more comfortable observing position than if it were nearer the zenith, where one must look directly upward. Of course, a storm of some kind may occur on that day to modify these general weather conditions and upset all calculations. While the cloud observations suggest that Georgia and Alabama have the best sites for the eclipse, it must be remembered that the duration is about one minute and twenty seconds in Alabama, and one minute and forty seconds in North Carolina. As a gain of twenty seconds in observing time will be considered by many of sufficient importance to take chances on the cloudiness, stations will be selected in North Carolina for that reason, although the probability for minimum cloudiness is
Chart II.—Probable State of the Sky along the Eclipse Track.
Average percentage of cloudiness in May and June.
twice as good in Georgia and Alabama. The table shows that the chances are only one to six against observers located in these States, while near the coast they are about two to six against them. On the whole, the general result is that observing in this region ought to be successful, because the favorable chances for good weather are above the average at that season of the year.
On Chart I there are six lines drawn across the track: No. 1 near New Orleans, and No. 6 on the ocean to the east of Norfolk, Va. These represent the places for which the times of the duration are computed in the American Nautical Almanac, with the following results:
|No.||h. m.||h. m.||m. s.|
|1.||At||1 30||Greenwich M. T.||= 7 27.||Local M. T. the duration is||1 12.6|
|2.||"||1 35||""||= 7 47.||"""""||1 19.6|
|3.||"||1 40||""||= 8 05.||"""""||1 26.0|
|4.||"||1 45||""||= 8 22.||"""""||1 31.7|
|5.||"||1 50||""||= 8 40.||"""""||1 37.0|
|6.||"||1 55||""||= 8 54.||"""""||1 41.9|
Chart III.— Fifteen Pictures of the Solar Corona, arranged in the Eleven-
Year Period, to show the Recurrence of Similar Types during this Period.
An observer at the intersection of these cross-lines with the central line will see the totality during the intervals given in the table.
The mode of the formation of the shadow cones of the moon, called the penumbra for the partial shadow and the umbra for the total shadow, are well illustrated in general works on astronomy, and good geometrical pictures of them can there be found, together with much useful information regarding the subject of eclipses. As we are here concerned chiefly with certain practical points about the eclipse of 1900, it will be well for the reader to consult such works for many details regarding the astronomical features attending an eclipse of the sun which must now be omitted.
There are many existing theories to account for the phenomenon of the sun's bright appendage, called the corona, which is visible only during eclipses, on account of the absorbing effects of the earth's atmosphere on its light. Is it electrical, or is it magnetic? Is it composed of fine stuff ejected from the sun, or of meteoric dust falling upon the sun? Is it merely an optical effect, as some suppose, or is it a portion of the newly discovered radiant matter streaming off to enormous distances into space? The answer to these questions is eagerly sought through observation, photography, and every other possible means, on the occasion of each total eclipse.
The efforts of astronomers have thus far secured a series of pictures of the solar corona, which, when compared together, show very distinctly that the corona, as well as the spots, the protuberances, and the faculæ, are going through a series of changes which seem to repeat themselves in the so-called eleven-year period. It has also been proven, with entire distinctness, that the earth's magnetic field, as marked by the changes in the intensity of the magnetic elements, in the auroral displays, and the earth electric currents show variations which synchronize closely with those observed on the sun; also that the weather elements of pressure, temperature, precipitation, and storm intensity all harmonize with the solar and the earth's magnetism in the same synchronism. All attempts of scientists to detect any variations in the sunshine which falls upon the tropics have been entirely futile; on the other hand, it has been shown that the magnetic forces having the characteristics just mentioned impinge upon the earth in a direction perpendicular to the plane of the earth's orbit, just as if the sun, being a magnet, throws out a field of force to the surface of the earth, which, by its variation depending upon the internal workings of the sun, produces the changes just enumerated in the earth's atmosphere and in its magnetic field, also throughout the planetary system, being, of course, strongest near the sun. The belief is gradually growing among scientists that the earth, the sun, and the planets are all magnetic bodies, and have these bonds of connection between them-in addition to the Newtonian gravitation. This is a most fascinating field of research, and, though full of difficulties, yet attracts the attention of many who are convinced that one of the most pressing duties of the hour is to clear up the problems connected with the transmission of energy from the sun to the earth in other forms than the ordinary or sunlight radiation. It is entirely probable that the secular variations of the weather changes from year to year, and even from month to month, are bound up with these solar forces, and that the solution of these questions will carry with them much information of practical use to civilized man.
The coronas of the past forty years are shown on Chart III, taken from the report of the eclipse of 1896 (August 9th), by A. Hansky. It arranges the coronas in the eleven-year period so far as the dates at which the eclipses occurred permit this to be done, and by comparing them in vertical lines the similarity is at once seen for the respective quarters of phases of the period. The forecast there given for 1900 is seen to resemble 1867, 1878, and 1889, but it differs in orientation from that on Chart IV, which was prepared by the author. The four coronas on the left in Chart III are taken at the sun-spot maximum, and the appearance is that of total confusion in the structure of the rays; the second and the fourth columns are for the sun's medium intensity at about halfway between the maximum and the minimum, and they show a system of polar rays taking on structural form, the second column being at a stage of diminishing and the fourth at one of increasing solar activity; the third column gives the corona when the spots are at a minimum of frequency and the sun is in a comparatively quiescent state, wherein the polar rifts are very distinct and the equatorial wings or extensions greatly developed.
The successful observation of a solar corona depends upon three conditions: the selection of the instrument, its proper mounting, and the photographic process, regarding each of which a few suggestions will be made. The instruments are divided into two classes, for visual and for photographic work. But in either case
Chart V.— Relative Images of the Corona as taken with Telescopes ranging from Forty Feet to Four Feet in Focal Length.
Some amateur observers have telescopes but no mountings suitable for eclipse work, and many astronomical telescopes have good equatorial mountings at home which are yet unavailable in the field for lack of proper foundations and supports. The ordinary telescope balanced near the center, with the eye end subject to all sorts of motions which may happen through jarring and rough handling in the hurry of shifting photographic plates, makes a very poor eclipse apparatus. All telescopes of any length should be held firmly at each end, so as to be perfectly steady, since the least vibration ruins a coronal picture devoted to delicate photographic effects. There are two ways of accomplishing this, and only two. Dispensing with an equatorial mounting, the lens must be set permanently on a base, and light reflected from a mirror must be utilized, which shall be concentrated on a plate also placed on a fixed base. This is the method employed by Schaeberle in Chili, April 16, 1893, to obtain No. III of Chart V. There is no objection to it if an observer possesses a perfectly plane mirror, which it is very difficult and also expensive to obtain; if the reflecting mirror should be imperfect it would distort the image of the corona. The second method, lacking a good mirror, is to mount the long-focus lens in a tube and point it directly at the sun. A forty-foot lens was thus mounted at Cape Ledo for December 22, 1889, and its action was very satisfactory. Of course, it was a cumbersome arrangement, and could not be employed by a small party. The foundation for the mounting of the forty-foot tube consisted of two casks filled with stones and cement, and set firmly in the ground. These made two good piers, since the narrowing tops of the casks held the bed plates of the telescope as in a vise. A triangle, whose base was parallel to the earth's axis and having the telescope tube itself for the long side, was fitted with an extension rod for adjustment in altitude on the third side, and the whole was made to revolve on ball bearings. This triangular support was rotated by a side rod of adjustable length, whose end terminated in a sand piston working with a regulating valve. The sand flowed out steadily like an hour-glass, and dropped the tube, keeping it central on the sun. The image was made to follow accurately for twenty minutes without tremor, all the time holding the solar disk tangent to fixed lines. The principle of a revolving triangle and a short piston, taking the place of an expensive reflecting mirror with a delicate clockwork or one carrying a telescope balanced on its center but subject to jars and side motions, is an important assistance in field work on account of its ready adaptability to all sorts of observations. Since time is limited, it is necessary to provide all operations with automatic arrangements as far as possible, by using such an apparatus as that described. What can be applied successfully to a forty-foot lens can serve for shorter telescopes. In combination with spectroscopes, polariscopes, and special instruments for photographing, an immense amount of work can be compressed into the few seconds allowed by mounting them all on such a movable frame as the triangle. The old-fashioned method of putting one observer to one telescope ought to be abandoned. Of course, for a rising sun during the forenoon a modification in the moving support must be employed. This should be such as to cause the objective of the telescope to rise from the ground toward the meridian, and it must be accomplished by attaching a heavy weight which in sinking draws the tube upward.
Of the two kinds of photographic plates, the wet and the dry, the dry plates are much more convenient in the field, and are good for certain purposes. The objections to them are that the rough granulations of the gelatin film sometimes overpower the fine tails of the corona itself; they burn out very near the disk by over-exposure while the faint outer extensions are being taken. Wet plates having the requisite quickness are harder to prepare,
Chart VI.—Tracks of the Eight Total Eclipses of the Sun seen on the North American Continent during the Nineteenth Century.
but are smoother and hold the coronal rays better from the base to the outer edge, and there is always plenty of time to give the necessary exposure. The No. 26 Seed plate requires from 0.5 to 2.0 seconds only, generally one second being about right; the wet plate will take the corona in eight seconds or less. The best time of exposure should be tested on a bright star of about the second magnitude, by trial before the eclipse. There is no rule about the photographic focus, except to discover it by a series of exposures at different distances near the supposed point. Eclipse work is a practical matter, and many rough-and-ready methods must necessarily be admitted. A good lens in a wooden light-tight tube, supported at each end, having the motion of the sun, the photographic focal plane carefully determined, the time exposures very short, and, finally, exceedingly slow development of the picture after the eclipse—these form the prime requisites. Expensive telescopes, clockwork on heavy iron piers, reflecting mirrors, and such like apparatus are not needed. Ingenuity in practical details, with great anxiety about the essential matter of the light itself, is what is needed for a successful eclipse expedition.
Those persons who have no telescope for viewing the sun, or camera for photographing it, can yet see the corona to great advantage by means of a good opera glass, and indeed this is really the most satisfactory way to thoroughly enjoy the spectacle. The object may be sketched on paper at once or from memory, and this picture may well be of value to astronomy.
The tracks of the eight North American eclipses seen since 1800 are shown on Chart VI. It is noted that three have paths very similarly located, and that five run in directions about parallel to one another, but almost at right angles to the first group. This comprises the eclipse of November 30, 1834, duration two minutes; August 7, 1869, two minutes and three quarters; July 29, 1878, two minutes and a half, which stretch from Alaska south-eastward in a fan-shape to the South Atlantic coast. The second group contains the tracks June 16, 1806, four minutes and a half; July 17, 1860, three minutes; January 11, 1880 thirty-two seconds; January 1, 1889, two minutes and a quarter; and May 28, 1900, two minutes. These tracks all trend from southwest to northeast, and cross the North American continent in different latitudes, that of May 28, 1900, being the most southerly and of rather short duration, lasting less than two minutes in the United States.
The path of the total eclipse of May 28, 1900, after leaving the United States, crosses the North Atlantic Ocean to Coimbra, Portugal, and continues over North Africa to its end at the Red Sea. Stations which are not situated on the path of the totality will see the sun partially eclipsed, in proportion to the distance of the locality from the central line to the northern or the southern limits. Thus New England, New York, the Ohio Valley, and the southern Rocky Mountain districts will see the sun about nine
Chart VII.—The Path of the Total Eclipse of may 28, 1900, with Times of Beginning and Ending at Several Places and the Northern and Southern Limits of the Partial Eclipse.
tenths covered; the Lake Region, the lower Missouri Valley, and Southern California will see an eight-tenths eclipse; and the northern Rocky Mountain region about six tenths or seven tenths. The best way to view the partially eclipsed sun is to secure three strips of thin colored glass, one and a half inch wide by five inches long—red, blue, and green; bind them over the eye end of a good opera glass, and adjust focus on the sun. This makes the light safe for the eyes and brings out the spherical aspect of the sun's ball. The time of the eclipse can be read by interpolating within the lines marked on Chart VII.
No other eclipse track will occur in this country till June 8, 1918, when one of the first kind will pass from Oregon to Florida, two minutes in duration. Another will occur in New England, January 24, 1925. Eclipses seven minutes in duration will occur in India in 1955, and in Africa in 1973, the longest for a thousand years. The remoteness of the last two, both in time and place, put them out of reckoning for most of us, but those of 1918 and 1925 give additional zest to the approaching eclipse of May 28th, as affording further opportunity for confirming facts and noting differences based upon the observations now made.