Pick from the Past
Natural History, July-August 1926

Personal Experiences at Eclipse Expeditions

WITH A SUPPLEMENT IN COLOR OF THE THREE SOLAR ECLIPSES
SEEN IN THE UNITED STATES IN 1918, 1923, AND 1925








This series of eclipse paintings had its inception in the engagement of Mr. Butler by Mr. Edward Dean Adams to paint the eclipse of 1918, and Mr. Adams’ subsequent gift of the painting to the Museum. The present Triptych has been designed by Mr. Butler for the Proposed Hall of Astronomy, plans of which are now exhibited for the first time, and represents the first example of American Museum methods of education applied to Astronomy. For additional information, click each panel to enlarge.

THE observation of a total eclipse of the sun is one of great excitement and nerve-racking tension. The life of an eclipse astronomer may be likened to that of a hunter after big game. Many months and even years are spent in quietly investigating the problems, a costly equipment is accumulated and each piece of delicate apparatus is carefully tested at home to see that it will properly perform its designated functions at the critical moment. After some weeks spent in the field erecting the instruments and most carefully adjusting the cameras and spectroscopes, the eventful day approaches. Each and every one of the observing party becomes more and more intense and keyed up for the great event. A successful attack lies only in taking care that every one of the possible chances of failure is obviated. When the “zero-hour” arrives, bringing with it the total eclipse, will the attack be successful or will some little blunder spoil everything or will cloudy skies render of no avail all the long months of preparation and show only the eclipse entirely eclipsed by clouds?

Today, as never before, our daily life follows its course surrounded by the wonders of science. But amongst all the wonders of all the wonderful sciences there is no science which deals with such a gorgeous spectacle as is exhibited by the queen of the sciences, Astronomy, at the moment when the earth is gradually shrouded in darkness and when around the smiling orb of day there appears the matchless crown of glory, the beautiful corona. Nor can any science duplicate the wonderful precision shown by the work of the astronomer in his capacity to predict hundreds of years in advance the exact hour and minute at which an eclipse will take place and the locality on the earth’s surface where such an eclipse will be visible.

These predictions are not the product of clairvoyance or necromancy, but come only as the result of long continued series of observations carefully carried out by astronomers of all ages and of all climes, and hence are the direct consequence of the faith kept by one generation of astronomers in handing down the torch to the succeeding generation.

If no high degree of accuracy is necessary, the predictions can be carried out with sufficient precision by means of the Saros, an interval of 6585 days, known to the Chaldeans three thousand years ago. An eclipse of the sun can take place only at the time of the new moon, and also when the moon is near the plane of the [Earth’s] orbit, called the ecliptic, i.e., when the moon is near her node. Owing to the rotation of the axis of the moon’s orbit, the sun passes the moon’s node after an interval of 346.620 days which is known as the “eclipse year.” Nineteen eclipse years amount to 6585 days and the same number of days are found in 223 ordinary lunar months.


THE LATEST PHOTOGRAPH OF THE SOLAR CORONA — Taken by the Swarthmore College Expedition in Sumatra on January 14, 1926, with a camera of 15-feet focus. The shape of the corona is nearly circular, exhibiting the type associated with a maximum of sun-spots.

Photograph by John A. Miller
After this interval the distance from sun to earth and from earth to moon (on which the conditions of the eclipse depend) are repeated very nearly the same as at the preceding Saros with the result that the length of the totality is nearly the same as it was at the eclipse 6585 days earlier. This interval amounts to 18 years, 11 days, if four leap years intervene, or 18 years and 10 days if the 29th of February has come five times.

By means of the Saros it is possible to trace the conditions which cause an eclipse. A partial eclipse of the sun is followed after eighteen years by a partial eclipse, a total eclipse by a total eclipse with a duration nearly the same as that of the eclipse preceding it in the Saros. In such an eclipse series there are anywhere from 68 to 75 solar eclipses depending on conditions, extending over some 1200 years. Of these 25 are partial eclipses, while 45 are central eclipses, of which only 18 are total and 27 annular.

The first eclipse in the cycle which includes that of January 24, 1925, appears to have been a partial eclipse visible on May 27, 933, only in far southern latitudes on the earth, while the last eclipse of the cycle will be on June 25, 2177 in high northern latitudes. The first total eclipse of the cycle was on June 8, 1564.

After the occurrence of three total eclipses of the sun in the United States in the short space of seven years, it is very surprising to find that in the remaining years of the twentieth century total eclipses visible under good conditions appear to shun both the United States and Canada. The next total eclipse in-this part of the world will be on August 31, 1932, but totality will be witnessed only along a short track in the United States in the state of Maine.

THE ECLIPSE OF JUNE, 1918

In NATURAL HISTORY, VOL. XIX, 244–263, an account is given of the first of the three American eclipses, that of June, 1918, and only brief reference need be made here to the scientific results sadly hampered by thin clouds that almost everywhere greeted the observing parties. The novel feature at this eclipse owes its inception to Edward Dean Adams of New York, a patron of science and art, who became a member of the eclipse expedition on the condition that he be “given a job.” Mr. Adams took upon himself the responsibility of trying, by some method, by photography, by a drawing, or by a painting, to procure a reproduction which would show the gorgeous beauty of the corona, and which should be true not only as to form but more especially as to color. Mr. Adams was very fortunate in finding Howard Russell Butler, a portrait painter of note who was well equipped for the task by having developed a shorthand method of noting both form and color. Unfortunately, the artist had never seen a total eclipse. When he arrived in Oregon ten days before the eclipse he had rather lurid ideas, obtained from reading astronomical books, of the appearance of the corona. It was necessary to take him in hand, tone down somewhat his vivid impressions, and show him the colors of the red and blue hydrogen lines in a powerful spectrograph and then criticize the trial paintings of a typical eclipse that he made in the days before the eclipse. He was an apt pupil and had his plans so well laid that he took full advantage of the 112 seconds of totality furnished him. His painting which Mr. Adams has presented to the American Museum of Natural History is exquisitely beautiful, a work of fine art in which the painter has not forgotten that scientific accuracy is the first desideratum when recording an astronomical event. Accordingly, the scientific world owes a great debt of gratitude to Mr. Butler for his painting, but even a still greater debt to Edward Dean Adams, through whose conception, generosity, and enthusiasm the painting of the corona became possible.

THE ECLIPSE OF SEPTEMBER, 1923

On September 10, 1923, the moon’s shadow touched the earth’s surface at sunrise in the Pacific off the coast of Japan. The shadow traversed the ocean at a speed well over a thousand miles per hour and appeared off the coast of southern California somewhat after noon. After crossing lower California, Mexico, and Yucatan, the eclipse ended at sunset in the Caribbean Sea somewhat north of British Guiana.

Fortunately the total eclipse track in 1923 passed over a portion of southern California where one naturally expects superb conditions of weather. According to a well-known American humorist, “When the scientists said that Los Angeles was only to get a 99 per cent eclipse (that is about the only thing I ever knew Los Angeles to fall down on, they are usually 100 per cent) it rather hurt their pride. It was the first time that Nature had ever handed them a mere 99. I don’t really think they would have gotten over it but San Francisco only received some 85 or 90 per cent, so that somewhat salved things over. But the Chamber of Commerce has held a meeting and voted resolutions to apply for the next eclipse in its entirety. They claim that it was due to the Club’s not giving the matter more thought that they lost the 1 per cent on this one.” Everyone, indeed, has heard of the boasted climate of southern California, the “land of sunshine and flowers.”

As the result of all the information available about the weather, the best spot for the University of Virginia eclipse expedition seemed to be San Diego, the only large city in the United States inside the path of totality.


Mount Wilson instruments being assembled for the eclipse of 1923
Within the memory of the oldest inhabitant there had not been a single cloudy day on the tenth of September. To make matters almost ideal, the total eclipse came at one o’clock when the danger from sea-fog was reduced to a minimum. A conservative estimate placed the chances of perfect conditions at about 90 per cent. In fact, the chances appeared so high that it seemed poor business to insure against clouds (which one of the insurance companies was willing to write).

On arrival at San Diego we found that arrangements had been made for the McCormick Observatory party to locate within the military reservation at Fort Rosecrans. Six weeks before the eclipse we found ourselves in a furnished house of ten rooms and two baths, the former home of a major. Meals were sent in from the company kitchen, —and Rosecrans boasted of the “best cook in the Army!”

San Diego has every reason to be proud of her city, located as it is in one of the finest spots of the globe and blessed with an agreeable climate that might well be the envy of any city in the world. We from Virginia, a state which has never been backward in painting a halo around everything connected with the Commonwealth, were much interested in the spirit of civic pride and aggressiveness, which at times may have bordered upon boastfulness. Our six weeks’ stay was filled with pleasant memories and we only wish that each and every total eclipse might be observed under the congenial surroundings of Fort Rosecrans.

Many are the problems that may be attacked at the time of a total eclipse. One of the most important is the confirmation of the bending of the rays of light from a star as these rays pass close to the edge of the sun, as predicted by Einstein. We had no equipment for this research and we preferred to let others tackle this problem while we devoted ourselves to following up the line of attack carried out at four previous eclipses. It was therefore decided to have two stations for observing the eclipse, each equipped with a powerful spectrograph for ascertaining the constitution of the gases forming the sun’s atmosphere and also the heights in miles that each of the vapors extend above the surface of the sun. One of these was located on Point Loma, near our temporary home, the other at Lakeside, twenty-five miles inland, and near the edge of the shadow cast by the moon. The essential part of the spectrograph was a concave grating ruled by Rowland of Johns Hopkins. Each of the gratings used was made by ruling on a spherical concave mirror of speculum metal 15,000 lines to each inch. The Point Loma grating was four inches in aperture and thus had 60,000 ruled lines. This grating was used by the writer at the eclipse of 1905 in Spain. The Lakeside grating of 90,000 lines was used at the 1918 eclipse.

The sun is the nearest of the fixed stars, and it is the only star which permits us to examine its atmosphere in detail at the time of a total eclipse. A knowledge of the heights attained by the solar vapors gives information regarding the pressures under which the spectroscopic lines take their origin.

At very reduced pressures in the sun’s chromosphere and at the high temperature found there it is readily possible for an atom to lose an external electron and become ionized. The spectrum of the ionized atom differs very much from that of the neutral atom which has not lost an electron. In the ionized spectrum certain lines are enhanced in intensity and these are the lines which are stronger in the spectrum of the electric spark than in the electric are. Knowledge of these things are of the very greatest importance in furthering our knowledge of the chemical atom, a quest in which astronomy, physics, and chemistry are vitally interested.

Other problems of the eclipse are connected with the investigation of the corona. What is it? and whence comes its light? Its feeble light has been traced to the enormous distance of ten millions of miles from the sun’s surface. It cannot be a true atmosphere consisting of gaseous particles for, if it were, the pressure at the sun’s surface would be colossal, and we know for certain that such pressures do not exist. As a further proof we know that occasionally comets come close to the sun’s surface, dashing by the sun at the rate of a hundred miles per second, —and yet the comet goes through the corona without any friction and without any perceptible impeding of its velocity, which would be impossible if the corona contained much matter. The only true atmosphere possessed by the sun is the chromosphere which stretches up to a maximum height of 10,000 miles.

We must rely mainly on the spectroscope to give information on the constitution of the corona. On account of the feebleness of its light it is almost impossible to use a dispersion exceeding that of a single prism. None of the lines in the spectrum of the corona, amounting to about forty, has been identified with terrestrial sources. The strongest line in the whole spectrum is in the green, at wave length 5303. This belongs to the mysterious and unknown element “coronium.” Part of the coronal light takes its origin from the gaseous coronium which gives a spectrum of bright lines, and part by sunlight reflected from the molecules of the corona, since a dark Fraunhofer spectrum is observed. It has always been difficult to decide whether the reflection and scattering of the ordinary sunlight is in the corona itself or whether it takes place in our terrestrial atmosphere.

Soon after our arrival, the San Diegans discovered us, and before long, invitations began to roll in upon us to address the University Club, Rotary and Kiwanis, the Lions’ Club, etc., at their weekly luncheons, and to give public lectures at night illustrated by lantern slides. At the first of the public lectures, given in one of the city schools, more than fifteen hundred people were turned away for lack of room. The following week a lecture was given in the Civic Auditorium in Balboa Park to an audience of five thousand.

As the day of the eclipse approached the interest aroused in the gorgeous spectacle was very great. If we had been so inclined, we might have spoken morning, noon, and night. But there was work that must be done, work that could not wait, —and much as we like to speak in public, we had to try and transfer to some other visiting astronomer the doubtful honor.

An unusual opportunity awaited science at this eclipse. At North Island was the powerful battle squadron of the Naval Aircraft forces. Here was a chance to employ photography from the air on any of the problems that could be solved by this method. It is manifestly impossible to use any but comparatively small cameras from an airplane and to give any but very brief exposures. On account of the short exposures permitted, no spectroscopic work could be attempted from the air, no investigation of the Einstein effect and no photography of the corona that demanded large focal scale. Airplane photographs could not compete with those taken from a fixed installation on terra firma. In the event of clouds and the possibility of soaring above them in a machine, airplane photographs might be taken, but there would be little of scientific value in photographing the corona on such a small scale. There seemed only one direction in which airplane photography could assist the astronomer, and that was in the attempt to find the position of the moon in the sky with greater accuracy, for in spite of the many refined researches of the mathematical astronomer, the motion of the moon is far from being known with the precision desired. The moon is erratic in her motions and quite in keeping with her feminine gender. The position of the moon affects the time of the prediction of the eclipse and the projection of the moon’s shadow on the earth’s surface.

The program finally adopted consisted in the attempt to photograph from five separate stations along the northern edge and one at the southern edge on the shore of Mexico, the edge of the shadow of the moon. For this purpose it was necessary to use the best of mapping cameras known to the photographers and to choose special sites to photograph where the terrain would offer as great contrast as possible between a point just inside and one just outside the moon’s shadow. Incidentally it must be admitted that so little was known of the amount of light to he expected a few feet outside of the moon’s shadow that there was some doubt as to the final success of the investigation. But there was nothing to do but to “try and see what happened.”

During the erection of our apparatus it was interesting to watch the gradual installation of the gigantic equipment to be used on eclipse day by the astronomers from the Mt. Wilson Observatory. This great observatory, the best equipped and most famous in the whole world, is located on Mt. Wilson near Pasadena. Being therefore only about 140 miles from the eclipse site, it was possible to transport all of their instruments by motor truck from observatory to eclipse camp.

In planning the work for the eclipse, Director Walter S. Adams utilized the interferometer mounting on which to place the cameras, spectroscopes, photometers, etc., of the eclipse program.


Solar eclipse of January 24, 1925

Photographed by Frederick Slocum, Van Vleck Observatory, Wesleyan University, Middletown, Connecticut.
The longest camera was thirty feet in focal length, used to photograph the Einstein effect. Another camera half this length was for the same purpose. There were cameras to portray the beauties of the corona in various scales and in different colors of light, spectroscopes to ascertain the constitution of the corona, instrument to photograph and to measure visually the intensity of the light of the corona at various angles from the edge of the sun. The instruments numbering fifteen made the most complete equipment that had probably ever been assembled in one place for photographing solar eclipse. The personnel included about thirty members of the Mt. Wilson staff, while an auxiliary party of twenty computers and friends of the staff were prepared to watch and measure shadow bands, etc.

Between this site and the Spanish lighthouse was the installation of the Department of Terrestrial Magnetism of the Carnegie Institution of Washington. Captain Ault, Mr. Johnson, and many assistants were prepared to measure the magnetic and electric elements connected with the coming of the eclipse. Fortunately for them they could look on the weather with unconcern for it made little difference to their measures whether the weather was clear or cloudy on eclipse day.

The most of us could have no such smug satisfaction as the day of the eclipse approached. The weather indeed seemed “unusual” (we understood this is the first word that a California baby learns at its mother’s knee). At any rate the cloudless skies that we had been led to expect at 1:00 P.M. were not always forthcoming. During the first two weeks of our stay at Point Loma high fog at noon was the rule rather than the exception. Still it was a long time to the eclipse, and conditions would undoubtedly improve, —and they did. The next two weeks gave perfect skies, an absence of wind and altogether ideal conditions. Would this last, or would another cloudy spell come? We were optimists and believed implicitly what our friends the Californians told us.

Saturday, September 8, was a gloomy day and the eclipse would have been under poor conditions. Sunday was even worse. What would Monday bring? At eight-thirty Monday morning we were off on our trip to Lakeside. As we got inland from the Pacific, out of reach of the high fog, and where ordinarily we got out of clouds, the conditions improved but little. At ten o’clock at Lakeside we found high cirrus and the conditions looked hopeless. But there were three good hours until the time of totality and much to be done in the final preparations! By twelve o’clock we heard the airplane aloft which was to observe over Lakeside and the pilot waved to us in friendly greeting. The clouds were not very heavy but there were no clear patches anywhere. We kept a stiff upper lip and refused to believe that after so much bragging California was going to get a black eye.

Still the clouds were not too thick to prevent us watching the diminishing crescent of the sun as totality approached. Nature was hushed but the cocks were crowing lustily as if night were falling. At the very second when expected, the sun was blotted out and a faint trace of corona appeared. But what a bitter disappointment! We carried through our program, exposing eight plates in the seventy seconds of totality, knowing full well that the developed plates would show not the slightest trace of light.

Well, there was no use crying over it (although one was tempted to). We had done our best and the fault was not ours. If misery likes company, it was evident that the clouds were general and not local. As quickly as a high-powered car could take us to San Diego we telephoned to Point Loma only to find as we expected, that conditions there were even worse and practically not a trace had been seen of the corona. Radio told us that the Yerkes party on Catalina Island had suffered a like fate and that the usual luck of the Lick party had deserted them, for conditions where Doctor Campbell and the Lick expedition were located at Ensenada were as bad as could be.

And this was the record for “sunny California”! Not a single expedition greeted with good conditions, and the whole scientific work a dismal failure! There was nothing left to do but pack up and go home, —and then begin to get ready for the next eclipse, that of January 24, 1925. It seemed foolish to expect good weather conditions at nine o’clock of a winter’s morning in New York and New England and with the sun low down toward the horizon. But nothing venture, nothing win!

THE ECLIPSE OF JANUARY, 1925

The day before the eclipse of January 24 was one of gorgeously perfect blue skies. Would the morrow provide as good skies? After all the long weeks of preparation and of hard work in installing the instruments, would the work be all of no avail by clouds blotting out the eclipse? As always we were optimists. We astronomer’s located at Middletown, Connecticut, at Wesleyan University, were all so keenly interested in what we were planning to accomplish that each of us slept the night before the eclipse with one eye open. At six o’clock on eclipse morning my friend Professor Slocum, the director of the Van Vleck Observatory, in whose home Mrs. Mitchell and I were staying, looked out and saw the stars on a brilliant clear sky. He gave voice to his elation by saying “We greet you, sunny California.” But this boasting was almost our undoing, for inside of quarter of an hour the sky was thickly overcast with very dense clouds.

What a dejected crowd of astronomers we were at eight o’clock when we had gathered at the Van Vleck Observatory to observe “first contact,” the beginning of the eclipse. There was nothing but clouds everywhere!

A quarter of an hour later a ray of hope appeared, there was a blue streak of sky low down in the northwest,—and the clouds were coming from that quarter. Would it clear off in time! Luck was with us. Fifteen minutes before totality the sun broke through the clouds. Five minutes before totality each observer was at his station and we waited in great expectation. A cloud, very thin and very fleecy, now hung over the sun. It was not thick enough to do much damage and it was moving slowly. We hoped it, too, would go. When the timers called out “Two minutes,” the cloud was almost gone. By now it was beginning to get quite dark, a weird and unnatural pall coming over the landscape. The observers outside noted shadow bands flickering over the snow. At one minute before totality, with the thin crescent of the sun growing very small, the atmospheric conditions seemed perfect, the thin cloud had gone.

The signal “Thirty seconds” rang out. Everything was hushed while we waited for the zero hour, the beginning of totality. In my right hand I had a pair of binoculars over the right glass of which was a grating for observing the flash spectrum visually. When I saw it flash out, I gave the signal, “Go”; totality had begun; and with my left hand I opened the shutter to begin the first exposure.

As a spectacle, the 1925 eclipse suffered from its taking place so early in the morning. If the darkening had come on during the middle of the day with the sun high up in the sky the psychological effect would have been all the greater. Still it was a gorgeous sight!

It will be impossible in this short article to give in detail an account of the scientific discoveries from the 1925 eclipse. Valuable information was gained about the intensity of the corona whose total light is about equal to the light of a standard candle at the distance of a meter. Compared with full noon sunlight the corona gives one millionth of the light of the sun and about half that of the full moon at its average distance from the earth. The surface brightness of the corona at the limb of the sun is about 1.4 times that of the full moon, and half the total light of the corona comes from a zone extending only 3’ [3 minutes] from the edge of the sun.

So far in the history of astronomy it has been impossible to detect the corona without an eclipse. The difficulty is that the coronal light has superimposed upon it the enormously greater illumination near the sun produced by the scattering of sunlight in our atmosphere and in the telescope. The effect of the illuminated atmosphere is ten thousand times more intense than the corona near the sun’s limb, and hence the astronomers hold out no hope of detecting the corona—a least with present observational mean—at any time other than at an eclipse.

With the passage of time additional knowledge of the corona will be accumulated with great slowness due to the paucity of time available for observations. There are a few astronomers in the world who


Photograph of the inner corona at the 1926 Sumatra eclipse. Taken by the Swarthmore College Expedition with a camera of 63-foot focus pointed directly at the sun. Note the prominences and the curved hoods covering the prominences.

Photographed by John A. Miller
have been on more eclipse expeditions than I have been on, but I do not know of any astronomers who have actually seen the corona more frequently than I have. My first eclipse was that of 1900, and in the quarter of a century that has elapsed I have traveled fifty thousand miles in order to witness six eclipses. The total time afforded for observations during the periods of totality of all of these eclipses has made altogether a paltry fourteen minutes. If in the balance of my active career as a professional astronomer I am permitted to observe each and every total eclipse visible anywhere in the world (a condition of affairs that is not at all likely) I would then be the one astronomer throughout the ages that has had the opportunity of observing total eclipses of the sun for the total length of time of one half an hour.

Astronomy, the grandest and most perfect of the sciences, exerts an appeal to the popular imagination that has no equal in any other science. The reason for the universal interest in astronomy is not because it teaches us that this earth of ours is a tiny and insignificant speck in the cosmos but rather for the opposite reason, that man, though he is so small and infinitesimal in size, is gifted with powers which are almost infinite. From this earth of ours the astronomer’s brain intelligence can reach out across millions of miles of space and can ascertain what the sun is made of, what its temperature is, what the conditions are in the solar atmosphere, and acquire this information with so much of certainty that we know more of conditions in the solar atmosphere than we do of the terrestrial. Nor are the investigations of the astronomer confined to ninety millions of miles of space. Light from the sun, traveling at the rate of 186,000 miles per second reaches the earth in 499 seconds, a little over eight minutes. Light from the nearest sun outside of our solar system takes more than four years to reach us. But still the astronomer reaches out farther and even farther and he tells with great positiveness and certainty that the nebula in the constellation of Andromeda is so far away that its light takes one million years to reach the earth!

The earth has existed all of this time and even very much longer, for the most competent authorities are confident that Mother Earth existed substantially in the condition in which she now is for at least one thousand millions of years. Truly it is a great stretch of time, measured by man’s standards, when we reach back far enough to be able to say, “In the beginning God created the world.”

[For a description of a more recent eclipse, see “Shades of Glory,” by Joe Rao, Natural History, October 2008.]

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