A Scientific Centennial for Dartmouth

PROFESSOR OF PHYSICS

NOT often can a college identify a specific event as a turning point in its history. For Dartmouth College one such event occurred 100 years ago on August 7, 1869. On that date a solar eclipse swept across the United States. Its path of totality began in northern Asia, crossed the ocean just south of Bering's Strait, and traveled southeastward from Alaska, across Canada, Montana, the Central and Mideastern States, to disappear over the Atlantic off the coast of North Carolina.

In this path at Burlington, lowa, Charles A. Young, Appleton Professor of Natural Philosophy and Professor of Astronomy at Dartmouth College, and his graduate assistant, Charles F. Emerson, had made ready a most unusual instrument for studying the sun's chromosphere. By the end of that momentous day Professor Young would have acquired international fame and Dartmouth College would be recognized as a leading institution in solar research.

Scientific achievements do not just happen; usually long-term preparations precede the crucial event. So it was for Professor Young. Even as early as 1866 when he returned to the Dartmouth campus to assume the professorships formerly held by his father, Prof. Ira Young, Charles Augustus began the special studies and training that were necessary for the successful outcome of that August day in 1869. This does not mean that science was a new field for him. Far from it. He had assisted his father in the observatory and the physics laboratories as early as the age of 10; had entered Dartmouth at the age of 14 and shown unusual talent in science as a college student; had accompanied his father to Europe where they visited astronomical observatories and scientific laboratories, purchased philosophical apparatus, and met many outstanding scientists; and soon after graduation in 1853 had published his first scientific article on "Climate" in a volume entitled New Hampshire As It Is. Daring the next three years, however, he taught classics and for one of those years attended Andover Theological Seminary with the intention of entering the mission field. Fortunately for astronomy and for Dartmouth this digression terminated in 1856 when he accepted the Professorship of Mathematics, Natural Philosophy and Astronomy at Western Reserve College in Hudson, Ohio. There he sharpened his abilities in these scientific disciplines and perfected his skills as a teacher.

In science as in every other human endeavor, the age that poses the questions often produces the talented men who can answer them. So it was in the 1860's. By 1866 the fundamental principles and methods of spectroscopy had become well established through the work of such eminent men as Fraunhofer, Herschel, Angstrom, Kirchhoff, and Bunsen. The spectroscope had become a powerful tool for studying the atomic constituency of glowing gases and hot bodies including the sun. Then, during the great solar eclipse that swept across India on August 17, 1868 came the first opportunity to examine spectroscopically the thin red ring of light around the sun, the chromosphere, with its curious protuberances which had been seen during earlier total eclipses. They had already been shown to have a solar rather than a lunar or terrestrial origin (1860); but their nature still remained a mystery. What were they? How were they produced? How fast did they move?

By means of the spectroscope several astronomers, but especially Janssen in India and independently Lockyer in England, demonstrated conclusively that the red ring of light came from glowing gases around the sun. In Professor Young's words, they "at once determined the before doubtful nature of the strange red protuberances which, when the body of the sun is obscured, are seen projected outward many thousand miles. They were shown to be immense masses of incandescent gases — mostly hydrogen — great pillars and clouds." Their forms and appearances were carefully observed and photographed "with good, but by no means perfect, success. Unfavorable weather interfered at several stations." Their enormous sizes, bizarre shapes, great speeds of ejection, and unknown mechanism of production continued to raise many unanswered questions. Therefore, astronomers waited eagerly for the 1869 eclipse, the next opportunity to make similar but more extensive observations of every kind.

When Professor Young arrived on the Dartmouth campus in February 1866 he immediately began making measurements of the moon, sun, and stars. One of the first entries in his notebooks is a record of times for an eclipse of the moon on March 30, 1866. Later, in September, with the help of Dr. Edward E. Phelps of the Dartmouth Med- ical School and seniors Warren G. Sanborn and William Hood, he observed to the nearest tenth of a second the times for the lunar occultation of Aldebaran, a brilliant red star in the constellation Taurus. On October 7, 1866, with the help of Prof. Elihu T. Quimby of the Chandler Scientific Department and seniors Sanborn, Hood, Doane B. Colcord, Sidney L. Rice, and Horace Goodhue, he carefully measured the immersion and emersion times during a partial eclipse of the sun. By continuing such measurements during the next few years Professor Young became highly skilled and well-prepared for the eclipse expedition of 1869.

After corresponding with Professor Cooke of Harvard, in the spring of 1866 Professor Young had ordered a large spectroscope of his own design from Alvan Clark, well-known maker of fine telescopes. A train of 6-8 prisms could be mounted on it in order to increase the separation of the dark Fraunhofer lines in the sun's spectrum. It was the telescope and collimator of this instrument, along with five prisms and the 4-inch comet-seeker, a wide angle telescope purchased by Prof. Ira Young in 1853, that Professor Young assembled and mounted on a board for observing the chromospheric spectrum during the 1869 solar eclipse. This unusual instrument can be seen in the photograph of the expedition party.

The cloudless sky was "darkly blue as on one of our clearest December days," Young wrote, "not a trace of haze or vapor of any kind obstructed vision. Every line in the spectrum was sharp, and the edge of the sun's image was steady and perfectly defined. I cannot say how it was with others, but as the event drew near, I was filled with indescribable exultation, which could hardly be suppressed enough to permit me to make the last adjustments and preliminary observations. ... Every observer was intent. In a few moments came the click from the spectroscope, and within the next ten seconds from all the other instruments, recording the first contact — within half a minute we heard the shouts from the city below us, whose multitudes had also seen it. As soon as he heard the magnets click, Professor Mayer touched the spring that uncovers for one-thirtieth of a second the photographic plate on the large telescope inside the shack, and thus secured a picture of the sun, while as yet the notch made by the moon was hardly perceptible. By a careful measurement of this notch it is possible to reckon backwards very accurately to the time of contact, and it was interesting to find that this instant as determined by the spectroscope, never before applied to such an observation, agreed with that computed from the pictures within less than one-third of a second. Rapidly the black notch grew larger. ... About ten minutes before the total obscuration, the sky appeared much as during a heavy thunderstorm. ...

"Little by little the silver thread narrowed, till all at once there was a flash of darkness; night was upon us, and the stars leaped into sight. The moon hung in the sky, dark, though not absolutely black. Fastened upon her edge there seemed to be several stars, of color like the planet Mars; around her was the mysterious corona, formed of rays extending in all directions, of varying length, like slender streamers of the Aurora Borealis. ... I saw it but a moment. Only while my assistant was handing me a lantern did I venture to look over my shoulder for an instant. ... I saw the eclipse but a moment, as has been said, but in my instrument had been observing something scarcely less beautiful. When the last ray of the sun disappeared the spectral lines belonging to the light of the so-called protuberances (which I have spoken of as hanging like stars upon the edge of the moon) only seen before with difficulty, blazed out magnificently, and with those I knew before came several others. One of them certainly was found to belong to the spectrum of the corona."

Here in Professor Young's words we have a firsthand account of his observations and especially of the two prin- cipal contributions that brought him international fame: the spectroscopic method of observing contacts during solar eclipses and the discovery of the green line in the corona spectrum.

With these discoveries of 100 years ago Professor Young and Dartmouth College were moved overnight from their provincial setting into the international arena of scientific research. The unusual homemade spectroscopic instrument was a notable success. Out of this experience Professor Young was inspired to invent his far-superior, highdispersion prominence spectroscope with which he discovered the flash spectrum of the sun in Jerez, Spain, the following year. Interest in science on the Dartmouth campus quickened, the Dartmouth Scientific Association was founded, a new and larger telescope was installed in Shattuck Observatory, and Professor Young became a world leader in solar research.

Photograph taken at Burlington, lowa, at the time of the August 7, 1869 eclipse.Inside the shack on the right are Prof. Charles A. Young, seated at the instrument,and C. F. Emerson (right), helping to control its position.

Some 30 Indian boys were enrolled in the ABC program at Dartmouth this pastsummer. Most of them are in this group assembled on the Baker Library steps.