Popular Science Monthly/Volume 64/April 1904/The Arequipa Station of the Harvard Observatory
|THE AREQUIPA STATION OF THE HARVARD OBSERVATORY.|
By Professor SOLON I. BAILEY,
DIRECTOR OF THE AREQUIPA STATION.
THE same restless energy which impelled the American people to become a world-power has led their men of science to extend the range of their researches. The possibilities of a nation's influence are bounded only by the whole earth; and in a similar way the field of astronomy is limited only by the whole sky. At the latitude of Cambridge, Mass., an observer can never see more than three fourths of the sky. In order to observe the remaining fourth, which lies about the south pole of the heavens, he must seek some station below the equator. A complete study of all the stars in the sky is imperatively demanded for the solution of many of the great questions which the astronomy of the future must answer. Only by bringing such completeness into astronomical research will the construction of the universe and the true place of our solar system become known.
The Arequipa Station of the Harvard College Observatory owes its foundation to the far-sighted policy of its present director. Professor Edward C. Pickering. Under his direction, in 1889, the writer of this article visited South America in order to make the preliminary studies necessary to the selection of a station for the observation of the southern sky. The west coast of South America was chosen for this purpose, since it offered the possibility of great altitude, in addition to a dry climate and a clear atmosphere. The funds for this enterprise had been bequeathed for such a purpose by Uriah A. Boyden, a Boston engineer.
From the best information which could be obtained in the United States, it was thought that the valley of the River Rimac, near Lima, Peru, would furnish conditions favorable for the proposed station. The valley itself, however, did not offer a sufficiently free horizon, being shut in everywhere by mountains. There was a wide range for choice in regard to elevation. The hills near Lima are only a few hundred feet in height, while the great mountains forming the western Andes rise eighteen thousand feet above sea level. The primary conditions were an open horizon and a clear sky. To obtain a free horizon it was only necessary to climb one of the steep and barren summits near the valley; but to determine where the sky was clearest was a more difficult problem. At this latitude the western Cordillera extends nearly parallel to the coast at a distance of perhaps sixty or seventy miles in an air line. The amount of cloudiness at different distances from the coast varies enormously. A different cloud system prevails on the coast from that in the mountains. The rainy season in the mountains is from November or December to March or April, more or less in different years. Toward the coast the rainfall grows less, while, in general, little or no rain falls within thirty or forty miles of the ocean. In Lima there is no rainy season, but there is an extremely cloudy season. This is due to the low cloud which is found more or less along the whole coast. This coast cloud is most prevalent from May to November. Throughout a large part of the year, however, the coast region of Peru, though almost rainless, is very cloudy. It seemed, therefore, that while the lofty clouds which cause the rainy season in the interior are gradually dissipated many miles from the coast, and the dense coast cloud never extends far away from the ocean, a situation chosen between these two, if such were possible, might, perhaps, escape both. With these ideas in mind a site was selected on an isolated summit, at a distance of about twenty-five miles from the coast, and at an elevation of six thousand five hundred feet.
The provisional station thus selected was at a distance of about eight miles from the village and hotel of Chosica, in the Rimac Valley, from which all supplies, of both food and water, were obtained. The residents of the hotel were our nearest neighbors, with the exception of occasional wandering herdsmen. In many ways it was an ideal location. It was named Mount Harvard, and became well known in Peru. The outlook was superb. To the east the ranges rose ever higher and higher to the Great Andes; to the west they fell away in numberless crests and wavy lines to the Pacific. Five miles away in a straight line a glimpse of green indicated the valley of the Rimac. The rest was hidden by mountains. In every direction nothing but barren mountains was to be seen. Where the buildings stood the soil was a hard sand, covered here and there with huge bowlders and with many varieties of cacti. To the north and south we looked down into gloomy ravines thousands of feet deep. To the east and west the slopes were more gradual, and there were charming little valleys needing only water to make them spots of beauty.
The buildings on Mount Harvard were portable structures, carried for the most part from the United States. They were made of a light framework of wood, covered with canvas and heavy paper. These houses and the instruments were conveyed from Chosica on muleback over a trail constructed for the purpose.
Life on Mount Harvard was somewhat lonely and monotonous, especially for Mrs. Bailey, who seldom enjoyed the society of any woman, except that of our amiable half-breed cook. Perhaps the most unique feature was its situation between two clouds. Below us to the west was the coast cloud, which reached a varying distance inland according to the topography. Up the Rimac Valley it flowed like a great river; occasionally it filled not only the valley, but the barren ravines that branched from it north and south, rising till it covered even the ridges at our feet and, flowing around us, formed islands of Mount Harvard and the other more lofty points. The upper surface of this cloud was very sharply defined, but of wave-like form, so that its resemblance to water was at times so perfect that we could with difficulty persuade ourselves that far beneath its under surface all the varied activities of ordinary life were going on cheerfully. If from this view of 'clouds wrong side up' we turned our eyes upward, we at times saw another cloud system far above us; so that frequently we were between two clouds in a wide but shallow world, ourselves, perhaps, the sole inhabitants.
Fortunately for our work, the clouds above us appeared but seldom during the first months of our residence on Mount Harvard. During this time the extension of the Harvard photometry to the southern sky was begun. This is a determination of the brightness of all stars visible to the naked eye, a work begun by Professor Pickering in 1879. Photographic work was also carried on. The climatic conditions from April to September were excellent, but later clouds became troublesome. This condition of the sky growing worse as the cloudy season approached, it was decided to devote the following months to a meteorological study of different localities along the coast, and incidentally to extend the work of the meridian photometer in some region more free from clouds. Of the clearness and steadiness of the atmosphere in these different localities there was no certain knowledge, and the only way was to investigate it for ourselves. We left the Mount Harvard Station early in November in charge of a Peruvian assistant. During the next four months a personal examination was made by Dr. M. H. Bailey and myself of what appeared to be the most desirable localities along the coast, including Arequipa, the region about Lake Titicaca, both in Peru and in Bolivia, the Desert of Atacama, Valparaiso, Santiago and various other places in Chili. Perhaps no spot in all America offers a clearer sky than the Desert of Atacama. More than a month was passed at Pampa Central, near the center of this desert, and a study was made of the meteorological conditions, while the work of the meridian photometer was rapidly extended. There is a striking difference during the Peruvian cloudy season, between the cloudiness at Mount Harvard or at Arequipa, where the conditions are similar, and that on the Desert of Atacama. This is well shown in the following brief table, where A represents a perfectly clear sky during the whole night; B, a clear sky for a portion of the night; C, sky partially cloudy all night, and D, sky completely cloudy all night.
A record of the cloudiness was kept not only at Mount Harvard, but at Arequipa and Pampa Central for some time after our residence in Chili. The cloudy season at Mount Harvard and Arequipa is in the southern summer, that of Pampa Central in the southern winter. By changing from one to the other of these localities one could keep in a region of clear sky nearly the whole time.
As a result of the investigations thus made the director selected Arequipa for the site of the permanent station, and the equipment on Mount Harvard was removed to that, city in October, 1890. In January of the following year Professor W. H. Pickering arrived in Arequipa, bringing with him the thirteen-inch Boyden telescope and other smaller instruments. Under his direction a residence for the astronomer in charge and his family was erected, and two additional buildings were received from the United States. One of these was designed for a laboratory and the other for the use of the Boyden telescope. During his two years' residence in Arequipa, Professor Pickering, assisted by Mr. Douglass, made a large number of observations of Mars, and of the satellites of Jupiter and Saturn, as well as of the lunar surface, all of which attracted wide attention. The steadiness of the atmosphere at Arequipa makes it an especially favorable spot
for the use of higher powers, and for the study of faint and difficult planetary details. For double star work also the conditions are extremely good, and a large number of new and interesting double stars have been discovered by Professor Pickering, Mr. H. C. Bailey and the other members of the observatory.
At the present time the equipment of the Arequipa Station of the observatory consists of the following instruments: the thirteen-inch Boyden telescope, an instrument so constructed that, by a change in the position of the lenses, it may be used either for visual or for photographic work; the twenty-four-inch Bruce photographic telescope, the most powerful instrument of its class in the world, a gift of the late Miss Catherine Bruce, of New York; the eight-inch Bache photographic telescope; a five-inch refractor, and several smaller instruments of different kinds.
In general the work carried on in Arequipa is the extension to the southern sky of that previously begun in Cambridge. This is well illustrated by the Harvard photometry. With the large meridian photometer alone more than a million light comparisons have been made. The greater part of this work was done in Cambridge by the director and his assistants, but about two hundred thousand observations have been made by the writer in Arequipa and elsewhere in South America. This work, planned and begun by Professor Pickering a quarter of a century ago, now furnishes not only precise determinations of the magnitudes of all the brighter stars in the sky, but also the magnitudes of certain zones of fainter stars, by which the estimated magnitudes of the stars included in the various great catalogues can be reduced to the photometric scale.
With another Pickering photometer, during the last year, several thousand light comparisons of Eros were made by the writer. Eros is that remarkably interesting little planet which at times comes so near the earth as to be our nearest celestial neighbor. Eros is a variable planet, undergoing striking changes in light. The above observations showed that during the year 1903 the complete light-cycle was only 2h 38m 6s.1. If these changes are due to the rotation of the planet, the true period may be that given above, or, more probably, twice that amount, 5h 16m 12s.2.
Visual observations of variable stars have been regularly carried on since the establishment of the station, although the results have not yet been published. These observations are now made by Messrs. Manson and Wyeth. A determination of the longitude and latitude of the station was made in 1897 by Professor Winslow Upton, of Brown University. The result was, longitude 4h 46m 12s west of Greenwich. The latitude is south 16° 22′ 28″. The longitude of the observatory in Cambridge is 4h 44m 31s. It follows, therefore, that Arequipa is about thirty miles west and four thousand miles south of Cambridge.
Photographic work has always occupied a large share of the time at Arequipa. Several photographic instruments are kept employed throughout the whole of every clear night. The photographs thus made are usually examined in Cambridge, where a number of assistants are employed for the purpose. Only in exceptional cases is more than a preliminary examination made in Arequipa,
The largest instrument in the observatory is the twenty-four-inch Bruce telescope. This telescope is a doublet, that is, it has a combination of four lenses, giving good definition over a large field. The scale is the same as that of the instrument used in the international photographic survey of the sky, but the region covered by each plate is six times as great, so that the work of covering the whole sky is much less. With such instruments the work of making a photographic Durchmusterung of the stars to any desired magnitude would be comparatively simple, since a pair of these telescopes, one in the northern, and the other in the southern, hemisphere could furnish all the plates needed within two or three years. The Bruce telescope, after a year's trial in Cambridge, was mounted in Arequipa, in 1895, by the writer. Nearly the whole sky has been photographed with exposures of ten minutes, showing stars to about the eleventh magnitude. Good progress has also been made on plates having exposures of sixty minutes, which show stars to about the fifteenth magnitude. A set of plates has also been begun, having exposures of four hours. These can only be made on moonless nights, and a number of years will be required to cover the whole sky. The approximate number of stars has been determined on some of these plates. The number varies, in general, from one thousand to ten thousand stars per square degree. Four hundred thousand stars have been photographed on a single plate. The whole number of stars which will be recorded in this splendid set when completed will probably approach one hundred millions. In addition to such vast numbers of stars, these plates will also contain numerous star clusters and nebulæ, together with occasional asteroids, comets and meteors. This set of plates alone would furnish two or three astronomers with materials for a lifetime of study. A large part of the plates thus far obtained with this instrument have been made by Dr. Stewart and Mr. Frost.
An instrument, which has been in constant use since the beginning of Professor Pickering's photographic researches in 1886 is the Bache telescope, which has an aperture of eight inches, and a focal length of four feet. It was employed for several years in Cambridge, then for a year and a half on Mount Harvard, and since that time in Arequipa. Altogether, more than thirty thousand photographs of the stars have been made with this instrument. By its use with an objective prism photographs of the spectra of all stars to about the eighth magnitude have been made. A study and classification of these spectra have been carried out by Professor Pickering as a memorial to the late Dr. Henry Draper, The funds for this research were furnished by Mrs. Draper. From a study of the spectral peculiarities of the stars thus photographed, Mrs. Fleming has discovered a large number of variable stars and several new stars. Charts of the southern sky are made with this instrument each year. This work is in extension of that done in Cambridge for the northern sky. This collection is now of great value in tracing the history of any newly discovered celestial object.
A similar but more frequent photographic survey of the sky is also made by means of a Cooke lens with an aperture of about one inch. Photographs are made each month with this instrument of the available sky. An exposure of one hour shows stars to about the eleventh magnitude, and a plate eight inches by ten covers a region more than thirty degrees square, or about one fortieth part of the whole sky.
The thirteen-inch Boyden telescope has been used photographically for the detailed study of the spectra of the brighter stars, and for charts of special regions. The power of this instrument is such that, by the use of a battery of two or three prisms, spectra of the bright stars are obtained several inches in length, which show hundreds of lines. By an examination of these spectra several spectroscopic binaries have been discovered. As the objective prisms employed do not permit the use of a comparison spectrum, the binary character is apparent only when both the components are bright. In such cases the lines of the spectrum are alternately single and double. A study of the spectra of the southern stars photographed with this telescope has been made by Miss Cannon as a part of the Henry Draper Memorial.
The focal length of this telescope is about sixteen feet, so that an arc a degree in length in the sky is represented on the photographic plate by a line more than three inches long. The scale of the instrument is thus very suitable for the details of nebulæ, and for nearly everything except the centers of the densest clusters. For long exposures on difficult objects, such as globular clusters, the telescope must follow the stars in their diurnal motion with great precision. This can only be accomplished with such an instrument by watching a star visually and keeping it constantly bisected by the lines of a reticle. Formerly, a secondary telescope was used for this purpose, but, due to the flexure between the two tubes, and perhaps for other causes, really fine photographs were not obtained with this telescope until a lens for following was inserted into the field of the main instrument, so that the other telescope was dispensed with. In all cases the mean movement of the telescope is provided for by carefully devised and well-constructed clockwork; ad in the case of small and rigid instruments this alone serves fairly well, unless the exposure is more than an hour.
More than five hundred variable stars have been discovered by the writer in the globular clusters, by means of charts made with this instrument. These constitute nearly one half of all the variable stars known, but they all occur in only one thirty-thousandth part of the sky. At the centers of some of these clusters, the stars are packed together so densely that there are one hundred stars to the square minute. If the stars were equally dense over the whole sky, their number would exceed ten billions, and the sky would be so luminous that there would be no real night. In one of these clusters. Messier 3, one hundred and thirty-two variables were found. These are all situated within a circle whose area is one fourth of a square degree, or only one one-hundred-and-sixty-thousandth part of the sky. In this cluster one star in seven is variable. The photographs used for this investigation must be made with the greatest care, and must then be enlarged, or else examined by a microscope, since the images of the stars on the original plates resemble thickly scattered grains of dust.
The duration of exposure employed varies enormously, according to the instrument and the object to be attained. They have been made from one second up to twenty-four hours. With the great Bruce lens, an exposure of one second is sufficient for the brightest stars, while an exposure of four hours, or more than fourteen thousand times as long, fails to record stars which will appear when an exposure of five or six hours is used. For the comparatively bright stars, the number increases approximately by the ratio three for each magnitude. For example, there are about three times as many stars of the second magnitude as of the first, and three times as many of the third magnitude as of the second. There are indications, however, that this ratio is not kept up for the fainter stars, that is, there are not three times as many stars of the sixteenth as of the fifteenth magnitude. No limit to the universe has yet been reached, however. With the Bruce telescope stars can be photographed too faint for vision in the greatest
telescopes of our day; but increase in exposure always brings out new and fainter stars, until the practical limit of the exposure is reached in the fogging of the plate by the diffused light of the sky. The longest exposure yet made in this observatory was with the Cooke lens, an exposure of twenty-four hours, on four different nights. Such an exposure in such an instrument brings out with great perfection the wonderful beauty of the cloud-forms of the Milky Way.
Since the establishment of the Peruvian station, meteorology has formed an important, though subordinate, part of the work. For about ten years a line of auxiliary stations was maintained, reaching from the Pacific across the Andes to the low country on the upper waters of the Amazon. The culmination of this series was the station on the summit of El Misti, at an elevation of nineteen thousand feet. The conditions for reaching great altitudes are exceptionally good in
this part of Peru, but even here it was found impossible, with the funds available for this purpose, to keep observers at the summit, in order to obtain systematic personal observations. It was necessary to depend chiefly on the self-recording instruments of Richard Fréres. The station was established by the writer in 1893, and was visited later by different members of the observatory, or by some person engaged especially for the purpose. At such visits the observer rewound the self-recording instruments and made personal observations. This station was continued for about seven years. The records were broken, and not always of the highest accuracy, but it is believed that they will be of service to meteorology. Personal observations of the highest precision at this station are much to be desired, but a special gift for this purpose would be necessary. Few persons could live, even for a few days, at such an altitude. Nearly every one suffers from mountain sickness, and sometimes very severely. Nevertheless, there are sufficiently well educated persons, born in Peru at a high altitude, who could be engaged for a reasonable sum to pass alternate weeks at the summit. In this way, for a few thousand dollars, complete records of great precision and value to science might be obtained. There are also problems in astronomy and physics, which could be investigated at a well-equipped station at such an altitude, which perhaps can never be solved at sea-level.
Arequipa is a city of about thirty thousand inhabitants. It lies on the western slope of the Cordillera at an elevation of seven thousand five hundred feet. No more beautiful view can well be imagined than that which is seen as one approaches Arequipa from the coast. It is built of a soft white volcanic stone, and in the distance appears to be a city of marble. It is surrounded by wide-spreading green fields of wheat, corn and alfalfa. It is in a region of volcanoes and earthquakes, but the danger from these is slight, either to observers or to instruments. The observatory is situated on rising ground, about two miles north of the city, at an elevation of eight thousand feet above sea-level. To the north rises the great range Chachani, about twenty thousand feet in elevation; to the northeast El Misti, a volcanic cone nineteen thousand feet high; and to the east Pichu-Pichu, over seventeen thousand feet high.
The climate of Arequipa is superb for those who do not object to a somewhat rarefied and dry atmosphere. There is scarcely any seasonal change in temperature during the year, though the diurnal range is fairly large. The mean maximum and minimum temperatures for the year 1902 were 68° and 49°. In the observatory residence, which is built of stone, the temperature without artificial heat ranges between 60° and 65° Fahrenheit. The rainfall is slight, amounting to only two or three inches during the year. Agricultural pursuits are possible only by means of irrigation. Around the fertile fields, in whose center lies the city, extend endless barren pampas. All the waters of the Chili River, however, are now well utilized, and there is no other convenient supply.
The people of Peru, and especially of Arequipa, have always taken great interest in the observatory, and have extended to it many favors and constant good-will. There is a university in the city of Arequipa which is supported by the Peruvian government. There is no lack of educated and refined Peruvian society, and in addition there are American, English and German colonies. The laboring and servant classes are half-breeds, usually illiterate and careless, but kindly and contented. Besides these, especially in the interior, are large numbers of civilized Indians. In the low lands lying about the head waters of the Amazon are also numerous groups of savages, but they are never seen near Arequipa, and are slowly becoming half-civilized or disappearing. Revolutions have been numerous in the past history of Peru, and one occurred since the establishment of the station in Arequipa, but not the slightest indignity was offered to the observatory or its members. The common people of Peru, even in time of revolution, are not dangerous, and those who are not actually engaged in warfare are seldom molested. It is a game of politics, carried on by bullets instead of ballots, and without permanent hatreds. For several years past no revolution of importance has occurred, and there are strong indications that Peru has at last entered on a career of peace and prosperity. Such, at least, is the earnest wish of her best citizens.