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Popular Science Monthly/Volume 29/May 1886/Photographing the Heavens

< Popular Science Monthly‎ | Volume 29‎ | May 1886


UNDOUBTEDLY one of the greatest achievements of modern days is the introduction of the exceedingly sensitive dry-plate in photography. By it one is enabled to picture the lightning's flash, the trotting horse, the surging wave, and the foliage swayed by the breeze. It is not to be foreseen what manifold applications this new method will eventually find in the natural sciences. Here we will consider only one of its numerous applications, namely, its use in photographing the starlit heavens.

Whoever has tried to form an idea of the number of stars, visible to the naked eye on a clear winter's night, almost invariably overestimates them. The layman declares he sees a hundred thousand, ay, a million stars. Such estimates, however, far exceed the truth, and, if anything is certain, it is the fact that the number of stars to be seen with the naked eye is very small. All stars discernible by the keenest of human sight, without the aid of a telescope, have long been noted down on charts, and their position in the vaulted dome exactly determined.

Should one count up all the stars in those parts of the heavens that become visible to us in the course of a year, even this sum would notPSM V29 D082 A spot in night sky seen with the naked eye.jpgFig. 1. by far approach seven thousand. However, if one resorts to a telescope, matters grow to be quite different; more and more stars then become visible, the number depending on the strength of the instrument in use. Fig. 1 represents a certain portion of the heavens as seen by the unaided eye. One discerns two brighter stars and several smaller ones. Fig. 2 shows this same spot, but as seen through a powerful telescope. This picture has not merely been drawn from fancy. Each point, even the smallest, was, after close observation, entered with the utmost care on a large chart, of which this illustration

PSM V29 D082 The night sky seen through a telescope.jpg

Fig. 2.

is a copy, but reduced in size. And each single one of these stars is a mighty body, in its sphere a shining sun, equaling ours in grandeur and splendor. From the beginning, each of these suns has traveled its prescribed round, and has filled its place in the vast universe. Such charts of the stars are leaves from the great volume of the history of the universe, a work which astronomy teaches us to read. On one of these pages, that has already been in part deciphered, is recorded the destiny of our planet.

It is, then, not surprising that astronomers seek to gain possession of as many reliable copies of such leaves from this history as possible; in other words, seek to own as exact and extensive star-maps as will include the very smallest luminous points in the heavens. What untold work the compiling of such charts entails may well be imagined; indeed, this is a task which is almost beyond human power. The chart from which the above picture is a copy was compiled at the observatory at Paris, and work at the same has already been continued for many decades. For years past, the two brothers, Paul and Prosper Henry, have been engaged in this exacting undertaking; but, notwithstanding the great experience which they in the course of time had gathered, their task almost came to a sudden end in the year 1884. At that time, while pursuing their observations, they came to that region of the heavens traversed by the milky-way. As is well known, the mild, lambent light of the milky-way is caused by a conglomeration of countless millions of stars placed behind one another to endless depths. To reproduce these millions of stars on charts proved to be utterly impossible.

The two observers then summoned the art of photography, recently so much improved, to their aid. Naturally they could not make use of the ordinary apparatus of the photographer; indeed, they were obliged to build a special telescope for their purpose. By means of clock-work, they succeeded in imparting to this a movement so prescribed and so regulated that the stars, though continuing in their unbroken course in the heavens, yet retain a stationary position with reference to the photographic plate. After many painstaking experiments, the enterprise was successful beyond expectation. Even the faintest of stars were plainly discernible on the plate, and in this manner more was accomplished in one hour than could be done by the old method of inscribing each star in many months.

These results incited to further progress. A new and very large telescope was constructed and directed toward the starry heavens. The plate now showed stars of the fifteenth magnitude, i. e., those whose light is so faint that only very few telescopes in all Europe can render them perceptible. In order to obtain this result, the plate, notwithstanding its extreme sensitiveness, had to be exposed to the light of these stars for fully an hour. If one were to carefully examine such a plate, or rather a cliché made therefrom, doubts might perhaps arise as to whether some of the little points thereon might not have been occasioned by particles accidentally present on the original plate. Such doubts might well be entertained, but Messrs. Henry have succeeded in meeting them in a most ingenious manner. After having exposed the plate for an hour, they shifted its position a trace to the right, and again exposed it for the same length of time. After this they lowered the plate with the telescope to the same extent as they had before shifted its position, and then, for a third time, exposed for an hour. If, after this, the original were to be examined with a microscope, it would be seen that each little star is really composed of three points, which form a small triangle. Thus any doubt is dispelled that might have been entertained as to whether an accidental blur had been pictured.

The advantage in preparing representations of the heavens by means of photography rests not only on the fact that by this means charts of the stars can be obtained much more readily than was the case when each star had to be separately noted, but the pictures thus obtained also seem to be absolutely correct; they contain no faulty entries, no mistakes. Even the most attentive observer is liable to error; he may overlook one or more stars, he may make a wrong entry, etc. All of these risks are not to be feared in employing a photographic plate; it is like a retina that sees everything as it is! This advantage can not be sufficiently appreciated, for it enables us to leave to coming generations an absolutely true and entirely correct picture of the starry heavens of to-day. The director of the observatory at Paris has for this reason suggested the obtaining of a complete photographic picture of the entire heavens by the systematic co-operation of different observatories in the northern and southern hemispheres. This is, indeed, a grand project; and to see it realized would, at all events, require a period of from eight to ten years—but what exceedingly important results would ensue from this!

With such charts from different times at his disposal, and equipped with a microscope and a micrometric apparatus to carry out his measurements, the investigator of the future will be enabled to make in his study astronomical discoveries that have hitherto escaped direct observation by the telescopes of the observatory. In his study he will be able to prove whether any, and, if so, which stars have changed their position in the heavens, whether among the countless number of the faintest little stars in the milky-way new ones have arisen, or old ones disappeared—in short, with the aid of such charts there opens to the mind a vista of research and discovery that seems well-nigh endless.

How much may be escaping astronomical science of to-day, simply because the eye of mortal explorer chances not to alight on that very point in the depths of the heavens where just then a most important event is taking place!

In future this will be different. Photographed charts of the heavens give an exact likeness of the appearance of the celestial dome at the time of their taking, and these may be examined and studied at any place and at any time, by day and by night. The most remote planet that revolves around the sun, known of to-day, is Neptune; yet it seems most probable that beyond this, one or even more planets are existing. As, however, they move but very slowly, and at the same time emit but little light, it has not yet been possible to discern them among the millions of little fixed stars. But, when once the entire heavens, even to the very smallest of visible stars, shall have been photographed, and if this work be repeated after a period of about ten years, the charts thus obtained will solve the problem as to the most remote planets, and the latter must be found. Ay—even more. The photographic plate is superior to the observant eye, in perceiving and reproducing the smallest stars, inasmuch as it shows objects in those places in the heavens where, with the most powerful telescopes, nothing more is to be seen.

In this connection the brothers Henry have recently made a most singular discovery. On the 16th of November they directed their large photographic telescope to that spot in the heavens where the star Maja is in the Pleiades, and afterward found on their plate, besides numerous stars, a spiral, nebulous spot, which, to a certain extent, seemed to come from the star Maja. As, even with the greatest telescopes of the observatory at Paris, no signs of such vapor could be perceived in that particular part of the heavens, a new photograph was taken on the 8th of December; this also showed the vapor, and a third picture, obtained the following day, once more bespoke its presence.

There can, then, be no doubt as to the existence of a spiral-like nebulous spot in the vicinity of that star, but of which the eye, even with the aid of a most powerful telescope, can perceive naught. What wonderful prospects for the future here open to view! A veritable astronomy of the invisible begins. Celestial orbs, ever veiled from our direct gaze, are rendered perceptible—ay, trace their own picture. Therein lies the highest triumph of the human mind, that it is able, in the true sense of the word, to force Nature to reveal her secrets; that a ray of light, called into being in the most remote depths of space, created at a time ere perhaps the foot of man had ever trodden the earth, should to-day itself trace on a plate the outline and the form of that orb from which it emanated myriads of years ago.—Translated for the Popular Science Monthly from Die Gartenlaube.