AS'TRO-PHOTOG'RAPHY (Gk. 5crrpop, as Iron, star + photography). In no department of astronomy has development in recent years been so marked as in the application of photo graphic processes of observation. If photog raphy could accomplish nothing more than what it has done and is doing for descriptive astronomy alone. we should still rank it among the most powerful weapons in the armory of the astronomer. It is but necessary, for in stance, to examine a series of eye and hand drawings of some total eclipse, executed by different skilled observers at the same time, to come at once to the conclusion that it is well nigh impossible to obtain correct information in this way; even the outlines of the corona and prominences will sometimes differ so greatly that one would scarcely believe an attempt had been made to delineate the same objects. Pho tography, of course, gives us a far more faith ful picture, and thus furnishes observational material of specially high value. But even more important, perhaps, arc the services it is capable of rendering to the astronomy of pre cise measurement, and photographic processes seem to be destined to replace the less facile visual methods which were employed exclusively until a few years ago. The heliometer (q.v.) is generally known as the most exact instru ment for executing measurements on the sky. Yet, in the opinion of some of the highest au thorities, the equality of photographic results with those of the heliometer may now be con sidered as conclusively demonstrated. The year 1892 saw the first complete publication of an extensive series of results. These were derived from excellent photographs of the cluster Plei ades, made in 1872 and 1S73 by Rutherford, in New York. These results have been followed in the last few years by many other star-cluster measures, so that an extensive mass of material is being gathered by photography—material of the highest precision and of the last importance in stellar astronomy.
It is only by a study of minute inter-stellar changes in the star-clusters that we may hope, within the limits of human time, to throw some light upon the problems of motion within the greater universe that lies beyond our own solar system; and it is to photography that we must look for our observational material. Generaliza tions of science can be secured only by the discussion of very large masses of observations; but although the heliometer can give us suffi cient precision, it involves so much labor that by its exclusive use astronomy could not hope to do more than touch the surface of those great problems.
An account of the immense advances made, since the introduction of photography, in our knowledge of planetoids is given in a special article concerning these bodies. Suffice it to mention here that the photographic revelations in this important field have culminated in the discovery of the planet Eros (q.v.), now- known to he, with the exception of the moon. our near est neighbor in space. Many comets, too, have been discovered, and many doubtless will be discovered with the aid of photography; and it is photography again that furnishes the means of determining their positions and meas u•ing their orbits with greater facility than could be obtained by any other method at pres ent known. There is no doubt, further, that photography will greatly increase the list of known distances between the earth and remote heavenly bodies ( see PARALLAX), and that be fore many years it will replace the eye even in noting the instant of a star's transit across the meridian of an observatory. See TRANSIT NSTRUMEN T.
Rut decidedly the most complete triumph of astro-photography is Gill and Kapteyn's Cape of Guod Hope photographic Du reli in ',stern »g, the results of which are comprised in a remark ably complete catalogue of the southern heavens. In the year 18S2, a very bright comet appeared in the sky, and was especially conspicuous in the Southern Hemisphere. Its brightness led Gill, at the ('ape of Good Hope Observatory. to attempt to photograph it. Having no special photographic apparatus. lie called to his aid a neighboring portrait photographer named Allis. An ordi nary portrait camera was employed by them, and. for convenience in mounting, it was simply strapped to the tube of a visual telescope in the observatory. With this apparatus they ob tained what is believed to be, in all probability, the earliest successful photograph of a cornet. In examining the negative, Gill's attention was attracted to the large number of stars appear ing on the plate as minute points or dots, and it was these that first suggested to him the possibility of making a complete examination of the heavens with the aid of photography.