Practical Talks by an Astronomer

by Harold Jacoby

Taking up first the photography of stars, we must begin by mentioning the work of Rutherfurd at New York. More than thirty years ago he had so far perfected methods of stellar photography that he was able to secure excellent pictures of stars as faint as the ninth magnitude. In those days the modern process of dry-plate photography had not been invented. To-day, plates exposed in the photographic telescope are made of glass covered with a perfectly dry film of sensitized gelatine. But in the old wet-plate process the sensitive film was first wetted with a chemical solution; and this solution could not be allowed to dry during the exposure. Consequently, Rutherfurd was limited to exposures a few minutes in length, while nowadays, as we have said, their duration can be prolonged at will.

When we add to this the fact that the old plates were far less sensitive to light than those now available, it is easy to see what were the difficulties in the way of photographing faint stars in Rutherfurd's time. Nor did he possess the modern ingenious device of a combined visual and photographic instrument. He had no electric controlling apparatus. In fact, the younger generation of astronomers can form no adequate idea of the patience and personal skill Rutherfurd must have had at his command. For he certainly did produce negatives that are but little inferior to the best that can be made to-day. His only limitation was that he could not obtain images of stars much below the ninth magnitude.

To understand just what is meant here by the ninth magnitude, it is necessary to go back in imagination to the time of Hipparchus, the father of sidereal astronomy. (See [page 39].) He adopted the convenient plan of dividing all the stars visible to the naked eye (of course, he had no telescope) into six classes, according to their brilliancy. The faintest visible stars were put in the sixth class, and all the others were assigned somewhat arbitrarily to one or the other of the brighter classes.

Modern astronomers have devised a more scientific system, which has been made to conform very nearly to that of Hipparchus, just as it has come down to us through the ages. We have adopted a certain arbitrary degree of luminosity as the standard "first-magnitude"; compared with sunlight, this may be represented roughly by a fraction of which the numerator is 1, and the denominator about eighty thousand millions. The standard second-magnitude star is one whose light, compared with a first-magnitude, may be represented approximately by the fraction ⅖. The third magnitude, in turn, may be compared with the second by the same fraction ⅖; and so the classification is extended to magnitudes below those visible to the unaided eye. Each magnitude compares with the one above it, as the light of two candles would compare with the light of five.

Rutherfurd did not stop with mere photographs. He realized very clearly the obvious truth that by making a picture of the sky we simply change the scene of our operations. Upon the photograph we can measure that which we might have studied directly in the heavens; but so long as they remain unmeasured, celestial pictures have a potential value only. Locked within them may lie hidden some secret of our universe. But it will not come forth unsought. Patient effort must precede discovery, in photography, as elsewhere in science. There is no royal road. Rutherfurd devised an elaborate measuring-machine in which his photographs could be examined under the microscope with the most minute exactness. With this machine he measured a large number of his pictures; and it has been shown quite recently that the results obtained from them are comparable in accuracy with those coming from the most highly accredited methods of direct eye-observation.

And photographs are far superior in ease of manipulation. Convenient day-observing under the microscope in a comfortable astronomical laboratory is substituted for all the discomforts of a midnight vigil under the stars. The work of measurement can proceed in all weathers, whereas formerly it was limited strictly to perfectly clear nights. Lastly, the negatives form a permanent record, to which we can always return to correct errors or re-examine doubtful points.

Rutherfurd's stellar work extended down to about 1877, and included especially parallax determinations and the photography of star-clusters. Each of these subjects is receiving close attention from later investigators, and, therefore, merits brief mention here. Stellar parallax is in one sense but another name for stellar distance. Its measurement has been one of the important problems of astronomy for centuries, ever since men recognized that the Copernican theory of our universe requires the determination of stellar distances for its complete demonstration.

If the earth is swinging around the sun once a year in a mighty path or orbit, there must be changes of its position in space comparable in size with the orbit itself. And the stars ought to shift their apparent places on the sky to correspond with these changes in the terrestrial observer's position. The phenomenon is analogous to what occurs when we look out of a room, first through one window, and then through another. Any object on the opposite side of the street will be seen in a changed direction, on account of the observer's having shifted his position from one window to the other. If the object seemed to be due north when seen from the first window, it will, perhaps, appear a little east of north from the other. But this change of direction will be comparatively small, if the object under observation is very far away, in comparison with the distance between the two windows.

This is what occurs with the stars. The earth's orbit, vast as it is, shrinks into almost absolute insignificance when compared with the profound distances by which we are sundered from even the nearest fixed stars. Consequently, the shifting of their positions is also very small—so small as to be near the extreme limit separating that which is measurable from that which is beyond human ken.