Within the last thirty years a remarkable change, long in preparation,[1] has conspicuously affected the methods and aims of astronomy; or, rather, beside the old astronomy—the astronomy of Laplace, of Bessel, of Airy, Adams, and Leverrier—has grown up a younger science, vigorous, inspiring, seductive, revolutionary, walking with hurried or halting footsteps along paths far removed from the staid courses of its predecessor. This new science concerns itself with the nature of the heavenly bodies; the elder regarded exclusively their movements. The aim of the one is description of the other prediction. The younger science inquires what sun, moon, stars, and nebulæ are made of, what stores of heat they possess, what changes are in progress within their substance, what vicissitudes they have undergone or are likely to undergo. The elder has attained its object when the theory of celestial motions shows no discrepancy with fact—when the calculus can be brought to agree perfectly with the telescope—when the coursers of the heavens come strictly up to time, and their observed places square to a hair's breadth with their predicted places.
It is evident that very different modes of investigation must be employed to further such different objects; in fact, the invention of novel modes of investigation has had a prime share in bringing about the change in question. Geometrical astronomy, or the astronomy of position, seeks above all to measure with exactness, and is thus more fundamentally interested in the accurate division and accurate centering of circles than in the development of optical appliances. Descriptive astronomy, on the other hand, seeks as the first condition of its existence to see clearly and fully. It has no "method of least squares" for making the best of bad observations—no process for eliminating errors by their multiplication in opposite directions; it is wholly dependent for its data on the quantity and quality of the rays focused by its telescopes, sifted by its spectroscopes, or printed in its photographic cameras. Therefore, the loss and disturbance suffered by those rays in traversing our atmosphere constitute an obstacle to progress far more serious now than when the exact determination of places was the primary and all-important task of an astronomical observer. This obstacle, which no ingenuity can avail to remove, may be reduced to less formidable dimensions. It may be diminished or partially evaded by anticipating the most detrimental part of the atmospheric transit—by carrying our instruments upward into a finer air—by meeting the light upon the mountains.
The study of the sun's composition, and of the nature of the stupendous processes by which his ample outflow of light and heat is kept up and diffused through surrounding space, has in our time separated,
- ↑ Sir W. Herschel's great undertakings, Bessel remarks ("Populäre Vorlesungen," p. 15), "were directed rather toward a physical description of the heavens than to astronomy proper."
