The Royal Observatory, Greenwich: A Glance at Its History and Work

by E. Walter Maunder

CHAPTER XI

THE SPECTROSCOPIC DEPARTMENT

Another department was set on foot by Airy at the same time as the Heliographic Department, and in connection with it; and it is the department which has the greatest of interest for the general public. This deals with astronomical physics, or astrophysics, as it is sometimes more shortly called; the astronomy, that is, which treats of the constitution and condition of the heavenly bodies, not with their movements.

The older astronomy, on the other hand, confined itself to the movements of the heavens so entirely that Bessel, the man whose practical genius revolutionized the science of observation, and whose influence may be traced throughout in Airy's great reconstitution of Greenwich Observatory, denied that anything but the study of the celestial movements had a right to the title of astronomy at all. Hardly more than sixty years ago he wrote:

'What astronomy is expected to accomplish is evidently at all times the same. It may lay down rules by which the movements of the celestial bodies, as they appear to us upon the earth, can be computed. All else which we may learn respecting these bodies, as, for example, their appearance, and the character of their surfaces, is, indeed, not undeserving of attention, but possesses no proper astronomical interest. Whether the mountains of the moon are arranged in this way or in that is no further an object of interest to astronomers than is a knowledge of the mountains of the earth to others. Whether Jupiter appears with dark stripes upon its surface, or is uniformly illuminated, pertains as little to the inquiries of the astronomer; and its four moons are interesting to him only for the motions they have. To learn so perfectly the motions of the celestial bodies that for any specified time an accurate computation of these can be given—that was, and is, the problem which astronomy has to solve.'

There is a curious irony of progress which seems to delight in falsifying the predictions of even master minds as to the limits beyond which it cannot advance. Bessel laid down his dictum as to the true subjects of astronomical inquiry, Comte declared that we could never learn what were the elements of which the stars were composed, at the very time that the first steps were being taken towards the creation of a research which should begin by demonstrating the existence in the heavenly bodies of the elements with which we are familiar on the earth, and should go on to prove itself a true astronomy, even in Bessel's restricted sense, by supplying the means for determining motion in a direction which he would have thought impossible—that is to say, directly to or from us.

The years that followed Kirchhoff's application of the spectroscope to the study of the sun, and his demonstration that sodium and iron existed in the solar atmosphere, were crowded with a succession of brilliant discoveries in the same field. Kirchhoff, Bunsen, Angström, Thal·n, added element after element to the list of those recognized in the sun. Huggins and Miller carried the same research into a far more difficult field, and showed us the same elements in the stars. Rutherfurd and Secchi grouped the stars according to the types of their spectra, and so laid the foundations of what may be termed stellar comparative anatomy. Huggins discovered true gaseous nebulæ, and so revived the nebular theory, which had been supposed crushed when the great telescope of Lord Rosse appeared to have resolved several portions of the Orion nebula into separate stars. The great riddle of 'new stars'—which still remains a riddle—was at least attacked, and glowing hydrogen was seen to be a feature in their constitution. Glowing hydrogen, again, was, in the observation of total eclipses, seen to be a principal constituent of those surroundings of our own sun which we now call prominences and chromosphere. Then the method was discovered of observing the prominences without an eclipse, and they were found to wax and wane in more or less sympathy with the solar spots. Sun-spots, planets, comets, meteors, variable stars, all were studied with the new instrument, and all yielded to it fresh and valuable, and often unexpected, information.

THE GREAT NEBULA IN ORION.
(From a photograph taken at the Royal Observatory Greenwich, December 1, 1899, with an exposure of 2¹/₄ hours.)