[235] Proctor, Saturn and its System, p. 64.
[236] Phil. Trans., vol. lxxvii., p. 125.
[237] Month. Not., vol. xi., p. 248.
[238] Ibid., vol. xxxv., pp. 16-22.
[239] Ibid., p. 26.
[240] Ibid., vol. xli., p. 190.
CHAPTER V
COMETS
Newton showed that the bodies known as "comets," or hirsute stars, obey the law of gravitation; but it was by no means certain that the individual of the species observed by him in 1680 formed a permanent member of the solar system. The velocity, in fact, of its rush round the sun was quite possibly sufficient to carry it off for ever into the depths of space, there to wander, a celestial casual, from star to star. With another comet, however, which appeared two years later, the case was different. Edmund Halley, who afterwards succeeded Flamsteed as Astronomer Royal, calculated the elements of its orbit on Newton's principles, and found them to resemble so closely those similarly arrived at for comets observed by Peter Apian in 1531, and by Kepler in 1607, as almost to compel the inference that all three were apparitions of a single body. This implied its revolution in a period of about seventy-six years, and Halley accordingly fixed its return for 1758-9. So fully alive was he to the importance of the announcement that he appealed to a "candid posterity," in the event of its verification, to acknowledge that the discovery was due to an Englishman. The prediction was one of the test-questions put by Science to Nature, on the replies to which largely depend both the development of knowledge and the conviction of its reality. In the present instance, the answer afforded may be said to have laid the foundation of this branch of astronomy. Halley's comet punctually reappeared on Christmas Day, 1758, and effected its perihelion passage on the 12th of March following, thus proving beyond dispute that some at least of these erratic bodies are domesticated within our system, and strictly conform, if not to its unwritten customs (so to speak), at any rate to its fundamental laws. Their movements, in short, were demonstrated by the most unanswerable of all arguments—that of verified calculation—to be calculable, and their investigation was erected into a legitimate department of astronomical science.
This notable advance was the chief result obtained in the field of inquiry just now under consideration during the eighteenth century. But before it closed, its cultivation had received a powerful stimulus through the invention of an improved method. The name of Olbers has already been brought prominently before our readers in connection with asteroidal discoveries; these, however, were but chance excursions from the path of cometary research which he steadily pursued through life. An early predilection for the heavens was fixed in this particular direction by one of the happy inspirations of genius. As he was watching, one night in the year 1779, by the sick-bed of a fellow-student in medicine at Göttingen, an important simplification in the mode of computing the paths of comets occurred to him. Although not made public until 1797, "Olbers's method" was then universally adopted, and is still regarded as the most expeditious and convenient in cases where absolute rigour is not required. By its introduction, not only many a toilsome and thankless hour was spared, but workers were multiplied, and encouraged in the prosecution of labours more useful than attractive.