The Astronomy of Milton's 'Paradise Lost' by Thomas Nathaniel Orchard (read along books .TXT) 📖

Records of the appearance of these remarkable objects have been handed down from earliest times; and when one of those mysterious visitors, travelling from out the depths of space, became visible in our skies, it was regarded with apprehension and dread as betokening the occurrence of calamities and direful events among the nations of the Earth.

The word comet is derived from the Greek κομη, signifying ‘hair,’ to which the hazy, luminous appearance of those objects bears some resemblance. A comet consists of a bright central part called the nucleus; this is surrounded by layers of nebulous matter called the coma, and both combined form the head, from which a long appendage extends called the tail. The nucleus and tail are not essential parts of a comet, for many have been observed in which both have been wanting. The tail is frequently very conspicuous, and presents considerable diversity both as regards its appearance and length. In some comets it is entirely absent, and in others it has been observed to stretch over an arc of sixty or seventy degrees, indicating a length of 100 to 150 million miles. Sometimes it is straight, and at other times it is curved at the extremity; it has been observed bifurcated into two branches; and, on rare occasions, comets have been seen with two or more tails. The tail of a comet is always directed away from the Sun; it increases in size as the comet approaches the orb, and diminishes as it recedes from him. This depends upon the degree of heat to which the comet is exposed, which has the effect of driving off or evaporating some of the matter composing the head. During the time the comet is travelling round the Sun there is a continuous emission of this highly attenuated matter, which is visible as the tail, but when the comet begins to recede from the orb and reaches cooler regions of space the tail diminishes in size as the temperature becomes reduced, and ultimately it disappears.

The appearance of a comet in the sky is often sudden and unexpected, and one of those erratic wanderers may become visible at any time and in any part of the heavens. It was remarked by Kepler that there are as many comets in the sky as there are fishes in the ocean. This may or may not be true, for they only become visible when they approach the Sun, and the time during which they remain so does not usually exceed a few weeks or months. Ancient astronomers were much perplexed with the motions of comets, which appeared to be much more irregular than those of other celestial bodies and unconformed to any known laws. Tycho Brahé believed that comets moved in circular orbits, and Kepler imagined that they travelled in straight lines outwards from the Sun. Newton, however, was able to demonstrate that any conic section can be described about the Sun consistent with the law of gravitation, and that the orbits of comets correspond with three of the four sections into which a cone can be divided. Consequently, they obey the laws of planetary motion. Comets which move in ellipses of known eccentricity and return with periodical regularity may be regarded as belonging to the solar system. Twenty of these are known, and eleven of them have more than once passed their perihelion. Those most familiarly known complete their periods in years as follows:—Encke’s 3·3; Swift’s, 5·5; Winnecke’s, 5·6; Tempel’s, 6; Brorsen’s, 5·5; Faye’s, 7·4; Tuttle’s, 13·8, and Halley’s, 76. Comets with parabolic and hyperbolic orbits may be regarded as stray objects which visit our system once, and depart never to return again. Besides those already mentioned there are many comets with orbits of such marked eccentricity that their ellipses when near perihelion cannot be distinguished from parabolæ. The great comets of 1780, 1811, 1843, 1858, 1861, and 1882 traverse orbits approaching this form, and some of them require hundreds and thousands of years to accomplish a circuit of their paths.

Numerous instances of the appearance of remarkable comets have been recorded in the annals of ancient nations. The earliest records of comets are by the Chinese, who were careful observers of celestial phenomena. A comet is said to have appeared at the time of the birth of Mithridates (134 B.C.), which had a disc as large as that of the Sun; a great comet also became visible in the heavens about the time of the death of Julius Cæsar (44 B.C.), and another was seen in the reign of Justinian (531 A.D.). A remarkable comet was observed in 1106, and in 1456, the year in which the Turks obtained possession of Constantinople and threatened to overrun Europe, a great comet appeared, which was regarded by Christendom with ominous forebodings. The celebrated astronomer Halley was the first to predict the return of a comet. Having become acquainted with Newton’s investigations, which showed that the forms of the orbits of comets were either parabolæ or extremely elongated ellipses, he subjected the next great comet, which appeared in 1682, to a series of observations, calculated its orbit, and predicted that it would return to perihelion in seventy-five or seventy-six years. On referring to past records he discovered that a great comet appeared in 1607, which pursued a path similar to the one traced out for his comet, another was seen in 1531, and one in 1456. Halley perceived that the intervals between those dates corresponded to a period of about seventy-six years, the time which he calculated would be required for his comet to complete a revolution of its orbit. He therefore had no hesitation in predicting that the comet would appear again in 1758. Halley knew that he would not be alive to witness the event, and alludes to it in the following sentence: ‘Wherefore if it should return according to our prediction about the year 1758, impartial posterity will not refuse to acknowledge that this was first discovered by an Englishman.’ As the time approached when the comet should be drawing near to our system, much interest was excited among astronomers, who would have an opportunity afforded them of testing the accuracy of Halley’s prediction. An eminent French mathematician named Clairaut computed anew, by a method rather different to that adopted by Halley, the retarding effect of the attraction of the planets upon the speed of the comet, and arrived at the conclusion that it would reach perihelion about the middle of April 1759; but, owing to unknown influences—Uranus and Neptune not having been discovered—it might be a month before or behind the calculated time. Clairaut made this announcement on November 14, 1758. Astronomers were now intently on the look-out for the comet, and night after night the sky was swept by telescopes in search of the expected visitor, which for upwards of seventy years had been pursuing its solitary path invisible to mortal eyes. But the mental vision of the mathematician did not fail to follow this celestial object, which was now announced as being on the confines of our system. The comet was first observed on December 25, 1758, it soon became conspicuous in the heavens, and reached perihelion on March 12, 1759, a month before the time assigned to it by Clairaut but within the limit of error allowed for unknown influences. Halley’s comet returned again in 1835, and may be expected about the year 1911. The periodic appearance of this comet has been traced back to the year 1305.

The celebrated comet of 1680 was noted as having been the one which afforded Newton an opportunity of making observations which led to his discovery that comets describe orbits round the Sun in conformity with the different sections of a cone. The comet of 1811 was observed for many weeks in the northern heavens as a brilliant object with a beautiful fan-shaped tail; it completes a revolution of its orbit in about 3,000 years. The comet of 1843 was also a splendid object. It possessed a tail 200 million miles in length, and approached within 32,000 miles of the Sun. The heat to which it was exposed was sufficient to volatilize the most infusible substances known to exist. Donati’s comet of 1858 will be long remembered as one of the most impressive of celestial spectacles: its tail extended over an area of forty degrees, and enveloped the star Arcturus, which could be seen shining through it with undiminished brilliancy. Its period is estimated to be 2,100 years. A great comet appeared in 1861, through the tail of which the Earth passed without any perceptible effect having resulted. No remarkable comets have appeared during recent years. In 1880, 1881, and 1882, several were observed, and that of 1881 was the first successfully photographed.

Comets consist of cosmical matter which exists in a condition of extreme tenuity, and especially so in the coma and tail. Sir John Herschel described them as almost spiritual in texture, and small stars have been seen shining through their densest parts without any perceptible diminution of their light. The