SECTION III

COMETS DURING THE RENAISSANCE AND UP TO THE TIME

OF NEWTON AND HALLEY.

Apian observes that the tails of comets are invariably directed from the sunObservations of Tycho Brahé; his views and hypotheses concerning the nature of comets-Kepler regards them as transient meteors, moving in straight lines through space-Galileo shares the opinion of Kepler-Systems of Cassini and Hevelius.

SIXTEEN CENTURIES passed away between the prediction of Seneca and its full realisation through the accumulated researches of many astronomers and the publication of the Principia, in which Newton demonstrated the law of cometary movements. There is nothing to tell of the history of comets and of systems during this long and dreary period in which the doctrine of Aristotle prevailed, except that it is entirely filled with astrological predictions. Our first chapter contains a résumé of all that the learned have found of interest concerning the apparition of comets and their formidable signification.

Towards the middle of the sixteenth century the movement of the Renaissance, so favourable to letters and the arts, extended its beneficent influence to the science of observation. At the end of the fifteenth century, we find Regiomontanus describing with care the movements of comets, Apian observing that cometary tails are always turned in a direction from the sun; Cardan remarking that comets are situated in a

region far beyond the moon, founding his opinion upon the smallness or absence of parallax. The time had arrived when, instead of proceeding by way of conjecture and hypothesis, astronomers began to multiply observations and to give them that character of exactness and precision which they had hitherto so much needed. Many erroneous hypotheses were yet to be made, but they were subjected to discussion, and the geometrical conclusions to which they led were compared with the facts of observation. Astronomers of high repute like Tycho Brahé, Kepler, Galileo, Hevelius and Cassini were to err as to the true nature of cometary orbits; philosophers like Descartes were to seek to connect them with their bold but false conceptions of the system of the world. But the great principle that was destined to bind in one majestic whole the entire edifice of accumulated astronomical knowledge, the principle of gravitation, was ere long to give Newton a right to regard these bodies as members of the solar system, or at least as bodies subject to the same laws as the planets. From this moment cometary astronomy begins, and rises rapidly to a degree of development comparable to that of other branches of astronomy.

We will first give a rapid sketch of the principal phases of this history up to the time of Newton, and then proceed to the study of comets in connexion with their movements, their physical and chemical constitution, &c.

The apparition of the comet of 1577 may be regarded as the starting-point of the new period. Tycho, who had carefully observed the temporary star of 1572, which had suddenly appeared in Cassiopeia, now applied himself to make numerous observations of the new comet; he determined its parallax, and thus proved beyond a doubt that comets move in regions more remote than the moon, as Cardan had already remarked. Tycho endeavoured to represent the movement of the comet

by making it describe around the sun an orbit external to Venus. With respect to its physical nature he regarded it as a meteor, but not an atmospheric meteor, since he supposed it to have been engendered in the depths of space. This was a first blow to the ideas of Aristotle, which other contemporary astronomers, such as Mæstlinus and Rothmann, continued to profess.

The comets of 1607 and 1618 furnished Kepler with an opportunity of explaining their apparent movements, and inventing an hypothesis which, although false, was ingenious. According to the immortal author of the three great laws of the planetary motions, comets traverse the solar system in rectilinear orbits, and Pingré justly remarks that the apparent movement of the comets of 1607 and 1618 is more naturally explained by this hypothesis than by that of Tycho, which is equivalent to saying that the paths of the two comets were more nearly straight lines than circles. As to the physical nature of comets, believed by Kepler to be as numerous in the heavens as fishes in the sea, his remarks on the subject taken from the second book of his work upon comets are as follows: They are not eternal, as Seneca imagined; they are formed of celestial matter. This matter is not always equally pure; it often collects like a kind of filth, tarnishing the brightness of the sun and stars. It is necessary that the air should be purified and discharge itself of this species of filth, and this is effected by means of an animal or vital faculty inherent in the substance of the ether itself. This gross matter collects under a spherical form; it receives and reflects the light of the sun, and is set in motion like a star. The direct rays of the sun strike upon it, penetrate its substance, draw away with them a portion of this matter, and issue thence to form the track of light which we call the tail of the comet. This action of the solar rays attenuates the particles which compose the body of

the comet.

It drives them away; it dissipates them. In this manner the comet is consumed by breathing out, so to speak, its own tail.' We see that although, in the opinion of Tycho and Kepler, comets are raised to the rank of heavenly bodies, they continue to regard them as stars of temporary origin, destined to disappear.

Some of the views of Kepler are affected by the singular and mystic conceptions of the great astronomer concerning the heavenly bodies; yet those relating to the formation of cometary tails, as we shall see further on, have been perfected and adopted by contemporary astronomers, and form the starting-point of one of the most accredited modern theories of cometary phenomena.

Galileo also believed that comets move in straight lines, but he was unable to rise above the common opinion, according to which they were mere transient meteors, exhalations of the earth.

The remarkable comets which appeared about the middle of the sixteenth century-namely, those of 1664, 1665, and 1680-attracted the attention of all men of science; the idea that they were veritable stars more and more gained ground, and, after the lapse of fifteen centuries, a definitive return was made to the system of Apollonius of Myndus; but modern astronomy was more exacting than the science of the ancient Greek philosophers. It was necessary to satisfy numerous and precise observations and to pass beyond vague ideas and conjectures. Henceforth the whole question reduced itself to the investigation of the geometrical form of the orbit described by comets and to the determination of the laws governing their

movement.

Cassini attacked this great problem, but he did not arrive at its solution, which is not surprising, when we bear in mind that this illustrious astronomer did not yet dare to abjure the

beliefs that Copernicus and Galileo had overthrown concerning the system of the world. By regarding the earth always as a fixed observatory he could not but confound the apparent motions of comets with their real motions. Cassini rightly supposed them to be stars, old as the world, but he made them describe circular orbits very eccentric to the earth, in order to account for the slight portion of the orbit that is visible during the brief durations of their apparitions.

Hevelius, a laborious observer, came back very nearly to Kepler's system, that is to say, to rectilinear orbits, or orbits sensibly rectilinear. Comets, in his opinion also, are the products of exhalations rising from the earth, the planets, or the sun. Drawn away at first by an ascensional movement, combined with the rotatory movement of the planet that has given it birth, the mass, after having described a spiral, finally attains the limit of the vortex which surrounds the planet; there it dies or escapes along the tangent to the limiting surface. The resistance opposed to it by the ether modifies the form of its orbit, which would otherwise be rectilinear, and causes it to take the form of a parabola. The whole of this system is purely imaginary, and must have made great demands upon the imagination of its author; it rests upon no solid basis of astronomical mechanics. The ideas of Hevelius found but few partisans amongst men of science; the work in which they are developed, valuable for the historic details it contains, and for various observations of comets, more especially those of 1652, 1664, and 1665, is little more than an object of curiosity in the history of science.

Newton, moreover, was about to put an end to all these hypotheses, by connecting the movements of comets with the laws that govern the motions of all the heavenly bodies which move within the sphere of the sun's attraction.