Notwithstanding the present improved state of astronomical science, it is evident that the philosophy of comets is very imperfect. Kepler, though in other respects a very great genius, and to whose useful labors astronomy is deeply indebted, indulged in the most extravagant conjectures; he imagined that the planets were large animals, swimming round the sun: and that the comets were animals still more huge and monstrous, which had been generated in the celestial spaces. Jean Bodin, a learned Frenchman of the 16th century, entertained an opinion, if possible, still more absurd and ridiculous. He maintained that the comets are spirits, which having lived on the earth innumerable ages, and being at last arrived on the confines of death, celebrated their last triumph, or are recalled to the firmament like shining stars! Mr. Whiston was of opinion, that comets are so many hells, appointed in their orbits alternately to carry the damned into the confines of the sun, there to be scorched by its violent heat, and then to return with them beyond the orb of Saturn, there to starve them in those cold and dismal regions. Thus
“Born in an age more curious than devout;
More fond to fix the place of heaven or hell,
Than studious this to shun, or that secure.”[131]
James Bernoulli, in his Systema Cometarum, says, that comets are no other than the satellites of some very distant planet, which is itself invisible to us on account of its vast distance, as are also the satellites, unless when they are in that part of their orbits which is nearest the earth. Comets, according to Des Cartes, were formerly fixed stars: but which becoming gradually covered with maculæ, and at length wholly deprived of their light, cannot keep their places, but are carried off by the vortices of the circumjacent stars; and in proportion to their magnitude and solidity, moved in such a manner as to be brought nearer the orb of Saturn; and thus, coming within reach of the sun’s light, are rendered visible.
Aristotle, Seneca, Plutarch, and others, testify, that the Pythagoreans, and the whole Italian sect, maintained, that a comet was a kind of planet or wandering star, which appeared again after a long interval of time. Hippocrates Chius was of the same opinion as Aristotle informs us. Democritus held also the same opinion, as Seneca tells us in his “Natural Questions;” book vii, chap. 3, “For,” says he, “Democritus, the most curious and subtle of all the ancients, suspected that there were many more stars which moved, meaning by this expression the comets; but he neither established their number, or their names, the courses of the five planets not having as yet been discovered.” Again, Seneca assures us, that Apollonius Myndius, one of the most skilful philosophers in the search of natural causes, asserted, that the Chaldeans reckoned comets among the other wandering stars, and that they knew their courses. Apollonius himself maintained, that a comet was a star of its own kind, as the sun and moon are, but that its course was not yet known; that by its motions it mounts very high in the heavens, and only appears when it descends into the lower part of its orbit. And Seneca himself embraces this opinion in the following truly philosophical words: “I cannot believe,” says he, “that a comet is a fire suddenly kindled, but that it ought to be ranked among the eternal works of nature. A comet has its proper place, and is not easily moved from thence; it goes its course, and is not extinguished, but runs off from us. But you will say, if it were a wandering star it would keep in the zodiac. But who can set one boundary to all the stars? Who can restrain the works of the Divinity to a narrow compass? For each of those bodies, which you imagine to be the only ones that have motion, have very different circles; why, therefore, may there not be some that have peculiar ways of their own, wherein they recede far from the rest? But that their courses may be known, it is necessary to have a collection of all the ancient observations about comets; for their appearances are so rare, that their orbits are not yet determined; nor can we as yet find whether they have their periods, or whether they return again in a certain order.”—“The time will come,” continues he, “wherein these things which are now hid from us will be discovered; which observation, and the diligence of after ages, will find out. For it is not one age that is sufficient for so great matters: the time will be when posterity will wonder that we were ignorant of things so plain; one will arise who will demonstrate in what regions of space the comets wander, why they recede so far from the other planets; how great and what sort of bodies they are.”[132] The period, predicted by Seneca, in the first century of the Christian era, is not yet arrived. “After all that has been done and written on the subject of comets,” says a late writer, “we must confess, that our knowledge of these wandering bodies is still very imperfect.” “It appears to me,” says La Lande, “that almost every thing depends on comets. The only thing that I recommend to my correspondents, is to look after and attend to comets: the knowledge of comets is alone wanting to complete the science of astronomy.”
Several ages elapsed before this prediction of Seneca seemed likely to be fulfilled. Tycho Brahé was the first who attempted to restore the comets to their proper rank in creation. Having diligently observed the comet of 1577, and finding that it had no sensible diurnal parallax, he very properly determined its true place to be among the other revolving bodies in the planetary regions, as appears by his book De Cometa, 1577. And Sir Isaac Newton, from his amazing discoveries, gives the following theory of comets: “They are,” says he, “compact, solid, fixed, and durable bodies; in fact, a kind of planets, which move in very oblique and eccentric orbits, every way with the greatest freedom; persevering in their motions even against the course and direction of the planets: and their tail is a very thin and slender vapor, emitted by the head or nucleus of the comet, ignited or heated by the sun.”
Various conjectures have been formed concerning the nature of the tails of comets. Dr. Hamilton, of Dublin, in the second of his Philosophical Essays, urges several objections against the Newtonian hypothesis: he remarks, that, since the tail of a comet, though exceedingly rare, meets with no resistance in its rapid motion round the sun (except so slight a one as can only cause a very small condensation on that side of it which moves foremost, and thereby may make it a little brighter than the other side), it cannot possibly move in a medium denser and heavier than itself, and therefore cannot be raised up from the sun by the superior gravity of such a medium. And since the stars seen through all parts of a comet’s tail appear in their proper places, and with their usual colors, he infers that the rays of light suffer no refraction in passing through the tail; therefore, since bodies reflect and refract light by one and the same power, he concludes that the matter of a comet’s tail has not the power of refracting or reflecting light, and is, of consequence, a lucid or self shining substance. Also from what astronomers say of the splendor of comets’ tails, it is manifest they do not shine with such a dull light as would be reflected to us by the clouds or vapors at so great a distance, but with a brisker though a glimmering light, such as would arise from a very thin, volatile, burning matter. Dr. Halley, speaking of the great streams of light in the remarkable aurora borealis seen in 1716, says, “they so much resembled the long tails of comets, that at first sight they might be taken for such:” and afterwards, “this light seems to have a great affinity to that which the effluvia of electric bodies emit in the dark.” Dr. Hamilton improves upon these hints: and since, as he shows, the tails of comets, the aurora borealis, and the electric fluid, agree remarkably, not only in their appearance, but also in such properties as we can observe of each of them, he concludes that they are substances of the same nature. And, because the electric matter, from its vast subtility and velocity, seems capable of making great excursions from the planetary system, he imagines that the several comets, in their long excursions from the sun in all directions, may overtake this matter; and by attracting it to themselves may come back replete with it, and being again heated by the sun, may disperse it among the planets, and so keep up a circulation of this matter, which there is reason to think is necessary in our system.[133]
Comets, descending from the remote parts of the system with great rapidity become visible to us in the lower parts of their orbits; and after a short stay, go off again to vast distances, and disappear. They move about the sun in very eccentric ellipses; and the velocity with which they seem to move is variable in every part of their path round the sun; when near to which they appear to move with great swiftness, and, when very remote, their motion is slow. They are opake bodies, but of a much greater density than the earth; for some of them are heated in every period to such a degree, as would vitrify or dissipate any substance known to us. Sir Isaac Newton computed the heat of the comet, which appeared in the year 1680, when nearest the sun, to be 2,000 times hotter than red hot iron, and that, being thus heated, it must retain its heat till it comes round again, although its period should be more than 20,000 years; and it is computed to be only 575.
The number of the comets is much greater than that of the planets belonging to our system. From the beginning of the Christian era, till now, there have appeared about five hundred. Before that time, we have accounts of about one hundred others. But, when it is considered that there may have been many that have not been seen, from being too near the sun, from appearing in moon-light, from being in the other hemisphere, or from being too small, or from not being recorded, the number is probably much greater. Miss Herschell, by means of the telescope, has, within the last twenty years, discovered several comets. The orbits of about one hundred comets have been calculated with sufficient accuracy for ascertaining their identity on any future appearance. Many of these orbits are inclined to the plane of the ecliptic in large angles, and many of them approach much nearer the sun than the earth does. Their motions are also different from those of the planets, some of them being direct and others retrograde, nearly half the number moving each way. The different motions of the comets, and the various inclinations of their orbits to the plane of the ecliptic, must not be regarded as the work of chance, but as calculated to answer beneficial purposes, or avoid baneful consequences; for if these orbits had been nearly coincident with that of the earth, both bodies might have arrived at the common point of intersection of their orbits at the same time; in which case a derangement of both motions must, at least, have been the necessary result.[134] But, according to all the observations that have been made respecting their present distribution and direction, there is not the least reason to apprehend any such consequence.