1.  There is no extravagance of fancy, how wild soever, but what hath been hazarded in different ages to account for the nature of comets, and the irregularity of their course. Even in the last age Kepler and Hevelius advanced conjectures entirely extravagant, respecting the cause of these phenomena. Mr. Cassini, and after him Sir Isaac Newton, have at length given certainty to the opinions of the philosophers in this respect, by observations and calculations most just and accurate; or to speak with more propriety, upon recalling and fixing our attention upon what had formerly been advanced by the Chaldean Egyptians, Anaxagoras, Democritue, Pythagoras, Hippocrates Seneca, Appolonius, Myndius, and Artemidorus. For in treating of the nature of these stars, their definitions of them, the reasons they assign for the rareness of their appearance, and the apologies they make for not having yet formed a more exact theory, are all in the very terms that Seneca had already used. With respect to the time of that philosopher, we have formerly taken notice, that the collecting together the observations anciently made by the returns of comets was not sufficient to establish the theory of them; because, their appearances were so very rare, that there had not been an opportunity of making a proper number of observations, to determine whether their course was regular or not; but that the Greeks, who had some time before observed this, were applying themselves to researches of this kind.

2.  Seneca, in the same place acquaints us, that the Chaldeans looked upon comets as planetary bodies; and Diodorus Siculus, in his history, giving an account of the extent of knowledge among the Egyptians, praises them for the application with which they studied the stars and their courses: where he remarks, that they had collect ed observations very ancient and very exact, fully informing them of the several motions, orbits and stations of the planets; adding also, that they could foretell earthquakes, inundations and the return of comets.

3.  Aristotle, in laying down the opinion of Anaxagoras and Demo critus, says of the first, that he apprehended comets to be an assemblage of many wandering stars; which, by their approximation, and the mutual blending of their rays, rendered themselves visible to us. This notion was far from being philosophical, yet was it preferable to that of some great moderns, such as Kepler and Hevelius. who would have it, that they were formed out of air, as fishes are out of water.Pythagoras, who approached very near to the times of Anaxagoras, taught. according to Aristotle’s account, an opinion worthy- of the most enlightened age; for he looked upon comets as stars, which circulated regularly, though elliptically about the sun, and which appeared- to us only in particular parts of their orbit, and at considerable distances of time; and the error which Aristotle falls into, in endeavouring- to explain Pythagoras’ sentiment, by a comparison referring to the planet Mercury, ought not to be imputed to the Pythagoric-school. Aristotle relates also, the testimonies of Hippocrates of Chois, and Aeschylus, in confirmation of this opinion. 

4.   Stoboeus presents us with Pythagoras’ sentiment in the very terms of Aristotle, though somewhat more clearly; for he says they imagined the comets to be wandering planets, which appeared only at certain times during their course.

5.   Upon the whole, Seneca, more than any other, hath discussed this subject like a true philosopher. in his seventh book of natural questions, he relates all the different opinions respecting comets, and seems to prefer that of Artemidorus, who imagined, “that there was an immense number of them, but that their orbits were so situated, that so far from being always within view, they could only be seen at one of the extremities.” He afterward reasons upon this with equal elegance and solidity. “Why should we be astonished, says he, that comets, which are so rare a spectacle in the world, have not yet come under certain rules: or that we have not hitherto been able to determine, where begins or ends the course of planets, as ancient as universe, and whose returns are at such distant intervals The time will come, cries he, that posterity will be amazed at our ignorance in. things so very evident; for what now appears so obscure, will one day or other, in the course of ages, and through the industry of our descendants, become manifestly clear; but a small number of years, passed between study and the indulgence of passion, will not avail for researches so important, as those which propose to themselves the compression of nature so remote.”

6.  Upon a review of the several passages which we have just now cited, it must now be admitted, that the moderns have said nothing solid with regard to comets, but what is to be found in the writings of the ancients: except what later observations have furnished them with, which Seneca judged to be so necessary, and which only can be the effect of a long succession of ages.

Chapter 18 - Of the Refraction of Light, and Astronomical Refraction; And of Perspective

Chapter 18