Cassini, and after him sir Isaac Newton, by their close observations and
accurate calculations respecting the nature and courses of comets, have
given certainty to the opinions of the old philosophers; or, to speak
with more propriety, they have recalled and fixed our attention upon
what had before been advanced by the ancients on these subjects. For, in
treating of the nature of these stars, their definitions of them, the
reasons they assign for the rarity 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. In the time of that
philosopher, the observations previously made of the returns of comets,
were not sufficiently collected to establish the theory of these
phenomena. Their appearances were so very rare, that they had not
afforded an opportunity to determine, whether their course was regular
or not. The Greeks, however, before Seneca’s time, had remarked to the
same effect, and were applying themselves to researches of this kind.
Seneca says, that the Chaldeans looked upon comets as planetary bodies;
and Diodorus Siculus, in 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; and remarks, that they had
collected, observations very ancient and very exact, fully informing
them of the several motions, orbits, stations, &c. of the planets. He
adds, that they could foretell earthquakes, inundations, and “the return
of comets.”
Aristotle says, that Anaxagoras 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,
though far from being philosophical, was yet far preferable to that of
some great moderns, such as Kepler and Hevelius, who supposed that
comets were formed out of air, as fishes are out of water.
Pythagoras, however, who approached very near to the times of
Anaxagoras, held an opinion worthy of the most enlightened age. 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.”
Seneca, more than any other, has discussed this subject like a true
philosopher. 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 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 the universe, and whose returns are at
such distant intervals_? The time will come,” he exclaims, with
enthusiasm, “when posterity will be amazed at our ignorance in things so
very evident; for what now appears to us 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, are not of avail for researches so
important, as those which propose to themselves the comprehension of
natures so remote.”
The moderns have said nothing satisfactory respecting 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 could be collected through a long succession
of ages.