In this chapter we shall briefly advert to a few historical remarks, relating to cometary astronomy.

Going back to the early ages of the world, we find that the Chaldæans considered comets to be permanent bodies analogous to planets, but revolving round the Sun in orbits so much more extensive, that they were therefore only visible when near the Earth. This opinion, which, by the by, is the earliest information we have of there being periodical comets, was also held by the Pythagorean school of philosophers. Yet Aristotle, who records this, insists that comets are merely mundane exhalations, carried up into the atmosphere, and there ignited.

Anaxagoras, Apollonius, Democritus, and Zeno considered that these bodies were formed by the clustering of many smaller planets.

It is a somewhat remarkable fact, that Ptolemy, so celebrated for his varied astronomical attainments, should nowhere have made any mention of comets; his omission is, however, made up for, by Pliny, who seems to have paid much attention to them. He enumerates 12 kinds, each class deriving its name from some physical peculiarity of the objects belonging to it.