Comæ and tails–Classification of the ancients according to apparent external form ; the twelve kinds of comets described by Pliny–The ‘ Guest-star ’ of the Chinese–Modern definitions : nucleus, nebulosity or atmosphere; tails.

What is the distinctive sign of a comet by which it is universally known, by which it is distinguished from all other celestial bodies? Everyone answers at once, it is the train of luminous vapour, the nebulosity of more or less length, which accompanies it or at least surrounds it; in other words, the tail and the coma

This is what the etymology implies, the word comet signifying long-haired or hairy. Armed with its tail, which appears brandished in the heavens like an uplifted sword or a flaming torch, the precursor of some untoward event, a comet is everywhere recognised on the instant of its appearance; it needs no passport signed by astronomers to prove its identity. But should the tail be absent, should no appendage or surrounding nebulosity distinguish the celestial visitor on its apparition, for the world at large it is no comet, but simply an ordinary star like any other.

Nevertheless, there are tailless comets. The comet of 1585 was equal to Jupiter in size, but less brilliant; its light was dull. It had neither beard nor tail, and it might have been compared to the nebula in Cancer (Pingré).