Ammianus Marcellinus Res Gestae 25.10.3

[3] For some think that they are so called because they are numerous stars united in one body, ¹ and send out writhing fires resembling hair. ² Others believe that they take fire from the dryer exhalations of the earth, which gradually rise higher. Others again think that the rays streaming from the sun are prevented by the interposition of a heavier cloud from going downward, and when the brightness is suffused through the thick substance, it presents to men’s eyes a kind of star-spangled light. Yet others have formed the opinion that this phenomenon occurs when an unusually high cloud is lit up by the nearness of the eternal fires, or at any rate, that comets are stars like the rest, the appointed times of whose rising and setting ³ are not understood by human minds. Many other theories about comets are to be found in the writings of those who are skilled in knowledge of the universe; but from discussing these I am prevented by my haste to continue my narrative.