De cometis

natural motion, acting as one to the motion of its container, yet dividing the air or fire. By its own celestial virtue, it is moved in likeness to the motion of its star, declining toward the south or toward the north, according to the succession of the signs, or sometimes retrograde against the succession of the signs. This regularity is nonetheless always maintained, that it describes a portion of a great circle or a parallel, either greater or smaller according to the velocity of its motion and the duration of the same. The position of this portion of the circle is the cause of why its latitude and longitude of motion are diversified in relation to the Ecliptic.

On the figure, color, and quantity of comets Chapter

To one diligently observing the writings of the ancients, it will be clear that diverse figures are appropriate to comets, according to the diversity of the natures of the stars providing them, just as with the species of animals. Similarly, a certain color is largely ascribed to them, but here, as in individuals of one species, it seems to vary greatly on account of the subject matter. Therefore, color determines the species less, but figure does so more, and motion most of all. In truth, the efficient causes are the celestial bodies. Quantity, also, as in individuals of other things, is diverse according to the scarcity or multitude of the matter, which does not diversify the species and nature, just as color does not do so much. From what precedes, it is clear that comets differ from other ignited impressions just as gems do from rocks, and as gold from ore, and almost as spirit from body. Addendum

A comet of the nature of Mercury, of a sky-blue color with a long tail, Alkindus a 9th-century Arab polymath says is much like a horn, and therefore he calls it "the horn."
A comet of the complexion of Mars, red with a long tail not as great as the preceding, is called "aurora" as in the morning.
A comet of the complexion of Venus, as great as the moon, having hair and a long, thick ray behind itself, which when it appears traverses twelve signs, is called "the cloud" because of its thickness.
A comet of the complexion of Jupiter, having very beautiful rays as white as the purest silver, whose brightness exceeds the brightness of the nocturnal stars, is called "silver."
A comet of the nature of Saturn is of a sky-blue color tending much toward the color of Saturn, namely leaden and somewhat black, and therefore it is called "the black."
The figure of these five is tailed, and they are named by the general name "pogomias."
The figure of the other four following is without a tail but hairy all around, and by general name and properly they are called "comets."
A great and round comet in which a human face is seen, having the color of silver mixed with gold, is called "the rose," whose star is not determined, though it is more ascribed to the sun.
A comet that goes near the sun, having a horrible aspect, is called "the dart" original: "veru", whose nature likewise is not determined, but it is more ascribed to the dragon's tail.
A comet having thick rays, and sometimes it is oblong, and sometimes...