De cometis

But if any of these, namely figure or color or motion, be lacking, it is necessary to mix other planets impeding or altering that by their power or mixture.

How the magnitude of comets is found 9

The quantity of comets is comprehended from its distance from the earth and the quantity of the angle of its visual pyramid. For to one degree in a great circle correspond sixteen miles of the circumference of the earth; when, therefore, the comet is distant from the surface of the earth as much as is the semidiameter of the earth, and its body chords one degree, the diameter of its body will be sixteen miles. If, however, it stands at double, its quantity will be double. For such is the proportion of the whole sine, namely sixty, to the distance of the star from the earth, as is the proportion of the chord to the diameter of the star. I say the chord that is subtended to the portion of the great circle in the sky which such a star chords, and such a chord is the base of the visual pyramid of that star. Therefore, let the base of the visual pyramid be multiplied by the distance of the comet from the earth, and the product be divided by sixty, and the quotient is the quantity of the diameter of the comet, the demonstration of which is fitting for another place. The distance of the comet from the earth is comprehended from the diversity of the aspect of that comet or any of its parts to any star neighboring it.

That the substances of comets and similar things have power to alter and permute bodies and spirits of virtues and to induce sicknesses and mortalities

For the principles of all natural, vital, and animal operations are virtues depending on bodies, Avicenna saying: "Every virtue of operation exists as a principle, nor does any operation proceed unless from virtue." Afterward he adds: "Vital virtue is that which preserves the existence of the spirits, which is the vehicle of motion, and renders it apt to receive the impressions of the same when it reaches the brain, and makes it powerful to give life wherever it expands." And the proper seat of this virtue and its operation is the heart. The heart therefore existing as the principle, things adulterated in it will alter all things depending on the altered ones. And since the instrument of all virtues is the spirit generated in the heart, whose power in individual limbs and organs causes operations according to their property, it will be necessary, the spirit itself being altered, that the operations proceeding from those spirits be altered and diversified. Regarding which, Avicenna similarly says in the Canon, third book, in the chapter on melancholy: "For the animal spirit is continuous with the vital spirit and of its own substance, wherefore its corrupted melancholy complexion corrupts also the complexion of the brain and communicates melancholy to it." Thus, therefore, with the spirits of the body changed, according to what the alteration will be, there arises dyscrasia a poor mixture of bodily humors and thence a mortal or healthy sickness, running the course of its star with its determined sharpness and motion. Whence in certain pestilences, the critical determination happened more in the seventh day; those who escaped this were made safe. In some others, the determination happened in the ninth. In some other times of pestilence, they were determined in the fourth. Truly, that spirit is altered from the air inspired, necessary for the eventilation of the spirits of its own heart, he says.