Astronomy

Astronomy and astrology seem to differ in this, that astronomy took its name from the law of the stars, astrology is so called as discoursing about the stars. For _nomos_ is interpreted law and _logos_ speech. So astronomy seems to be that which treats of the law of the stars and the revolution of the heaven, investigating the regions, circles, courses, risings and settings of the stars, and why each is so called. Astrology, however, considers the stars according to the observation of birth and death and of any other events whatever; which is partly natural, partly superstitious. Natural, in the complexions of bodies, which vary according to the temperament of the higher bodies, as health, sickness, storm, fair weather, fertility and barrenness. Superstitious, in contingent matters and in those that lie under free will; this part the mathematicians treat.

What is astronomy? It is the science that sets forth the causes of the things which appear to us, dwelling on the Earth, in the heavens and the stars, and which produce the changing of the seasons; once these are understood, we may be able to foretell the face of the heavens, that is, the celestial appearances, for the future, and to assign the fixed times of those that are past.

Whence is astronomy so called? From the law or governance of the stars, that is, of the motions by which the stars are moved, as economy is named from the management of a household, and a pedagogue from the guiding of boys.

What relationship has this science with the others? First, it is a part of physics, because it inquires into the causes of natural things and events, because the motions of the heavenly bodies are among its subjects, and because one of its aims is to search out the structure of the edifice of the world and of its parts. … Sixth, it nevertheless belongs to the class of the mathematical disciplines, and uses geometry and arithmetic as its two wings, considering the quantities and figures of the bodies and motions of the world, numbering the times, and by these despatching its demonstrations and bringing the whole speculation to use, that is, to practice.

How manifold, then, is the care and office of the astronomer? The parts of the astronomical office are chiefly five: the historical, concerning observations; the optical, concerning hypotheses; the physical, concerning the causes of the hypotheses; the arithmetical, concerning tables and calculation; the mechanical, concerning instruments.

How do they differ from one another? Although none of them can do without the geometrical demonstrations, which serve theory, and the numbers, which serve practice, since these are, as it were, the common speech of geometers, yet the first three belong more to theory, the last two more to practice.