What is _astronomy_? It is the science of treating of the causes of those celestial appearances which we who live on the earth observe and which mark the changes of times and seasons; by the studying of which we are able to predict for the future the face of the heavens, that is, the stellar phenomena, and to assign fixed dates for those which have occurred in the past. _Why is it called astronomy?_ From the law (νουος) or governance of the stars (ἀστρα), that is, of the motions in which the stars move, just as economy is named from the law of domestic affairs (οἰκονουία) and paedonomy (παιδονουία) from the ruling of youths. _What is the relation of this science to the other sciences?_ 1) It is a branch of physics because it investigates the causes of natural objects and events, and because among its subjects are the motions of the heavenly bodies, and because it has the same end as physics, to inquire into the conformation of the world and its parts. 2) Astronomy is the soul of geography and hydrography, for the various appearances of the sky in various districts and regions of the earth and sea are known only by astronomy. 3) Chronology is dependent upon it, because the movements of the heavenly bodies prescribe seasons and years and date the histories. 4) Meteorology is also its subordinate, for the stars move and influence this sublunary nature and even men themselves. 5) It includes a large part of optics, because it has a subject in common with that; that is, the light of the heavenly bodies, and because it corrects many errors of sight in regard to the character of the earth and its motions. 6) It is, however, subordinate to the general subject of mathematics and uses arithmetic and geometry as its two wings, studying the extent and form of the bodies and motions of the universe and computing the periods, by these means expediting its demonstrations and reducing them to use and practical value. _How many, then, are the branches of astronomical study?_ The departments of the study of astronomy are five; historical, in the matter of observations, optical as to the hypothesis, physical as to the causes of the hypotheses, arithmetical as to the tables and calculations, mechanical as to its instruments. * * * * * _Since we must begin with appearances, explain how the world seems to be made up._ The world is commonly thought, accepting the testimony of the eyes, to be an immense structure consisting of two parts, the earth and the sky. _What do men imagine concerning the figure of the earth?_ The earth seems to be a broad plane extending in a circle in every direction around the spectator. And from this appearance of a plane bounded by a great circle the appellation, _orbis terrarum_, the circle of the earth, has arisen, and has been taken over by the Scripture and among other nations.