JOHANN KEPLER 31 



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 f From the law (vovo<i) or govern- 

 ance of the stars (dorpa), that is, of the motions in which the stars 

 move, just as economy is named from the law of domestic affairs 

 (oLKovovia) and paedonomy (-n-aiSovovia) from the ruling of youths. 



What is the relation of this science to the other sciences f i) 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 in- 

 fluence 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 mathe- 

 matics 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 cal- 

 culations, 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. 



