334 GREEK SCIENCE. 



having begun to plant colonies in Italy, Gaul, and Egypt, became ac- 

 quainted with the Pythagorean system, and the notions of the ancient 

 Druids concerning astronomy. 



Autoiycus. Passing over the names of various other astronomers of this period, 

 who appear to have done very little towards the advancement of the 

 science, we come to Autoiycus, the most ancient writer whose works 

 have been handed down to our time. He wrote two books, viz., ' Of 

 the Sphere which moves,' and the other, * On the Risings and Settings 

 of the Stars.' These works were composed about 300 B. c. 



We have now passed over a period of three hundred years from the 

 time of Thales, and, therefore, by making a few extracts from these 

 works of Autoiycus, we shall be enabled to form some idea of the 

 progress of astronomy during this period. In the work on the move- 

 able sphere, we have several propositions, of which the following are 

 the most important : 



Earliest work 1. If a sphere move uniformly about its axis, all the points on its 

 sur ^ ace which are not in its axis, will describe parallel circles, having 

 for their common poles, those of the sphere itself, and of which all the 

 planes will be perpendicular to the axis. 



2. All these points will describe, of their respective circles, similar 

 arcs in equal times. 



3. Reciprocally, similar arcs will indicate equal time. 



4. If a great fixed circle, perpendicular to the axis, divide the sphere 

 into two hemispheres, the one visible, the other invisible, and that the 

 sphere turns about its axis, those points on the surface that are 

 hidden will never rise, and those that are visible will never set. This 

 is what we now denominate a parallel sphere ; the great fixed circle 

 corresponding with our equator. 



5. If a great circle pass through the poles, all the points of the 

 surface will rise and set alternately. This corresponds to our horizon, 

 and to our right sphere. 



6. If the great circle be oblique to the axis, it will touch two equal 

 parallel circles ; of which, that adjacent to the one pole will be always 

 apparent, the other always invisible. 



The first of these circles was called by the Greeks (although not by 

 this author), as we still denominate it, the arctic circle, and the other 

 the antarctic circle. 



7. If the horizon be oblique, the circles, perpendicular to the axis, 

 will always have their points of rising and setting in the same points 

 of the horizon, to which they are all equally inclined. 



8. The great circles which touch the arctic and antarctic circle, will, 

 during the complete revolution of the sphere, twice coincide with the 

 horizon. 



9. In the oblique sphere, of all the points which rise at the same 

 instant, those which are nearest to the visible pole will set last ; and 

 of the points which set at the same instant, those that are nearest the 

 same pole will rise first. 



