216 THE GREEK ASTRONOMY. 



line drawn by astronomical methods on the floor on 

 which they stood. The plane of the instrument 

 was made vertical by means of a plumb-line : the 

 bounding radius, from which angles were measured, 

 was also adjusted by the plumb-line 11 . 



In this manner, the places of the sun and of the 

 moon could be observed by means of the shadows 

 which they cast. In order to observe the stars 12 , 

 the observer looked along the face of the circle 

 of the armil, so as to see its two edges apparently 

 brought together, and the star apparently touch- 

 ing them 13 . 



It was afterwards found important to ascertain 

 the position of the sun with regard to the ecliptic : 

 and, for this purpose, an instrument, called an 

 astrolabe, was invented, of which we have a de- 

 scription in Ptolemy 14 . This also consisted of cir- 

 cular rims, moveable within one another, or about 

 poles; and contained circles which were to be 

 brought into the position of the ecliptic, and of 

 a plane passing through the sun and the poles of 

 the ecliptic. The position of the moon with regard 

 to the ecliptic, and its position in longitude with 



11 The curvature of the plane of the circle, by warping, was 

 noticed. Ptol. iii. 2. p. 155, observes that his equatorial circle 

 was illuminated on the hollow side twice in the same day. (He 

 did not know that this might arise from refraction.) 



18 Delamb. A. A. i. 185. 



13 Ptol. Synt. i. 1. *Cl<rirep KCKoAAfjjuieVoe dn<j>orfpai<; av-rtav 

 Tat? 7rt<J)aveiats o dffTtjp ev Tia Si' avrtav eViTre'Sw 



14 Synt. v. 1. 



