164 THE GREEK ASTRONOMY. 



was not wanted for such observations. Ptolemy 10 says that he found 

 it more convenient to observe altitudes by means of a square flat piece 

 of stone or wood, with a quadrant of a circle described on one of its 

 flat faces, about a centre near one of the angles. A peg was placed at 

 the centre, and one of the extreme radii of the quadrant being perpen- 

 dicular to the horizon, the elevation of the sun above the horizon was 

 determined by observing the point of the arc of the quadrant on which 

 the shadow of the peg fell. 



As the necessity of accuracy in the observations was more and more 

 felt, various adjustments of such instruments were practised. The 

 instruments were placed in the meridian by means of a meridian 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? 1 



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 oi 

 the armil, so as to see its two edges apparently brought together, and 

 the star apparently touching them. 12 



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 description in 

 Ptolemy. 14 This also consisted of circular rims, movable 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 regard to the 

 sun or a star, were thus determined. 



The astrolabe continued long in use, but not so long as the quadrant 

 described by Ptolemy ; this, in a larger form, is the mural quadrant, 

 which has been used up to the most recent times. 



It may be considered surprising, 15 that Hipparchus, after having 



J0 Synt. i. 1. 



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.) 



"Delanib. A. A. \. 185. 



13 Ptol. Synt. i. 1. T fl<r:r/) KoAA);/'io; dufiorepais avrwi' raTf ixiipaviiai; 5 darr,p 11 



. v. 1. Del. A. A. 151. 



