86 SCIENCE IN GRECO-ROMAN ANTIQUITY 



ported by a small pillar, to which it was fixed by its 

 lowest point ; another circle, movable about the axis 

 of the earth, could always be brought to coincide with 

 the ecliptic ; a third circle turned round the poles of 

 the ecliptic on two cylinders which were fixed thereto 

 and marked the longitudes ; finally, a fourth circle, 

 placed inside the three others, carried two sights used 

 for sighting the heavenly body, whose position it was 

 desired to determine." The second, or planispherical, 

 astrolabe was quite different from the spherical 

 astrolabe, or armillary sphere ; it was composed of a 

 disc which could be suspended vertically or placed 

 horizontally ; it was used for taking the altitude of the 

 stars and for solving triangles. " Thus the same name 

 has been given to two things which have no resemblance 

 and thereby regrettable confusions have arisen." 1 

 Hipparchus also invented trigonometry, but, in order 

 to solve a triangle, he always supposes it to be in- 

 scribed in a circle ; the sides of this triangle are then 

 chords which are calculated as a function of the radius 

 of the circle. This being so, Hipparchus calculated a 

 table of chords and laid down the formulae by which 

 the problems of spherical astronomy can be solved. 

 He is thus more truly than Aristarchus the creator of 

 trigonometry. 



Having seen a new star appear, he had the idea of 

 making for posterity a catalogue of the positions of the 

 stars and principal constellations. One can never, 

 said Pliny, praise him enough for this undertaking, 

 which would have made even a god shrink back 

 (Nat. Hist., I, p. 159, 10). Thanks to the accuracy of 

 his observations, which he compared with those of his 

 predecessors, Hipparchus proved that, if the latitudes 

 of the stars have remained constant, their longitudes 

 have all increased by the same amount. He concluded 



1 Doublet, Histoire de VAstronomie, Doin, Paris, 1923, p. 

 105. 



