130 THE GREEK ASTRONOMY. 



star can ever be seen at the same time with the sun. If the whole 

 circuit of the sky be divided into twelve parts or signs, it is estimated 

 by Autolycus, the oldest writer on these subjects whose works remain 

 to us, 38 that the stars which occupy one of these parts are absorbed by 

 the solar rays, so that they cannot be seen. Hence the stars which 

 are seen nearest to the place of the setting and the rising sun in the 

 evening and in the morning, are distant from him by the half of a 

 sign : the evening stars being to the west, and the morning stars to 

 the east of him. If the observer had previously obtained a knowledge 

 of the places of all the principal stars, he might in this way deter- 

 mine the position of the sun each night, and thus trace his path in a 

 year. 



In this, or some such way, the sun's path was determined by the 

 early astronomers of Egypt. Thales, who is mentioned as the father 

 of Greek astronomy, probably learnt among the Egyptians the results 

 of such speculations, and introduced them into his own country. His 

 knowledge, indeed, must have been a great deal more advanced than 

 that which we are now describing, if it be true, as is asserted, that he 

 predicted an eclipse. But his having done so is not very consistent with 

 what we are told of the steps which his successors had still to make. 



The Circle of the Signs, in which the sun moves among the stars, 

 is obliquely situated with regard to the circles in which the stars move 

 about the poles. Pliny 39 states that Anaximander, 40 a scholar of Thales, 

 was the first person who pointed out this obliquity, and thus, as he 

 says, " opened the gate of nature." Certainly, the person who first 

 had a clear view of the nature of the sun's path in the celestial sphere, 

 made that step which led to all the rest ; but it is difficult to conceive 

 that the Egyptians and Chaldeans had not already advanced so far. 



The diurnal motion of the celestial sphere, and the motion of the 

 moon in the circle of the signs, gave rise to a mathematical science, 

 the Doctrine of the Sphere, which was one of the earliest branches of 

 applied mathematics. A number of technical conceptions and terms 

 were soon introduced. The Sphere of the heavens was conceived to be 

 complete, though we see but a part of it ; it was supposed to turn about 

 the visible pole and another pole opposite to this, and these poles were 

 connected by an imaginary Axis. The circle which divided the sphere 

 exactly mid way between these poles was called the Equator 



38 Delamb. A. A. p. xiii. 39 Lib. ii. c. (viii.) 



40 Plutarch, De Plac. Phil. lib. ii. cap. xii. says Pythagoras was the author of 

 :his discovery. 



