3 i8 THE POPULAR SCIENCE MONTHLY 



mastering these treasures was too great, perhaps his studies in anatomy 

 and metaphysics prevented, but be that as it may, it must be acknowl- 

 edged that his few pronouncements on physical science were for the 

 most part erroneous and proved hindrances and not helps to those 

 who followed him. 



With the founding of the schools of Alexandria in the palmy days 

 of the Ptolemys astronomy became a science. It was during this 

 period that simultaneous measurements were made upon the altitude 

 of the sun at Alexandria and Syene. At the latter place the 

 sun at the summer solstice was on the zenith and at the former place 

 7° 12' therefrom. From this and the known distance between stations 

 the circumference of the earth was calculated as 250,000 stadia or 28,- 

 000 miles. This measurement was first made by Eratosthenes (276- 

 196) and places him in the first rank in the Hall of Fame. 



The centuries immediately preceding and following the beginning 

 of the Christian era mark the rise both of astronomy and of geometry. 

 It was probably due to progress in the latter science that Hipparchus 

 (190-120 B.C.) and, later, Ptolemy (a.d. 120-170), were led to pro- 

 pound the system that still bears their names. This was a far more 

 ambitious system than any that had preceded it, and sought, for the 

 first time perhaps, to describe the exact path of the heavenly bodies. 

 From our vantage point of wider knowledge it is easy to see its 

 absurdities. Hasty judgment, however, must not be passed either 

 upon its founders or upon the system itself. 



In the first place, it was based upon observations; in other words, 

 it was a generalization from data. 



Secondly, it satisfactorily explained the observations contained in 

 the premises. 



Thirdly, it made possible the forecasting of eclipses. 



The characteristics of the system may be given as follows : 



1. The earth is a globe set immovable at the center of the celestial 

 sphere — which sphere carries the fixed stars and revolves once per day. 



2. The size of the earth is insignificant in comparison with that 

 of the celestial sphere. 



3. Seven planets revolve around the earth in the following order — 

 the moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn. 



4. The moon and sun move in excentric circles, the rest in epicycles. 

 Of the several considerations that must have induced the Alex- 

 andrian school to adopt this system we may note : 



I. The Pythagorean system called for a moving earth, the Ptolemaic 

 did not. 



II. The observed motions of the planets were explained by the 

 Ptolemaic system, and, while it is true that the Pythagorean system 

 was capable of this also, it does not appear that the test by actual calcu- 

 lation had been made. 



