INFLUENCE OF THE PTOLEMAIC EPOCH. 545 



a, century subsequent to this period, endeavoured to esta- 

 blish the hypothesis of the Samian philosopher, which, re- 

 sembling the views of Copernicus, met with but little atten- 

 tion during that age ; and lastly, to Hipparchus, the founder 

 of scientific astronomy, and the greatest astronomical observer 

 of antiquity. Hipparchus was the actual originator of astro- 

 nomical tables amongst the Greeks,* and was also the dis- 

 coverer of the precession of the equinoxes. On comparing his 

 own observations of fixed stars (made at Rhodes and not at 

 Alexandria), with those made by Timochares and Aristyllus 

 he was led, probably, without the apparition of a new star,f 

 to this great discovery, to which indeed the earlier Egyptians 

 might have attained by a long continued observation of the 

 heliacal rising of Sirius.| 



A peculiar characteristic of the labours of Hipparchus is 

 the use he made of his observations of celestial phenomena 

 for the determination of geographical position. Such a con- 

 nection between the study of the earth and of the celestial 

 regions, mutually reflected on each other, animated through 

 its uniting influences the great idea of the Cosmos. In the 

 new map of the world constructed by Hipparchus, and 

 founded upon that of Eratosthenes, the geographical degrees 

 of longitude and latitude were based on lunar observations 

 and on the measurements of shadows, wherever such an 

 application of astronomical observations was admissible. While 

 the hydraulic clock of Ctesibius, an improvement on the earlier 

 clepsydra, must have yielded more exact measurements of 

 time, determinations in space must likewise have improved 

 in accuracy, in consequence of the better modes of measuring 

 angles, which the Alexandrian astronomers gradually pos- 

 sessed, from the period of the ancient gnomon and the scaphe 

 to the invention of astrolabes, solstitial armils, and linear 

 dioptrics. It was thus that man, and step by step as it were, 



* Ideler, Handbuch der Chronologic, bd. i. s. 212 und 329. 



f Delambre, Histoire de I'Astronomie ancienne, t. i. p. 290. 



Bokh has entered into a discussion, in his Philolaos, s. 118, as to 

 whether the Pythagoreans were early acquainted, through Egyptian 

 sources, with the precession, under the name of the motion of the 

 heavens of the fixed stars. Letronne (Observations sur les Represen- 

 tations zodiacales qui nous restent de VAntiquite, 1824, p. 62), and 

 Ideler (in his Handbuch der Chronol., bd. i. s. 192), vindicate the exclu- 

 sive claim of Hipparchus to this discovery. 



2N 



