336 



GREEK SCIENCE. 



i.e. 230. 



Ancient 



armillary 



sphere. 



Determina- 

 tion of the 

 equinoxes. 



knowledge in the time of Aristarchus, who lived about two hundred 

 and sixty-four years before the Christian era. 



Eratosther.es. In order of time we pass now to Eratosthenes, who may, perhaps, 

 with more propriety than Autolycus, be considered as the founder of 

 astronomical science ; particularly if it be true that he placed in the 

 portico of Alexandria certain armillary spheres ; of which so much 

 use was afterwards made, and which, it is said, he owed to the mu- 

 nificence of Ptolemy Euergetes, who called him to Alexandria, and 

 gave him the charge and direction of his library. 



According to the description given of these armillaries by Ptolemy, 

 they were assemblages of different circles ; the principal one of which 

 served as a meridian ; the equator, the ecliptic, and the two colures, 

 constituted an interior assemblage, which turned on the poles of the 

 equator. There was another circle, which turned on the poles of the 

 ecliptic, and carried an index to point out the division at which it 

 stopped. The instrument of which the above appears to be the ge- 

 neral construction was applied to various uses ; amongst others, it 

 served to determine the equinoxes, after the following manner : The 

 equator of the instrument being pointed with great care in the plane of 

 the celestial equator, the observer ascertained, by watching the mo- 

 ment when neither the upper nor the lower surface was enlightened by 

 the sun; or rather, which was less liable to error, when the shadow 

 of the anterior convex portion of the circle completely covered the con- 

 cave part on which it. was projected, This instant of time was evi- 

 dently that of the equinox. And if this did not happen at a time 

 when the sun shone, two observations were selected, in which the 

 shadow was projected on the concave part of the circle in opposite 

 directions ; and the mean of the interval between these observations 

 was accounted the time of the equinox. At this time we find enu- 

 merated five planets, viz., Qaivtav, QaiQuv, Hvpoeidri^ which appear 

 to indicate Jupiter, Saturn, and Mars ; and to which were added 

 Venus and Mercury. 



Magnitude of Eratosthenes not only taught the spherical figure of the earth, but 

 irth> attempted to ascertain its actual circumference, by measuring, as exactly 

 as could be done in his time, the length of a certain terrestrial arc, and 

 then finding the astronomical arc in degrees intercepted between the 

 zeniths of the two places. The segment of the meridian which he 

 fixed upon for this purpose, was that between Alexandria and Syene. 

 The measured distance of which was found to be 5,000 stadia. The 

 angle of the shadow upon the scaphia, which was observed at Alex- 

 andria, was equal to the fiftieth part of the circle ; and at Syene there 

 was no shadow from this gnomon at noonday of the summer solstice. 

 That this last observation might be the more accurately taken, they 

 dug a deep well, which, being perpendicular, was completely illumi- 

 nated at the bottom when the sun was on the meridian. The exact 

 quantity which this philosopher assigned to the circumference of the 

 earth is not known ; at least, different opinions have been advanced : 



