108 
ERATOSTHENES 
out of 83 contained in the whole meridian circle’. 1 The 
mean of Ptolemy’s estimates, 4 7° 42' 30", is of course nearly 
ll/83rds of 360°. It is consistent with Ptolemy’s language 
to suppose that Eratosthenes adhered to the value of the 
obliquity of the ecliptic discovered before Euclid’s time, 
namely 24°, and Hipparchus does, in his extant Covimentary 
on the Phaenomena of Aratus and Eudoxus, say that the 
summer tropic is ‘ very nearly 24° north of the equator ’. 
The Doxographi state that Eratosthenes estimated the 
distance of the moon from the earth at 780,000 stades and 
the distance of the sun from the earth at 804,000,000 stades 
(the versions of Stobaeus and Joannes Lydus admit 4,080,000 
as an alternative for the latter figure, but this obviously 
cannot be right). Macrobius 2 says that Eratosthenes made 
the ‘measure’ of the sun to be 27 times that of the earth. 
It is not certain whether measure means ‘ solid content ’ or 
‘ diameter ’ in this case ; the other figures on record make the 
former more probable, in which case the diameter of the sun 
would be three times that of the earth. Macrobius also tells 
us that Eratosthenes’s estimates of the distances of the sun 
and moon were obtained by means of lunar eclipses. 
Another observation by Eratosthenes, namely that at Syene 
(which is under the summer tropic) and throughout a circle 
round it with a radius of 300 stades the upright gnomon 
throws no shadow at noon, was afterwards made use of by 
Posidonius in his calculation of the size of the sun. Assuming 
that the circle in which the sun apparently moves round the 
earth is 10,000 times the size of a circular section of the earth 
through its centre, and combining with this hypothesis the 
datum just mentioned, Posidonius arrived at 3,000,000 stades 
as the diameter of the sun. 
Eratosthenes wrote a poem called Hermes containing a good 
•deal of descriptive astronomy; only fragments of this have 
survived. The work Catasterismi (literally ‘ placings among 
the stars ’) which is extant can hardly be genuine in the form 
in which it has reached us ; it goes back, however, to a genuine 
work by Eratosthenes which apparently bore the same name ; 
alternatively it is alluded to as KaTdXoyoL or by the general 
1 Ptolemy, Syntaxis, i. 12, pp. 67. 22-68. 6. 
2 Macrobius, In Sonin. Scip. i. 20. 9.