ERATO'STHENES, a distinguished contemporary of Archimedes, is said to have been born at Cyrene in the year n.c. 276. 1Ie possessed a variety of talents seldom united in the same individual, but not all in the same eminent degree; his mathematical, astronomical, and geo graphical labours, aro those which have rescued his name from oblivion.
The Alexandrian school of sciences, which flourished under the first Ptolemies, had already produced Timochares and Aristyllus, whose solstitial observations, made probably by the shadows of a gnomon, and by the armillary circles imitative of those of the celestial vault, retained considerable credit for centuries afterwards, though from these methods of observation they must have been extremely rude and Imperfect.
Eratosthenes had not only the advantage, arising from the instru ments and observations of his predecessors, but the great Alexandrian library, which probably contained all the Phoenician, Chaldaic, Egyptian, and Greek learning of the time, was entrusted to his superintendence by the third Ptolemy (Euergetes), who invited him to Alexandria; and we have proof in the scattered fragments which remain to ua of this great man, that these advantages were duly cultivated to his own happiness and the progress of infant astronomy.
The only work attributed to Eratosthenes, which has come down to us entire, is entitled 'Catasterismi,' and is merely a catalogue of the names of forty-four constellations, and the situations in each oonstella tion of the principal stars, of which he enumerates nearly five hundred, but without one reference to astronomical measurement : we find Hipparchus quoted in it, and mention made of themotion of the pole, that of the polar star having been recognised by Pytheas. These circumstances, taken in conjunction with the vagueness of the descriptions, render its genuineness extremely doubtful; at all events, It is a work of little value. It may be seen in the Oxford edition of • Arians,' and was republished by Shaubach, with notes by ileyne (Oott., 1795). A more correct edition of the text was published by F. K. Slatthise, in his edition of Aratns' (Frankfurt, 1817, Svo); and
by A. W'eshrmaan, in his 'Scriptores Hietorim Poetics() °mei,' pp. 239.267.
If Eratosthenes be really the author of the treatise 'Catssterismi,' It must have been composed merely as a 'vade mecum,' for we find him engaged in astronomical researches far more exact and more worthy of his genius. By his observations ho determined that the distance between the tropics, that is, twice the obliquity of the ecliptic, was a of an entire circumference, or 47° 42' 39", which makes the obliquity to be 23° 51' 1D•5", nearly tho same as that supposed by Ilipparchus and Ptoleminus. As the means of observation were at that time very imperfect, the instrumeuts divided only to intervals of 10', and corrections for the greater refraction at the winter solstice, for the diameter of the solar disc, &c., then unknown, we must regard this conclusion as highly creditable to Eratosthenes.
His next achievement was to measure the circumference of the earth. He knew that at Syene (now Assouan) the sun was vertical at -noon in the summer solstice; while at Alexandria, at the same moment, it was below the zenith by the fiftieth part of a circumference : the two places are nearly on the same meridian (error 2°); neglecting the solar parallax, be concluded that the distance from Alexandria to Syene is the fiftieth part of the circumference of the earth; this distance he estimated at 5000 stadia, which gives 250,000 stadia for the circum ference : the following diagram will explain the principle of this admeasurement C the centre of the earth, A Alexandria, s Syene, S the sun,
Thus Eratosthenes has the merit of pointing out a method for finding the circumference of the earth : but his data were not suffi ciently exact, nor had he the means of measuring the distance As with sufficient precision.