POSIDONIUS, a celebrated Stoic philosopher of Apameia in Syria, was the disciple and successor of Panætius, (Strab. xiv. 655.) The exact date of his birth and death are unknown, but he flourished B. C. 78, and came to Rome B. C. 51. (Suidas.) He resided principally at the island of Rhodes, and his reputation was so high that many of the young Roman nobility attended his lectures. He was visited by Cicero, B. C. 78, (Plut. Cic. 4. Cic. N. D. 1, 3), and by Pompey, B. C. 62. (Plin. vii. and Plut. Pomp. 42.) Posidonius went to Rome on an embassy respecting the affairs of the Rhodians, (Plut. Mar. 45), and probably at this time visited Spain, (Strab. xiii. 614), Gaul, (iv. 197), and Liguria, (iii. 165). He lived to the age of eighty-four, (Lucian, Macrob. 20), and was succeeded by his grandson Jason, (Suid.) Cicero (Div. i. 3), alludes to a work of his on divination, in five books; but he was not merely a philosopher, he was also an astronomer and geographer, who is frequently quoted by Strabo. He made an attempt to calculate the circumference of the earth, and also the diameter of the sun. (Strab. ii. 95, 138.) He treated at considerable length the phenomena of tides, and observed that they had diurnal, monthly, and annual periods, like the moon, (138, 174.) The scattered fragments of Posidonius have been collected and published, under the title, Posidonii Rhodii reliquiae doctrinae, collegit atque illustravit J. B. Baker accedit Vrytlembaechii annotatio, 1810.