It is curious that Posidonius (135–51 B.C.), who was much less of an astronomer, made a much better guess at the distance of the sun from the earth. He made it 500,000,000 stades. As he also estimated the circumference of the earth at 240,000 stades, we may take the diameter of the earth to be, according to Posidonius, about 76,400 stades; consequently, if _D_ be that diameter, Posidonius made the distance of the sun to be equal to 6545_D_ as compared with Hipparchus’s 1245_D_.
Ptolemy does not mention Hipparchus’s figures. His own estimate of the sun’s distance was 605_D_, so that Hipparchus was far nearer the truth. But Hipparchus’s estimate remained unknown and Ptolemy’s held the field for many centuries; even Copernicus only made the distance of the sun 750 times the earth’s diameter, and it was not till 1671–3 that a substantial improvement was made; observations of Mars carried out in those years by Richer enabled Cassini to conclude that the sun’s parallax was about 9·5″ corresponding to a distance between the sun and the earth of 87,000,000 miles.
Ptolemy made the distance of the moon from the earth to be 29½ times the earth’s diameter, and the diameter of the earth to be 3⅖ times that of the moon. He estimated the diameter of the sun at 18⅘ times that of the moon and therefore about 5½ times that of the earth, a figure again much inferior to that given by Hipparchus.
On the astronomy of the early Greek philosophers much information is given by Aristotle (especially in the _De Caelo_); for Aristotle was fortunately in the habit of stating the views of earlier thinkers as a preliminary to enunciating his own. Apart from what we learn from Aristotle, we are mainly dependent on the fragmentary accounts of the opinions of philosophers which were collected in the _Doxographi Graeci_ of Hermann Diels (Berlin, 1879), to which must be added, for the period before Socrates, _Die Fragmente der Vorsokratiker_ by the same editor (2nd edition, with index, 1906–10, 3rd edition, 1912). The doxographic data and the fragments for the period from Thales to Empedocles were translated and explained by Paul Tannery in _Pour l’histoire de la Science Hellène_ (Paris, 1887). A History of Astronomy was written by Eudemus of Rhodes, a pupil of Aristotle; this is lost, but a fair number of fragments are preserved by later writers. Of the theory of concentric spheres we have a short account in Aristotle’s _Metaphysics_, but a more elaborate and detailed description is contained in Simplicius’s commentary on the _De Caelo_; Simplicius quotes largely from Sosigenes the Peripatetic (second century A.D.) who drew upon Eudemus.