Ptolemy elected to use the circumference of the earth proposed by Poseidonius rather than the accurate size determined by Eratosthenes (fl. 240-200 B.C.).

Eratosthenes calculated it to be (in modern equivalents) 25,000 miles, which is very close to its actual circumference of 24,902 miles. Like Ptolemy, Eratosthenes had been head of the library at Alexandria. He had used ingenious computations to make his measurements of the earth, and devised a system of parallels and meridians for locating places on a map. The later astronomer Poseidonius had calculated the circumference to be approximately 18,000 miles, about three-fourths its true measure. This smaller figure was adopted by Ptolemy.

Hipparchus divided the equator into 360 degrees, a figure accepted by Ptolemy, whose world map covered the half of the world that he knew. He noted that much of the information he had received from travelers and writings was unverified. This made it impossible to map the world accurately, and Ptolemy admitted it. As Peter Whitfield put it: "Ptolemy’s aims and methods were rigorous and scientific, but his materials were not equal to the demands he placed on them" (Whitfield 1998, 11).

In his thinking Ptolemy adopted the geocentric universe propounded by Aristotle (384-322 B.C.) and others, rather than the heliocentric universe of Aristarchus of Samos (d. 230 B.C.). This is curious, in light of his professed thesis of adopting the simplest explanation possible for any phenomenon. It is tempting to wonder why he did this or what difference it would have made to the history of astronomy if he had followed Aristarchus.