Ptolemy argued that if the earth were not at the center of the universe then it would surely be falling towards the center. Here he was following the teachings of Aristotle. Ptolemy argued that the earth was round by observing that when a ship sails toward shore, mountains may appear to rise from the sea, proving that they were hidden by the earth's curvature. Here he was following a long line of reasoning regarding the spherical shape of the earth.
Pythagoras and his followers argued that the earth was round circa 500 B.C. 350 years prior to Ptolemy, Eratosthenes accepted the roundness of the earth and devised a way to measure its circumference. With an ingenious and complicated experiment, Eratosthenes concluded that the circumference of the earth was 29.000 miles. Its actual circumference is 25,000 miles.
Ptolemy is in debt to another astronomer, as well. Hipparchus was born in what is now western Turkey and did most of his work on the island of Rhodes in the Aegean Sea. Many argue that Ptolemy's best work is simply a copy of Hipparchus. Hipparchus was the first to record the position of the stars; he claimed he did this so that he could tell time at night - another first.
'We shall try to note down everything which we think we have discovered up to the present time; we shall do this as concisely as possible and in a manner which can be followed by those who have already made some progress in the field. For the sake of completeness we shall set out everything useful for the theory of the heavens in the proper order, but to avoid undue length we shall merely recount what has been adequately established by the ancients. However, those topics which have not been dealt with at all by our predecessors, or not as useful as they might have been, will be discussed at length to the best of our ability.'
G. J. Toomer says about the Almagest:
"As a didactic work, the "Almagest" is a masterpiece of clarity and method, superior to any ancient scientific textbook and with few peers of any period. But it is much more than that. Far from being a mere "systemisation" of earlier Greek astronomy, as it is sometimes described, it is in many respects an original work."