Ptolemy as imagined in an old drawing, with a quadrant and the muse Astronomia.

Was Ptolemy the greatest astronomer of antiquity or the most successful fraud in the history of science?

His observations match his tables in an uncanny way; agreement between theory and observation are too good to be true. Seemingly Ptolemy fabricated his stated observations.

Agreement between his numerical parameters and modern values is, however, too close to be fortuitous. Probably Ptolemy had a large number of observations, and errors largely cancelled each other out in calculations of parameters. Next, Ptolemy might have selected from among his observations a few in best agreement with the theory, and then presented these examples to illustrate the theory.

Ptolemy lacked our modern understanding of error ranges, standard deviations, and the use of mean values from repeated observations - concepts that would have enabled him to propound a theory not necessarily in total and absolute agreement with every data point obtained, but in a less strict agreement with all the data points to within a statistically defined interval around a mean value. Instead, absent any tolerable fluctuation in the agreement between theory and observation, every measurement would be understood as an exact result. Consequently, judicious selection from among many measurements was required.

We also should remember that the Almagest is not a modern research paper, but was a textbook. Ptolemy's intent was to present procedures and sample data, showing future astronomers how it was done and enabling them to introduce their own observations over a longer temporal baseline in order to derive even better parameters.

Any fudging or fabrication by Ptolemy may have been understood by him as little fibs allowable in the neatening up of his pedagogy, not as lies intended to mislead his readers about crucial matters. The Greek astronomical tradition was far more concerned with general geometrical procedures than with specific numerical results. Modern norms of science did not yet exist.