During the intervening period the precession of the equinoxes meant that some of Hipparchos's observations needed updating, and a major complaint against Ptolemy was that he may have used and miscorrected them for precession, rather than make new observations of his own. Certainly he made an error in the length of the seasonal year, and Tycho Brahe pointed out a consistent error in the longitude of stars given in Ptolemy's catalogue. More than a century later, the renowned French astronomer Delambre made pointed criticisms of Ptolemy's work, saying that although the errors might have come in a complicated way from original data by Hipparchos, "One could explain everything in a less favourable but all the simpler manner by denying Ptolemy the observation of the stars and equinoxes, and by claiming that he assimilated everything from Hipparchos, using the minimal value of the latter for the precession motion."
The most pungent criticism came in the 20th century from Robert Russell Newton, who in 1977 published The Crime of Claudius Ptolemy, in which he states: "[Ptolemy] developed certain astronomical theories and discovered that they were not consistent with observation. Instead of abandoning the theories, he deliberately fabricated observations from [them]." Certainly the work of Tycho Brahe and Delambre shows that the errors were not random, though Newton's claim of a crime may be unfairly using modern notions of scientific fidelity to judge the ancient world. Newton's trenchant criticisms, implying that Ptolemy's errors had a terrible effect on scientific progress, have caused some scholars to row back in his defence, but the main point must be that in true scientific inquiry the observations come first, the theory later.
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