Ptolemy's inheritance 

Around AD 150, in Alexandria, Claudius Ptolemy completed the work Arabic tradition would name the Almagest, the greatest. Thirteen books of mathematical astronomy, with a geometric model of the heavens built on circles and epicycles, tables for computing positions, and in books seven and eight a catalog of one thousand and twenty-two stars arranged in forty-eight constellations, each with ecliptic coordinates and a brightness grade.

It worked well enough to dominate. For roughly fourteen centuries this was the reference sky: translated into Arabic in the ninth century, corrected and extended by astronomers such as al-Sufi, whose Book of Fixed Stars added careful drawings and independent brightness estimates, passed into Latin in the twelfth century, and still the framework Copernicus argued against in 1543. Most of the constellation names in use today reach us through this line.

There is a long argument about where the numbers came from. Ptolemy states that he took precession into account, and his catalog's longitudes are consistent with an epoch around AD 137. But scholars from Tycho Brahe onward have noticed problems: the implied precession constant matches the value Hipparchus reported rather than a fresh measurement, and the errors in the positions show patterns suggesting many entries were converted from an earlier dataset rather than measured anew. In 1977 Robert Newton pressed this into an accusation of fraud. Others, including Owen Gingerich and Dennis Rawlins, argued for a more ordinary explanation: heavy reliance on Hipparchus, honest use of a flawed precession value, and observational work that was real but partial.

The scholarly consensus today is cautious rather than dramatic. Ptolemy's catalog is a composite. Some of it he measured, much of it he inherited and adjusted, and he did not always make the distinction visible in the way a modern paper would demand. That is a failure of citation, not necessarily of honesty, and it is worth remembering that the norms he is being judged against were invented long after him.

For Hipparchus, the consequence was double. His work survived, embedded in a book that everybody used, which is why we know his results at all. It also became invisible, absorbed into someone else's authority, which is why the extent of his contribution has been arguable for centuries. Being cited badly is a better fate than being lost, but only just.

That left historians with an uncomfortable position. The most important star catalog of antiquity was known almost entirely through a later author's version of it, and any direct trace of the original had apparently vanished.

Then, in the twenty-first century, a page of reused parchment turned out to have been hiding something underneath.