Claudius Ptolemy's Almagest, compiled in the second century, catalogued forty eight constellations, most of them drawn from Greek myth with older Babylonian antecedents behind several. For the better part of two thousand years after Ptolemy, that set grew as later astronomers added constellations to cover the southern sky he never saw and to fill gaps between his own figures, but no constellation had a fixed edge. Two stars near a shared border could be assigned to either neighboring figure depending on which astronomer's chart was consulted, a genuine ambiguity rather than a rounding error.
The International Astronomical Union closed that ambiguity in 1930, adopting a definitive set of eighty eight constellations with precisely drawn boundaries, the work chiefly of the Belgian astronomer Eugene Delporte, published as Delimitation Scientifique des Constellations. The reorganization had casualties as well as fixes: Ptolemy's enormous ship, Argo Navis, too large and unwieldy to survive as one modern figure, was broken into four, the keel Carina, the stern Puppis, the sails Vela and the compass Pyxis, none of which appears among the ancient forty eight under its own name. Perseus, standing nearby, had its own boundaries fixed in the same 1930 pass, so a date attached to one figure in that reorganization can as easily belong to its vanished neighbor as to itself.