Crux is the smallest constellation in the modern sky, four stars forming a compact cross low over the southern horizon, and it is younger as an independent figure than most of the star patterns around it. Ptolemy catalogued its stars in the Almagest around 150 CE, but not as a cross: he placed them within Centaurus, reading them as part of the centaur's hind legs. For over a millennium after Ptolemy, nobody looked at this patch of sky and saw a cross, because for the astronomers of the Mediterranean world it had never been separated from the figure beside it.
The change came with the age of European ocean navigation. Sailors and travelers reaching far southern latitudes, where Crux climbs high enough to be strikingly visible, began describing the four stars as their own distinct shape from the early sixteenth century, the Italian traveler Andrea Corsali among the first to record it in writing in 1516. Celestial cartographers followed: the Dutch clergyman and astronomer Petrus Plancius and the German uranographer Johann Bayer both mapped Crux as an independent constellation around 1598 to 1603, formally detaching it from Centaurus for the first time in the history of Western astronomy.
Once separated, Crux acquired a cultural weight far beyond its size. It sits near the south celestial pole and has served navigators as a rough southern counterpart to Polaris ever since its identification, and it has become one of the most widely adopted national symbols in the sky, appearing on the flags of Australia, New Zealand, Papua New Guinea, Samoa and Brazil.