The Southern Cross or Crux can be observed from the south of 26°N, and it is always visible south of 26°S. On a flat Earth, it should be visible from the outer parts on Earth at the same time, but not from the central areas, disproving the flat model. The visibility of Crux is only consistent with the spherical Earth model.
Some flat-Earthers claim that if Earth is a sphere, then Crux should be visible from every longitude in the southern hemisphere. In reality, it depends on the latitude. Crux is circumpolar –visible all night, every night– south of 26°S. Only people in southern ends of Australia, New Zealand, southern parts of Chile & Argentina, and the southern tip of South Africa can observe Crux all night, every night.
References
- Determining the Visibility of a Star From Its Declination and the Observer’s Latitude – FlatEarth.ws
- Crux – Wikipedia