/u/Schwartz210

This all already covered in several posts here and in the FAQ. That the circumference of the earth was around 40,000 km was already well known in his lifetime. He instead chose to cite works that said it was 30,000 km. All through his life he insisted that India or China or Cathay was just around the corner.

You can say that Ptolemy's error was understandable given the tools he had to work with. But you can't say Columbus' error was understandable because he cited very old work in the face of contemporary evidence.