The Pythagoreans introduced the idea of a spherical Earth, but not based on scientific measurement. As worshippers of numbers and geometry, the Pythagoreans insisted that the Earth must take on the most perfect shape: the sphere. The idea of a flat or cylindrical earth persisted through the 5th century BCE, but the argument remained focused on questions of philosophy and theology, not on scientific observation. Plato surveys this range of views, and Garwood notes: “by the time [Plato’s] pupil Aristotle was writing, later in the fourth century BC, the globe concept seems to have become widely accepted among educated people.”
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By the time of the Scientific Revolution and the Enlightenment, there was no educated dispute about the shape of the earth, though estimates of its size still varied. Columbus’ novelty didn’t rest in advancing a spherical earth, but in proposing a size for it that was far too small; this strategic misestimation was necessary to justify his belief that well-provisioned ships could reach the Indies by sailing west.
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I think we can safely regard the flat-Earth cosmologies of the ancient Middle East as pre-scientific, and not reflecting a scientific consensus of the sort that [is proposed today]. The debates over the shape of the Earth that prevailed through the age of the Pythagoreans were also not primarily scientific. Indeed, it’d be about 2000 years before anything we’d recognize as science and true scientific discourse really became established in the West. Aristotle, Thales, and other Ancient Greeks set the stage for science and established some critical early results, but the notion of scientific consensus—a shared vision among a community of scholars united through a process of peer review and valuing evidence-driven, repeatable, testable hypotheses—would have to wait for a few millennia. In summary, then, there was probably never a scientific consensus that the Earth was flat, and that idea was first overturned not by science, but by a different philosophical system. Any lessons for modern climate science in that history 2500 years ago are hard to identify.