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Educated people even in ancient times knew the earth was round and even roughly how large.

By the time of Columbus I suspect everyone who had even the slightest education knew it.

Yes, I’ve seen links to that effect. And after all, it’s not difficult to visualize how to estimate the radius of the earth. One need only travel a measured distance north or south of a given location, and measure the elevation of Polaris at both locations. This would be more accurate at high latitudes and at greater differences in latitudes, but it would certainly give you a good ballpark figure. It could also be done by measuring the noontime elevation of the sun at two different low latitudes on the same day.
With instruments then available it would have been easy to estimate that sailing west from Spain to China would require a voyage of over 10,000 miles. Far beyond the range of the Nina, Pinta, and Santa Maria without resupply. Therefore if Columbus had been correct in his assumption that there was no land west of Spain until you got to China, he would never have been able to make the voyage. In that light it’s easy to see why the crew thought Columbus was nuts - he was.

One need only travel a measured distance north or south of a given location, and measure the elevation of Polaris at both locations.

Born and raised in Indianaplis.

First trip to southern Florida left me quite disoriented at night; due to the star locations being shifted.

I was ready for it when I visited cancun.

With instruments then available it would have been easy to estimate that sailing west from Spain to China would require a voyage of over 10,000 miles. Far beyond the range of the Nina, Pinta, and Santa Maria without resupply. Therefore if Columbus had been correct in his assumption that there was no land west of Spain until you got to China, he would never have been able to make the voyage. In that light it’s easy to see why the crew thought Columbus was nuts - he was.

Not necessarily so. At that time, they did not know exactly where China was when it came to longitude. It was not until the 18th century and the invention of the Marine Chronometer that sailors could tell how far east or west they were.

Columbus did not underestimate the size of the Earth. He mis estimated how far west China was from Europe.