I might be wrong, but I think I read somewhere that Colombus did know how large the earth was, but that he thought the distance from Europe to China was twice as long as it really is, because Marco Polo supposedly should have miscalculated it (of course Marco Polo did not follow a straight line, but had to follow the terrain, which might be part of the explanation of a miscalculation).
No, he made BOTH errors.
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See also
Effects of No Columbus: Counterfactual
kasumigenx
Columbus' Ships Sink, When is America discovered?
Anekcahdep
WI Columbus never returned to Spain? (Multi-page thread 1 2)
phildup
WI: no Support for Columbus
Sian
Spain denied Columbus?
MrNumbahOne
WI: Columbus doesn't make it back (Multi-page thread 1 2 3 4)
AllActionMan
1492 Columbus sank in the ocean blue
docfl
Failed Columbus - Effects on Europe?
Emperor-of-New-Zealand
Immediate Effects of Failed Columbus? (Multi-page thread 1 2)
Beedok
WI: Columbus never arrived America?
Chörnyj Orel
WI: Columbus falls off the map
Amedras
WI: Columbus Failed? (Multi-page thread 1 2 3)
TMS2224
WI - 1492, Indians Discover Columbus Lost At Sea.
Forever_found
WI Columbus Failed? (Multi-page thread 1 2)
OneUp
Columbus Never Returns
Codae
Christopher Columbus lost at sea
Niceguy
Columbus turns back
Peter Cowan
WI Hurricane maxima at time of Columbus
NapoleonXIV
The ships of Columbus mutiny! (Multi-page thread 1 2)
Darkest
WI Columbus Fails
Fearless Leader