The Age of Exploration: Chapter 3, Exercise 2 — Possible Solution
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THE ERROR
Columbus believed the Earth was about 75 percent of its true diameter (75
percent of Eratosthenes's calculation), so he estimated Asia was only about
2,400 nautical miles west of the Canaries, roughly a quarter of the true
distance.

WHY THE PLAN LOOKED MORE FEASIBLE
A much shorter Atlantic made a westward voyage to Asia seem within reach of
the ships of the day.

CAUTION
The source does not say what his contemporaries thought of the estimate.