Final answer:
Christopher Columbus embarked on his journey to eventually find an alternate route to the East. The most probable astronomical reason for his sailing was to prove the true circumference of the Earth, which he greatly underestimated. The calculations of Earth's circumference by Eratosthenes already existed, but were not widely accepted.
Step-by-step explanation:
The unprecedented journey of Christopher Columbus was primarily embarked upon to find an alternate trading route to the East. The most probable astronomical reason behind his voyage is to prove what the true circumference of the earth was. Since it was widely known that the Earth was round, Columbus's idea of sailing west to reach the East seemed plausible. However, Columbus greatly underestimated the size of the Earth and had no way of accurately judging where he had landed with respect to his destination. Despite knowing the nearly accurate circumference as calculated by Eratosthenes in the second century BCE, most scholars did not find these calculations dependable.
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