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Why did Portuguese navigators name Rio de Janeiro in 1502?

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Aerial view of Rio de Janeiro, Brazil, showing the city's buildings, Sugarloaf Mountain, Guanabara Bay with boats, and surrounding green hills. — Rio de Janeiro Rio de Janeiro, Brazil.

Portuguese navigators named the site Rio de Janeiro in 1502 because they arrived at the entrance of the bay on January 1 of that year and mistook it for the mouth of a river. In Portuguese, rio means "river" and janeiro means "January".1