Why did Columbus think he had reached Asia in October 1492?

12 October 1492 : Columbus reaches the New World
After sailing across the Atlantic Ocean, Italian explorer Christopher Columbus sights a Bahamian island, believing he has reached East Asia. His expedition went ashore the same day and claimed the land for Isabella and Ferdinand of Spain, who sponsored his attempt to find a western ocean route to China, India, and the fabled gold and spice islands of Asia ..........................
After sailing across the Atlantic Ocean, Italian explorer Christopher Columbus sights a Bahamian island, believing he has reached East Asia. His expedition went ashore the same day and claimed the land for Isabella and Ferdinand of Spain, who sponsored his attempt to find a western ocean route to China, India, and the fabled gold and spice islands of Asia.