What year was Mercury called a planet?

Answer 1

Since it is visible to the naked eye, Mercury was observed by the ancients, the first known reference by the Assyrians about 14th century BC. The term "planet" derives from a term which means wanderer, simply because it changes position in the sky (owing to its solar orbit) as opposed to the stars which appeared to have fixed positions. Mercury was thus always considered a planet, whether its true nature was fully known or not.