When was mercury first discovered as a planet?

It has been known since ancient times.

Naked eye planets tend to have that ability.

Probably tens of thousands of years ago when the first ancestors to humans looked up, and noticed a "star" close to the sun that did not stay fixed like the others.

The Mariner 10 spacecraft passed Mercury twice in 1974 and once in 1975. It sent back pictures of a moonlike, crater-pocked surface and reported temperatures to be about 430°C (about 810°F) on the sunlit side and about -180°C (about -290°F) on the dark side. Although Mercury’s surface appears very similar to the surface of the Moon, there are some significant differences. The smooth, lava-like plains on Mercury, for example, are not as dark as the smooth plains (maria) of the Moon. Also unlike the surface of the Moon, the surface of Mercury is crisscrossed by long escarpments, or cliffs, indicating a period of contraction as the planet cooled early in its history.

Jondalar the caveman was the first to notice the planet Mercury in 24946 BC. He was a member of the Zelandonii tribe which lived in what is today western France. His history survives to the present in fictionalized form as Jean M. Auel's "Earth's Children" series. A considerable number of astronomical discoveries is attributed to Jondalar, by the way. He must have been a very curious and observant fellow.

Recent research has revealed that Jondalar was a phony. A research assistant actually discovered Mercury while looking for better ways to hit birds with rocks, and Jondalar took credit for it.

http://www.universetoday.com/guide-to-space/mercur...

google.com, great place for answers.