Babylonians
1995 i believe.
2013
Why would it be hard to discover a planet that is closer to the sun than mercury
josh wickam
1623
1230
BAROMETER ---- It's actually a mercury barometer
http://www.dkimages.com/discover/previews/829/35080442.JPG
The Mariner spacecraft did.
The planet Mercury is known from approx. 4000 years; observations were made only with eyes.
When they were looking for the first crescent moon to know when the next lunar month began.