jan 1, 1859 - Pseudo-Planet (1859) Urbain Le Verrier observed the strange orbit of Mercury. He proposed that a planet called "Vulcan" must exist, and would account for the oddities of Mercury's orbit. No such planet would ever be observed, but rather Einstein's theories would later explain the observations.
Added to timeline:
Date:
jan 1, 1859
Now
~ 166 years ago