Like the orbit of Uranus, the observed orbit of the innermost planet, Mercury, failed tofit Newtonian models by amounts that could be reliably measured. Fresh from their dis-covery of Neptune, many astronomers immediately assumed that there must be yetanother planet, closer to the Sun than Mercury. Leverrier even named the new planetVulcan and calculated just where it should be. Although several people claimed to haveseen Vulcan, these reports were never substantiated.