[SunkenCiv]

Jay W: The planet Vulcan is supposed to be in the same orbit as Earth, just directly opposite the sun, and thus perpetually invisible to us from Earth. Well, that's the so-called theory, or notion, at any rate! ;-)

Vulcan was supposed to be a planet orbiting inside the orbit of Mercury and perturbing it. There were sightings of sun-crossing asteroids (none of which were ever seen again, suggesting they were nowhere near Mercury, but were in fact near the Earth) attributed to Vulcan. The precession of Mercury was given a different origin by Einstein, and the search for Vulcan effectively ended.

The Nine Planets: Hypothetical Planets

[RightWhale]

Oppenheimer, Einstein's boss at Princeton, thought Einstein was a crackpot. Except for 1905, it might be so viewed. Maybe including 1905.