I was today years old when I learned that amateur astronomer Percival Lowell, now infamous because of his spurious observations of an elaborate network of canals on Mars, did the same thing to Venus—which is a clear indicator that his "canals" were entoptic phenomena, i.e. Lowell was glimpsing the network of blood vessels in his eye's retina (the "Purkinje tree") and drawing that onto the disc of both Mars and Venus.
(btw, "aphroditographic work" is how astronomer Eugenios Antoniadi described the business of observing and drawing Venus, in a criticism of Lowell. "aphroditographic". great, huh?)
what's particularly upsetting about the story, though, is that Lowell roped in other true believers: the staff of his Flagstaff observatory (which was, apparently, badly placed for astronomical observation, prone to atmospheric turbulence) would see the same canals or spokes that he did, even though nobody else could. one of those assistants, however, a man named Andrew Douglass, conducted an experiment in which pointed Lowell's telescope at spheres placed a long distance away—and he observed the same "canals". when Douglass made the mistake of confessing his doubts about Lowell's observations to Lowell's brother-in-law, Lowell sacked him. yay!
well, at least now we know roughly how Elon Musk must be running his "Starship" operations....
~Chara