Schiaparelli draws the canali
Schiaparelli mapped Mars through the 1877 opposition and crossed it with fine dark lines he called canali — channels. English made them canals, and the word did the rest.
Giovanni Schiaparelli, director of the Brera Observatory in Milan, went after the 1877 opposition with a 22-centimetre Merz refractor and a micrometer. His goal was unglamorous and useful: measure the positions of Martian surface features precisely enough to build a proper coordinate system and a proper map. He did exactly that, and the nomenclature he invented — Syrtis Major, Hellas, Elysium, Solis Lacus, Nix Olympica — is still on the charts the Mars Reconnaissance Orbiter team uses. Nix Olympica turned out to be the largest volcano in the solar system.
He also drew dozens of thin, straight lines crossing the bright orange areas and labelled them canali — Italian for channels or grooves, the word you would use for a natural watercourse. It was not even a new usage. Angelo Secchi, making the first serious colour drawings of Mars from the Collegio Romano in 1858, had labelled the broad dark wedge now called Syrtis Major Canale Atlantico, and meant nothing more by it than an English geographer means by the Channel. Translated into English as "canals" — in 1877, eight years after the Suez Canal opened — it meant something dug on purpose. Schiaparelli did not make that claim. He wrote that the lines might well be natural, and he declined for decades to say who or what had made them, which is not the same as saying nobody had.
Then it got worse. Between 1879 and 1882 he reported gemination: canals that doubled, running as clean parallel pairs where one line had been. That is a strong signal that the eye is generating the pattern rather than recording it — a real trench does not split in two between oppositions — but at the time it read as the network becoming better resolved.
The observations themselves were honest and careful. The trouble sat in the chain between the eyepiece and the conclusion. Schiaparelli was working at the resolution limit of his telescope on a small, dazzling disc through a churning atmosphere, sketching from memory between moments of steady seeing. His Merz had overcorrected chromatic aberration, and he was reportedly red-green colour-blind, which may have sharpened his sense of contrast on a rust-coloured planet. None of that is fraud. It is what it looks like when a good instrument is pushed past what it can actually deliver.
Schiaparelli's eyesight failed in the early 1890s and he gave up planetary observing. By then the canals had left Milan entirely. What he had drawn as tentative markings on a working map was, in English, already a piece of engineering built by somebody.