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, observed the 1877 opposition with a 22-centimetre Merz refractor and a micrometer. His aim was to measure the positions of surface features precisely enough to build a coordinate system and a map. 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, natural or artificial. The usage was not new. 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, meaning no more by it than an English geographer means by the Channel. Translated into English as "canals" — the Suez Canal had opened in 1869 — the word meant something dug on purpose. Schiaparelli made no such claim. He wrote that the lines might well be natural, and declined for decades to say who or what had made them.
Between 1879 and 1882 he reported gemination: canals that doubled, running as clean parallel pairs where one line had been. A real trench does not split in two between oppositions; doubling is a strong sign that the eye is generating a pattern rather than recording it. Schiaparelli thought the doubling would change opinions on the planet's physical constitution.
He was working at the resolution limit of his telescope, on a small, dazzling disc seen 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. His eyesight failed in the early 1890s and he gave up planetary observing.