Which 1877 observation of Mars produced a century of belief in artificial canals?

Schiaparelli's, at a close approach in 1877. He mapped linear features and called them canali, which in Italian means channels and carries no implication of engineering. Translated into English as canals, it acquired one.

Percival Lowell took it and ran. He built an observatory in Arizona, produced increasingly elaborate maps of a global irrigation network, and published books arguing that a dying civilisation was moving meltwater from the poles to its deserts - which entered popular culture so thoroughly that it shaped a century of Martian fiction, including Wells.

The lines were not there. Better telescopes failed to confirm them, and the explanation is a quirk of vision at the limit of resolution: the eye connects scattered indistinct features into straight lines. A 1903 experiment had schoolchildren draw a disc of blurred dots from a distance, and they drew canals. Mariner 4 photographed a cratered surface in 1965. Lowell's observatory is nonetheless where Pluto was found - his wrong prediction of a ninth planet funded the search that stumbled on one.