Percival Lowell staked his observatory on Martian canals — they were an optical illusion

Giovanni Schiaparelli described fine dark lines on Mars during the 1877 opposition, calling them 'canali' — channels. English translators wrote 'canals', a word that implies construction. Percival Lowell, a Boston Brahmin who had run a cotton mill and written books on Japan, read Camille Flammarion's La planète Mars in 1893, learned that Schiaparelli's eyesight was failing, and decided the canals were his project. He founded Lowell Observatory at Flagstaff, Arizona — the first observatory deliberately placed at high altitude for clear seeing — and pointed it at Mars.

Lowell's Mars was a dying planet whose advanced civilization built canals to carry water from the melting polar caps to the equatorial deserts. He published three books — Mars (1895), Mars and Its Canals (1906), Mars as the Abode of Life (1908) — and produced intricate maps of 'canals' and 'oases'. The maps sold newspapers and the books sold well. But his own staff could not see the markings he drew, and colleagues across Europe and America could not confirm them either; some even reported 'gemination', the mysterious doubling of the canals.

The disproofs were on the table while the theory was still fashionable. In 1903 Joseph Evans and Edward Maunder had schoolboys draw what they saw through small telescopes: point-like spots resolved into straight lines — the canals were an optical illusion of an eye joining dots. Alfred Russel Wallace's 1907 book Is Mars Habitable? showed the planet was far too cold and dry for flowing water. In 1909 Eugène Antoniadi, using the great 83 cm telescope at Meudon, saw no canals at all — the dark features were irregular patches. Photographs from Pic du Midi agreed.

Lowell never conceded. He died in 1916, and the observatory outlived the theory: his Planet X search ended with Clyde Tombaugh's discovery of Pluto in 1930 — a prediction that was itself wrong, corrected after Voyager 2 in 1989. Mariner 4's 1965 flyby photographed a cratered, barren Mars with a surface pressure of a few millibars and ended the canals debate outright. The canal myth now stands as the canonical example of expectation-driven observation — Carl Sagan himself listed 'a canal-infested Mars' among classic science-fiction devices built on earlier misapprehensions.