Mars and Its Canals
In the days before telescope photography, astronomers had to draw what they thought they saw through the eyepiece throughout the long dark nights. Sometimes they saw saw more than there really was to see, and a bit over 100 years ago Percival Lowell published books on what he was sure were canals on Mars, signs of intelligent civilization. (In case you too are skeptical, we also have at Librivox a criticism of Lowell's theories in a book published a year later (in 1907) by Alfred Russel Wallace.) - Summary by ToddHW