Among Schiaparelli's contributions are his telescopic observations of Mars. In his initial observations, he named the "seas" and "continents" of Mars. During the planet's "Great Opposition" of 1877, he observed a dense network of linear structures on the surface of Mars which he called "canali" in Italian, meaning "channels" but the term was mistranslated into English as "canals". While the term "canals" indicates an artificial construction, the term "channels" connotes that the observed features were natural configurations of the planetary surface. From the incorrect translation into the term "canals", various assumptions were made about life on Mars; as these assumptions were popularized, the "canals" of Mars became famous, giving rise to waves of hypotheses, speculation, and folklore about the possibility of intelligent life on Mars, the Martians. Among the most fervent supporters of the artificial-canal hypothesis was the American astronomer Percival Lowell, who spent much of his life trying to prove the existence of intelligent life on the red planet. After Lowell's death in 1916, astronomers developed a consensus against the canal hypothesis, but the popular concept of Martian canals excavated by intelligent Martians remained in the public mind for the first half of the 20th century, and inspired a corpus of works of classic science fiction. Later, with notable thanks to the observations of the Italian astronomer Vincenzo Cerulli, scientists came to the conclusion that the famous channels were actually mere optical illusions. The last popular speculations about canals were finally put to rest during the spaceflight era beginning in the 1960s, when visiting spacecraft such as Mariner 4 photographed the surface with much higher resolution than Earth-based telescopes, confirming that there are no structures resembling "canals". In his book Life on Mars, Schiaparelli wrote: "Rather than true channels in a form familiar to us, we must imagine depressions in the soil that are not very deep, extended in a straight direction for thousands of miles, over a width of 100, 200 kilometers and maybe more. I have already pointed out that, in the absence of rain on Mars, these channels are probably the main mechanism by which the water can spread on the dry surface of the planet."
An observer of objects in the Solar System, Schiaparelli worked on binary stars, discovered the large main-belt asteroid 69 Hesperia on 29 April 1861, and demonstrated that the meteor showers were associated with comets. He proved, for example, that the orbit of the Leonid meteor shower coincided with that of the comet Tempel-Tuttle. These observations led the astronomer to formulate the hypothesis, subsequently proved to be correct, that the meteor showers could be the trails of comets. He was also a keen observer of the inner planets Mercury and Venus. He made several drawings and determined their rotation periods. In 1965, it was shown that his and most other subsequent measurements of Mercury's period were incorrect. Schiaparelli was a scholar of the history of classical astronomy. He was the first to realize that the concentric spheres of Eudoxus of Cnidus and Callippus, unlike those used by many astronomers of later times, were not to be taken as material objects, but only as part of an algorithm similar to the modern Fourier series.
