Martian canals

Early observers noticed first "seas" and later "canals" on the surface of Mars, starting with the discovery of Syrtis Major by Huygens.

These turned out to be optical illusions, from several sources. Some were caused by the telescopes, where flaws caused high contrast areas to have faint doubling edges. More importantly, the human eye and brain have evolved to detect edges, and random blobs at the edge of discernible vision, will tend to be connected by lines by the human nervous system.

While illusory, the canals remain visible in telescopes.

History

Giovanni Schiaparelli first observed straight lines on Mars and called them 'canali' which is channels in Italian. He thought that they might be water ways. However, this term was mis-translated into 'canals' in English. As telescopes improved, it became clear that Mars changed color, sometimes dark, and sometimes brighter. (This is caused by dust storms and the movement of dust.) Unsurprisingly, people thought it might be vegetation growing and dying out with the Martian seasons. Some astronomers saw lines which they thought were canals, and others did not see them.

The American astronomer Percival Lowell took things a step further. He thought that the canals were large engineering projects to move polar water into desert regions. He built an observatory in Flagstaff Arizona and spent many hours observing Mars. He wrote a popular book, promoting his ideas, and drew very complex maps showing dozens of canals.

As telescopes improved, by 1903 more and more people thought that there were no canals, but Lowell remained convinced until his death in 1916.

It became clear that the dozens of canals were not there, but the possibility of some channel like features remained until the Mariner 4 fly by in 1965, which proved they didn't exist. However the Mariner 9 spacecraft orbited Mars and found river valleys, giving strong evidence that Mars at one time had running water.