Is there a good explanation for the observation of Martian canals? Martian "canals" have been observed by independent observers after their first description. Now, they are attributed to "optical illusion", but I think that this is not a good choice of word, because an optical illusion should be visible today as well. It would have to be a psychological effect, but it is rather astonishing to have people draw whole maps of non-existent canal systems.
Is there a good explanation (optical or psychological) for the observation of Martian canals?
 A: The human brain likes to find patterns in what it observes. That's why we see patterns in the stars which we call "constellations." The canals of Mars are similar. For most of them, there is something there, just at the edge of vision, and the human tendency is to "connect the dots" and see lines were only vague streaks exist.
It's worth noting that some experienced observers, notably E. M. Antoniadi, were never able to see anything resembling canals. There's some thought that Percival Lowell, the main proponent of canals, may in fact have had some defect in his vision, because he saw canals on Venus as well, which most observers see as a perfectly plain white ball.
I've observed Mars for decades visually with excellent telescopes, and have never ever seen anything even vaguely resembling a canal.
A: The Wikipedia article on Martian Canals is very good.
The existence of canals in the "martian made" sense, instead of the natural sense, was primarily due to wishful thinking on the part of Percival Lowell.  The original intention based on observations made in 1877 was probably natural channels.  Quoting a snippet from the Wikipedia article:

The Italian word canale (plural
  canali) can mean "canals" (including
  artificial canals or ducts) or
  "channels" or "gullies". The first
  person to use the word canale in
  connection with Mars was Angelo Secchi
  in 1858, although he did not see any
  straight lines and applied the term to
  large features —for example, he used
  the name "Atlantic Canale" for what
  Blockquote
  later came to be called Syrtis Major
  Planum.
It is often stated that Schiaparelli
  intended the meaning "channels" and
  that "canals" was a misunderstanding
  or mistranslation into English.
  Nevertheless, the English term
  "canals" was used from the very
  earliest accounts in English, and as
  far as is known, Schiaparelli made no
  effort to correct the supposed
  misunderstanding if he was aware of
  it. As the word "canal" can also mean
  "channel" or "watercourse" – not
  necessarily artificial – in English,
  the charge of "mistranslation" is
  unwarranted.

Lowell was practically obsessed with the idea of there being life on Mars and strongly supported the idea of canals.  He was also rich and could afford to build his own observatory and promote the idea.  It wasn't right, but he believed (or wanted to believe) it was. I can't find the reference, but I've seen side-by-side comparisons of images of Mars taken at the same time that Lowell made his canal maps and the maps that Lowell drew show no relation to the markings on the images.
As for an optical explanation another quote from the Wikipedia article:

William Kenneth Hartmann, a Mars
  imaging scientist from the 1960s to
  the 2000s, explains the "canals" as
  streaks of dust caused by wind on the
  leeward side of mountains and craters.

Remember that these "canal" observations were being made with relatively small, ground-based telescopes.  The resolution wasn't that great and many observers never saw "canals" at all.  And everyone that did, made different maps which could be explained by the shifting dust blown by the Martian wind from year to year.
