Glossary of Astronomical Terms/canals

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Canals

For over 100 years, observers saw a series of lines on the surface of Mars. In 1877 the Italian astronomer Schiparelli described them as "channels", canali in Italian. This was imperfectly translated as "canals" into English.

In the late 1800s, American astronomer Percival Lowell popularized the concept that the canals of Mars were a means by which a race of intelligent Martians irrigated the dry, dying planet with melted water from the polar ice caps:

[W]e saw how badly off for water Mars, to all appearance, is; so badly off that inhabitants of that other world would have to irrigate to live. As to the actual presence there of such folk, the broad physical characteristics of the planet express no opinion beyond the silence of consent, but they have something very vital to say about the conditions under which alone their life could be led. They show that these conditions must be such that in the Martian mind there would be one question perpetually paramount... How to procure water enough to support life would be the great communal problem of the day... If cultivation there be, it must be cultivation largely dependent upon a system of irrigation, and therefore much more systematic than any we have as yet been forced to adopt. — Percival Lowell, Mars (1895)(public domain)

This, in turn, led to the popular science fiction concept of Mars as inhabited. However, other astronomers, such as American Edward Barnard and France's Eugene Antoniadi, stated that in their observations of Mars, they could not see any evidence of canals. Lowell's response was: to observe canals required exceptionally still air and perseverance of the observer. Lowell continued, "It is interesting to recall, in connection with this incredulity about the canals, that precisely the same thing happened in the case of the discovery of Jupiter's satellites and with Huyghens' explanation of Saturn's ring." However, Barnard used a 32-inch refractor and Antoniadi the 33-inch Grand Lunette at the Paris observatory in remarkably clear air. Antoniadi's 1909 map of Mars does not show canals; it is very close in detail to the current map of Mars.

This concept was quelled finally in 1964 with the flyby of the U.S. space probe Mariner 4, which showed no canals. However, the probe, and subsequent probes, did show evidence that in the past, Mars had surface water which formed rivers and eroded martian terrain. Unfortunately for the romatic image of Mars many held dearly, there is no evidence of intelligent Martians to enjoy these rivers.

What causes observers to see the canals and green hectares of martian soil? The green is explainable as an optical illusion caused by orange soil adjacent to gray rock. The canals themselves are a natural phenomenon, but of the human mind, which tends to "connect the dots" and make complete patterns where incomplete patterns lay.