The Planet Mars: A History of Observation and Discovery by William Sheehan

Author:William Sheehan [Sheehan, William]
Language: eng
Format: epub
Publisher: The University of Arizona Press
Published: 0101-01-01T00:00:00+00:00

We now know from Earth-based charge-coupled device (CCD) images (see chapter 15) and spacecraft photographs that Antoniadi was more or less right and Lowell was wrong. There are no genuine canals on Mars. Schiaparelli, Lowell, and countless others were victims of the "Grand Illusion": under certain conditions of observation, the really complex Martian details appear as a r�seau of fine lines.

Ultimately, the solution to the canal mystery belonged more to the realm of perceptual psychology than to astronomy. The canals were seen by glimpses; they were fragmentary perceptions. To quote Antoniadi, who had seen many of them, as both single and double lines, at Juvisy: "A glimpsed object is not as certain as an object held steadily, and, however self-evident or trite such a remark may be, yet it is a very important one to make."39 The periods when a large wavefront of uniform refractive index passes across the telescope aperture typically last on the order of a fraction of a second. Thus, the best seeing often occurs by what Percival Lowell once called "revelation peeps." The effect is exactly like that produced by a tachistoscope, a device used by perceptual psychologists since the turn of the century to study what takes place during interrupted or brief perceptions. The planetary observer waits for the tachistoscope flash, and waits with an expectant mind---just as Sir Ernst Gombrich listened to weak and static-filled radio transmissions during World War II; the interpretation is of "whiffs" of information coming over the airwaves (or in this case light waves). What appeared to many observers as canals and oases was actually the brain's shorthand rendering of what the eye vouchsafed to it in glimpses obtained through telescopes with modest apertures. But there was a further stage along what Schiaparelli had described as the ascending stairway of perception, the stage Antoniadi rendered in the sketches and maps he made with the Grand Lunette. Instead of lines and dots, the characteristic markings appeared as winding, knotted, irregular bands, jagged edges of halftones, and sooty patches. In turn, Antoniadi's view of the planet was itself a tentative---and blurred---vision of what would later be revealed when spacecraft visited the planet and sent back images of the surface.

The canals have been disproved, but they will never be forgotten. They will always remain an important chapter in the history of Martian exploration, not least because of the literature they inspired. Beginning with H. G. Wells's interplanetary invaders and Edgar Rice Burroughs's eerie invocations of Barsoom, the canals and the dying world they were supposedly meant to save inspired much of our early science fiction---and there, at least, they will live on.40 Moreover, observers, especially those using small instruments, will continue to have fleeting glimpses of the canals from time to time---tantalizing Lowellian moments.

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