NavList:

Frank, you wrote:

"So why doesn't averted vision work on Venus? I don't know. It has to be some property of human vision in bright light, which apparently does not have a common name"

Your assumption is correct. Averted vision works in night vision (so called "scotopic" conditions, with very low background brightness). Humans are able to detect dimmer stimuli at an eccentricity of about 10 degrees from the center than when using central vision.  Note that this not at the extreme periphery of the visual field, but in the near periphery, not very far from the central area of the retina, the fovea. This detection is mediated by rod photoreceptors and their pathway to the brain, which have evolved for maximum sensitivity at the expense of resolution.

During the transition from night through twilight to day, the sensitivity of the rod pathway decreases as background (sky) brightness increases. In these conditions, the rod pathway is said to "saturate", and loses its ability to mediate the detection of dim stimuli.  If you wish, the "common name" for this property of human vision in bright light, is "rod saturation".  In daylight (so called "photopic" conditions, with bright background brightness), maximum light sensitivity is mediated by cone photoreceptors and their pathway to the brain. This reaches its peak sensitivity in the fovea at the very center of the retina. Therefore, averted vison does not help when attempting to see a small and dim target (e.g., Venus) in daylight, and foveal vision is ideal for this task.

Those interested in a more technical (and much better) description about the transition from night vision to day vision may wish to read a relatively recent review paper by A. Stockman and L. Sharpe: "Into the twilight zone: the complexities of mesopic vision and luminous efficiency" (Ophthalmic and Physiological Optics : 2006, v26: 225-239). It may be accessed without charge from Andrew Stockman's lab website: http://www.cvrl.org.

Best regards,
Rafael