Wow! Thank you for the information on bird vision. I did not realize how many more colors they see!
Ronn!Blankenship <[EMAIL PROTECTED]> wrote
From <<http://www.bio.bris.ac.uk/research/vision/4d.htm>>:
"Bird colour vision differs from that of humans in two main ways. First, birds can see ultraviolet light. .... It is mammals, including humans, that have poor colour vision! Whilst UV reception increases the range of wavelengths over which birds can see, increased dimensionality produces a qualitative change in the nature of colour perception that probably cannot be translated into human experience. ...
Also, if you ever are able to reproduce the
charts of the area around the constellation Orion showing what it would look like to each of those (based only on the sensitivity)
I would be very interested. This is all new to me.
I generated them with Chris Marriott's SkyMap Pro (<<www.skymap.com>>) by successively setting the limiting magnitude (recall that a difference of 5 magnitudes means a ration of 100 times in brightness) to 4.5 (human from a light-polluted area like campus), 6.5 (human in a dark sky), 8.3 or so (cat: 6x as sensitive as a human), 11.5 (owl: 100x as sensitive as a human), printing the results, and making transparencies of them. Unlike many other available sky-charting programs, which restrict the limiting magnitude depending on the field of view, so one can only see lots of faint stars if one restricts the chart size to something corresponding to a telescopic field of view, SkyMap Pro allows one to generate maps of larger portions of the sky with the limiting magnitude set quite faint.
I made these charts awhile back, and have thought about sometime re-doing them when I have the time. Also, I made them before I got a copy of the full USNO catalog which is complete down to about 19th magnitude or so over the whole sky, and might be more complete (I think SkyMap Pro uses the SAO catalog, which is complete only down to 9th magnitude or so, for basic data, supplementing it with the HST GSCC and the Hipparcos/Tycho catalogs, but I am not absolutely sure that the charts I produced, particularly for the owl, are complete. There are enough faint stars all over it the entire field that you can't even identify Orion without some concentration, though, so it is suitably impressive to students as it is . . . )
Another statement I came across from somewhere is that a rabbit's eyes are so sensitive to movement that she could actually see the Sun moving across the sky.
Hah! That is amazing.
I did a Google search for "rabbit vision" but didn't find any information to support or deny that assertion, which IIRC was made in a _Sky & Telescope_ article about the eye several years ago. Most of the stuff I found was all quoted from the same source which was directed at answering the question "What does my rabbit see?" for young people who have pet rabbits. (The Internet came along about a quarter of a century too late to be useful when I was in that category, so I am unable to confirm any of that information from personal observation.)
Another question, what kind of resolution do birds have in ordinary light? I know, from observation, that I can readily see a dark vertical line, a tower, against a light background, the sky, when its width is one in three-thousandsth of a radian or about one minute of arc. (Roger Clark, in `Visual Astronomy of the Deep Sky', provides more detailed information that involves the surface magnitude of the background. Where I have made comparisons, his information agrees with my personal experience.)
One arc minute is about the limit of resolution of the average human eye. Recall, frex, that the maximum discrepancy between Tycho's observations of Mars and positions predicted by Kepler's best-fit circular orbit was only 8 arc minutes, and it was only because Kepler had worked for Tycho for about one year and was certain that his observations were accurate to the limit of naked-eye resolution that convinced him that a circular orbit was not adequate. And about the same time, Galileo's observations of the phases of Venus provided strong evidence of the heliocentric model. Venus is just under 1 arc minute in diameter, so IOW the critical observations were just out of reach of the naked eye. (Some people with sharper-than-average vision can indeed see that Venus is not a circle when it is nearest Earth and hence less than half-"full". However, astigmatism and air turbulence can also make it appear out-of-round, and in any case only a small percentage of people have sharp-enough vision, so anyone who reported that Venus looked crescent-shaped at times was probably ignored, as was anyone who reported seeing Jupiter's moons Ganymede and Callisto with his naked eye, although that is possible for people with sharper-than-average vision when Jupiter is near opposition in a dark sky. BTW . . . I also have an attention-grabbing illustration for when the topic of "naked-eye observing" comes up . . . )
-- Ronn! :)
"Earth is the cradle of humanity, but one cannot remain in the cradle forever." -- Konstantin E. Tsiolkovskiy
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