Re: I can't recall if I ever posted this here

In article <1240619069@xxxxxxxxx>, Wayne Throop <throopw@xxxxxxxxx> wrote:

[quoting, I think, George Haley?]

: Here's a thread from AFU where one Keith Lynch describes being able to
: see glowing powerlines, fireflies a mile away, has read a newspaper by
: the light from Venus, and can see the phosphors on a TV that hasn't
: been on in a week, which is all pretty impressive.

I'm pretty sure *I* couldn't read by venuslight no matter how
dark adapted my eyes. At least, not without some additional
artificial aid... or if the type was exceedingly large.

Venus light is a misleading concept.

We see by sunlight at night, to some extent, and more when the sun is
just down than at midnight. Well, you can only see by Venus light
when the sun is just down. So ...

That said, Venus is by far the brightest object in the sky after
the sun and moon. I believe it's brighter than all starlight put
together, though I can't be sure, because estimates for the total
brightness of starlight vary by several orders of magnitude. (I wish
I were making this up. It wouldn't be terribly hard to use modern
star maps to come up with an ideal number for starlight depending on
location and date/time, but all kinds of meteorological and positional
issues interfere, and apparently astronomers don't think it's worth
bothering.)

But anyhows, the human visual system, under proper conditions,
is supposed to be able detect light flashes contining fewer than
a hundred photons. Which is impressive.

The problem with the fact that the human eye can pick up a single
photon (and, yes, convey that information to the brain) is that the
human eye flashes by mistake rather more than a single photon's
worth per, say, second.

Pigment molecules are some of the stablest complex organic molecules
around, but that doesn't mean they're perfect, and humans in
particular have a LOT of pigment molecules.

<http://en.wikipedia.org/wiki/eigengrau>

This article said, before I edited it, that there's about one firing
per 100 seconds. Um, no. If you follow the reference link, what you
find is "one photoisomerization *per rod* per 100 sec" (emphasis added).
We have millions of rods - again per Wikipedia, about 120m - which
gives us an eigengrau of about 1m firings per second for someone with
two normal eyes.

The reason we can quasi-reliably detect a light source that we get,
say, 90 photons from, is that it's a *light source*. So the
particular rods that see those photons are firing considerably more
often than once per 100 sec, and the brain notices this. We can't
see background light of total 90 photons entering the eye, however.
And I'm skeptical of your "light flashes". This implies a very brief
background light, rather than a very brief discrete source; but I
don't see how the brain is supposed to notice 90 extra rods firing
when (again per Wikipedia) rods sum over 100 ms, so our window has
about 100k eigengrau firings in it.

If you want fantasy critters for whom, say, starlight is like daylight
to us, you have two options. Give 'em magic, or give 'em sufficiently
advanced something or other. I really don't see a way, within normal
evolution, to get something like that. (In sf, you'd want to give
them owl eyes or some such, but that's not seeing like us, that's a
different way of seeing, with its own costs in, for example, daylight.)

Joe Bernstein

--
Joe Bernstein, tax preparer, bookkeeper and writer joe@xxxxxxxxxxx
<http://www.panix.com/~josephb/>
.

Relevant Pages

  • Re: Jupiter, seen from Vesta
    ... My eyes have never been that good, ... But if Venus can be seen as a crescent by normal eyeballs, ... the images you report, which appear to be spots upon the sun, ...
    (rec.arts.sf.science)
  • Re: What really is "Light" (assuming SR is correct)??
    ... These folks absolutely worship Einstein and the likes of Plank ... the available photons, is that photons amount to absolutely nothing, ... before doing Mars or Venus ... Lunar/Moon Space Elevator, plus another ISS within the CM ...
    (sci.physics)
  • Re: What really is "Light" (assuming SR is correct)??
    ... These folks absolutely worship Einstein and the likes of Plank ... the available photons, is that photons amount to absolutely nothing, ... before doing Mars or Venus ... Lunar/Moon Space Elevator, plus another ISS within the CM ...
    (sci.physics.relativity)
  • Re: I cant recall if I ever posted this here
    ... When my eyes are fully ... I can slowly read the largest of newspaper *headlines* by the light ... of Venus, or at least I could the last time I checked. ...
    (rec.arts.sf.written)
  • Re: Paper - Optical SETI with Imaging Cherenkov Telescopes
    ... that your usage of detecting gamma rays on behalf of a scientific ... mutated DNA/RNA worth of the humanity that's situated upon Earth? ... Few if any stars are offering us the sort of photons other than of ... Life upon Venus offers a Township, ...
    (sci.astro.seti)
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