20 ASTRONOMY • SEPTEMBER 2017lthough our limited
eyes cannot detect
purple stars, the
color purple is
nevertheless part
of a skywatcher’s vocabulary. It
becomes especially noticeable
when we deal with atmospher-
ic aberrations, lensing effects,
and the scattering of light.
Examples of the last include
the violet f lash at sunset, the
purple light of twilight, the
indigo bands of a rainbow, or
the face of New Moon during a
total solar eclipse. All these are
fantastically purple, yet the
opportunities to see them are
generally uncommon.
Even simultaneous color-
contrast illusions combined
with a nervous atmosphere can
trick us into believing we can
see the color purple in some
stars. The 19th-century
English astronomer William
Henry Smyth was a master of
simultaneous color-contrast
perception. He noted, for
example, that double star Eta
(η) Cassiopeiae has a bright
white primary with a lilac
secondary.Back to nature
Nevertheless, although the
color purple is a rarity in the
sky, I wonder how much of its
light escapes our gaze. This
could be because we either
do not consciously look for it,
or perhaps we do not know
how to use our eyes efficiently
enough to perceive it. I don’t
know the answer, but I have
seen something that makes me
ponder.
Occasionally I write about
how we can train our eyes to
perceive light and color moreeffectively in the open air by
observing the world around us
during the day or in the twi-
light (dawn and dusk). Doing
so helps us to learn how our
eyes react under certain light-
ing conditions.
For instance, one misty
morning last April (in the
Southern Hemisphere) I was
walking outside in a gray and
cloudy dawn when my eyes
caught the sight of a black
house spider’s web in a tree.
When I looked directly at the
dense and messy web, which
was about 50 feet (15.2 meters)
away, it looked white.
As I passed by it, I saw out
of the corner of my eye a beau-
tiful patch of lavender light in
the direction of the web. When
I stopped and studied the web,
I noticed that its thinner sec-
tions did have a slight aqua
sheen when seen with direct
vision, which turned purple
when I used peripheral vision.
Later I matched my visual
impression to a spectrum of
light, and I estimated that the
light I perceived from the spi-
derweb with averted vision had
a wavelength of about 440
nanometers, putting it in the
blue-violet region. This dif-
fered from my estimate of
about 480nm — in the green-
blue region — that I saw using
direct vision. That’s quite a
spectral shift toward the violet!
And this was on a gray morn-
ing with little (if any) blue in
the sky.Color mixing?
In bright twilight, the eyes’
night-sensitive rod cells are
still active and can affect color
appearance by desaturatingSECRETSKY
The color purple
There’s wonderful stuff at one end of the rainbow.long wavelengths of light in
our peripheral vision. In addi-
tion, the cone cells that line the
eye’s periphery are dominated
by short-wavelength S cones,
which are most sensitive to
blue-violet wavelengths at
around 445nm.
Furthermore, S cones in the
human eye are more than four
times less sensitive than light
shining directly into the center
of the eye. Because of this
decreased sensitivity, they act
like a filter of sorts: They cut
down on intrusive glare whileenhancing our perception of
blue-violet light.
Just as no two fingerprints
are the same, our visual experi-
ences are highly subjective (in
fact, unique). By sharing my
experiences with you, all I can
hope to do is inspire you to
have your own. As always, send
your thoughts to sjomeara31@
gmail.com.BY STEPHEN JAMES O’MEARAA
BROWSE THE “SECRET SKY” ARCHIVE AT http://www.Astronomy.com/OMeara.Compare the two views of the web of a black house spider, which the author
encountered last April. The one at the top, viewed directly, shows hints of green,
albeit ever so faintly. When viewed obliquely (bottom image), the strands take on
a definite purple cast. STEPHEN JAMES O’MEARAStephen James O’Meara
is a globe-trotting observer
who is always looking for the
next great celestial event.