From: Amara Graps (amara@amara.com)
Date: Tue Sep 19 2000 - 15:16:46 MDT
From: scerir <scerir@libero.it>
>This halo is called the heiligenschein,
>and is due to the brightest light return
>ocurring at the antisolar point.
>It is caused both by the reflection of
>the sun's light from the far side of the dew
>drops, and by the fact that you are able to see
>illuminated grass stems which would be
>masked from view from any other direction.
>
>pictures of heiligenschein
>http://www.cchem.berkeley.edu/~wljeme/Chapt2.html
Lovely!
When I first read your post, I thought it was related to the
"gegenschein", (German for counter-glow) which is a brightening of
the night sky in the region of the ecliptic directly opposite the
sun's position at the time of observation. It is caused by the
reflection of sunlight by small particles that lie in the plane of
the solar system. The light-scattering from the particles is in the
forward direction.
"Zodiacal Light and the Gegenschein"
http://www.as.wvu.edu/~jel/skywatch/skw9810h.html
and if you're curious about one aspect of the work of us
"dusty astronomers":
"Zodiacal Light"
http://sn-callisto.jsc.nasa.gov/sn3_other/zodiac/zodiac.html
But it's not related. Heiligenschein is mostly a daytime pheonomena
and it usually needs dew.
Minnaert, in _The Nature of Light and Color in the Open Air_ (a
wonderful 1950s classic text available in a Dover paperback) says:
"In the early morning, when the sun is still low and casts our long
shadow on the dwy grass, we perceive a remarkable aureole of light,
uncoloured, lying near and above the shadow of our head. No, it is
not an optical illusion, nor a contrast phenomenon, for when the
same shadow falls on a gravel path, you no longer see this aureole
of light.
This phenomenon is at its best when the length of the shadow is at
least 15 yards, and when it falls on short grass or clover,
greyish-white from the heavy dew. Under such circumstances the
Heiligenschein is very pronounced. It is less distinct in the middle
of the day after a shower, or at night in the light of strong
electric lamps. If there is any doubt about this phenomenon, the
best way to be certain is: (i) Survey the whole lawn, and note how
the light increases near your shadow. (ii) Take a few steps: you
will see the glow of light go with you, and places where the grass
was not particularly bright become illuminated as your shadow
approaches. (iii) Compare your shadow with that of other people; you
will see the Heiligenschein sourrounding only your own head."
Here is another reference about it:
http://www.weather-photography.com/Atmospheric_Optics/heiligenschein.html
which says:
"Heiligenschein is a bright spot of light around the shadow of the
observer's head, cast on dewy grass land. The dew droplets on the
grass act as lenses, focusing sunlight on the grass leaves, strongly
illuminating them, and working again as a lens for the backscattered
light. This effect is enhanced by the fact that in the antisolar
point (the point opposite the sun, i.e. the shadow of one's head),
no other shadows are present, and thus the region looks much
brighter because the light and dark details of the surface are
integrated by the eye. Look for the Heiligenschein early in the
morning, when the sun is low and thus your shadow long, and move
yourself to let the shadow be cast on wet dewy grass. The effect is
very common, so you should be likely to see it."
And here is another beautiful site:
http://micro.magnet.fsu.edu/primer/lightandcolor/diffraction.html
These kinds of phenenomena, for me, is about Art, Science and the
Beauty of the Universe.
"The universe: a device contrived for the perpetual astonishment of
astronomers." --Arthur C. Clark
Thanks very much for the tip about the Heiligenschein, scecir.
Amara
"That's the whole problem with science. You've got a bunch of
empiricists trying to describe things of unimaginable wonder."
--Calvin (& Hobbes)
(Oops!)
*****************************************************************
Amara Graps | Max-Planck-Institut fuer Kernphysik
Interplanetary Dust Group | Saupfercheckweg 1
+49-6221-516-543 | 69117 Heidelberg, GERMANY
Amara.Graps@mpi-hd.mpg.de * http://galileo.mpi-hd.mpg.de/~graps
*****************************************************************
"Never fight an inanimate object." - P. J. O'Rourke
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