Re: [asa] Antisolar Sky

Thank you very much, George. I've learned a lot from this thread and I look forward to continuing experimental observation to understand it all better.

This discussion started with the question of whether the ANE culture might have deduced the sphericity of the earth from visual observations. I confess I don't think I could have figured it out from any observations--it's tough enough to interpret observations given the assumption of a spherical earth. A lot of credit goes to those who did first figure it out, which if I understand it correctly was around the 5th century BC?

Randy
  ----- Original Message -----
  From: George Cooper
  To: ASA
  Sent: Sunday, August 24, 2008 7:40 PM
  Subject: [asa] Antisolar Sky

  The Antisolar Sky

  For the colorful view of the eastern sky during the latter portion of a colorful sunset, Randy, your backscattering viewpoint was correct, of course. Allow me to attempt to get this right. [Some of this is my own reasoning based on what little I have found.]

  The pinkish-orange band of color that is opposite either a sunrise or sunset results from Mie scattering (back scattering) of sunlight off water molecules, primarily. Since the light from this colorful region has traveled through additional atmosphere, more reds and oranges will be favored due to the Rayleigh scattering effect.

  The region below this is blue-gray due partially to the Chappuis effect (I did spell it right, btw). This effect describes the transmittance of visible light through ozone. The ozone allows more blues to pass than the other colors, so it seems to play a significant roll during twilight time for the sky above – where the sky overhead appears blue at twilight -- and the blue-gray region in the Belt of Venus (Earth’s shadow cast upon the sky).

  History lesson: There seems to be very little history to this, surprisingly. According to Hoeppe (editor of a German physic’s magazine and author of Why the Sky is Blue), the first record for an observation of Earth’s shadow came from priest Johann Funck of Germany in 1716, but only mentioned the pinkish glow, and missed the idea of the Earth casting its shadow.. The shadow was identified by Horace Benedict de Saussure (inventor of the cyanometer – another cool story, btw), in 1787. In the mid 19th century, mountain climbing got popular, so these observations of Earth’s shadow became more common. After climbing the Matterhorn in 1868, John Tyndall also recorded the twilight phenomenon.

  Enter Jean Dubois took a spectrum of the blue-gray region in the 1940’s and determined this region was a result of Chappuis absorption. The sharpness of the bands of color seemed implausible to Dubois who expected a more diffuse effect. According to Hoeppe, “it is the oblique incidence of the sun’s rays along the ozone layer that makes the Earth shadow visible”. It is this sharpness in color contrast that makes the arch of the Earth’s curvature so noticeable.

  This is a bit remarkable since if we could take all the ozone in a vertical column of sky and compress it to one atmosphere, it would only be 3mm thick (300 Dobson units)!

  I have found very little regarding the Chappuis Effect, but I assume the above is likely correct. Another site that seems to indicate an alternative view that supports a simple Earth shadow effect can be found here ( near the bottom of the page):

  http://www.weatherscapes.com/techniques.php?cat=optics&page=twilight

  A great paper on atmospheric effects can be found here by Craig Bohren: http://www.pro-physik.de/Phy/pdfs/OE004_1.pdf

  Have a nice sunset!

  “Coope”

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Received on Mon Aug 25 09:51:02 2008