RE: [asa] Rudwick does it again (back to Adam)

 In reading my prior message I should have clarified that my understanding
of the belt is as follows: the lower, darker region is the Earth's shadow;
the next band above is the blue-gray region caused by ozone absorption
effects; next is the adjacent pinkish band; then blue sky above.

I am giving you a simplified explanation for the Belt of Venus because I
have forgotten some of the details, so even this simple description could be
in error. Mie scattering would be a reasonable explanation for the pink
band in the Belt, but I think Rayleigh scattering is, by in large, describes
the greatest contribution to the pinkness. Have you ever noticed how the
more distant thunderstorm cloud tops are pinkish? This is due to the
extensive amount of atmosphere sunlight travels that causes more blues and
greens to scatter away (as per Rayleigh scattering that is in addition to
the Mie scattering off the cloud). [Cloud water vapor allows Mie
scattering to essentially "reflect" all the incident visible wavelengths
equally.] However, the pink band in the Belt offers little or no Mie
scattering-sized particles, contrary to the cloud analogy. In both cases,
pink Belt band and pink cloud tops, it is the optical depth that is likely
the key issue; more atmosphere will scatter more blues and greens resulting
in a pinkish color for any distant observer.

I will get back to you on this after I have a chance to straighten out the
all too typical knowledge phenomenon I exhibit: George Scattering. J

George Cooper

From: asa-owner@lists.calvin.edu [mailto:asa-owner@lists.calvin.edu] On
Behalf Of Randy Isaac
Sent: Friday, August 22, 2008 10:54 AM
To: asa@calvin.edu
Subject: Re: [asa] Rudwick does it again (back to Adam)

Very interesting. Thank you, George. I had been looking for a book on
optical effects in the atmosphere so I appreciate the reference.

Thanks for the clarification. I had misunderstood your previous note to
imply that the shadow of the earth was somehow connected with the Belt of
Venus but now I understand what you said. Is it correct that the pink sky
near the sun at sunset is related to Mie scattering but the Belt of Venus is
a reflection of such light from Mie scattering?

Randy

----- Original Message -----

From: George Cooper <mailto:georgecooper@sbcglobal.net>

To: asa@calvin.edu

Sent: Friday, August 22, 2008 10:59 AM

Subject: RE: [asa] Rudwick does it again (back to Adam)

Hi Randy,

If you can observe an unobstructed horizon opposite the setting or rising
Sun, you should have little trouble seeing an arch in the dark blue-grey
color band that hugs the horizon. This darker region is the shadow of the
Earth cast upon the horizon before the Sun actually rises or just after it
has set. Because of the miles of shadow you are observing, you will see
Earth's curvature as a result. The higher you are and the broader your
horizon, the easier, it is to see, but it is still fairly easy to see the
curvature in this color band without being on a mountain, as you correctly
stated.

The pink color band in the Belt of Venus is due to "reflected" light of the
reddened Sun. The color of the blue-gray band in the belt is due to our
very thin ozone layer. Ozone has the surprising ability to absorb the
longer wavelength visible colors, so a dim blue color results when sunlight
has traveled through enough of the ozone layer. This ozone color absorption
also explains why you still see a blue sky directly above you when the Sun
is on the horizon. The Rayleigh scattering effect alone would cause the sky
directly above to appear more yellow or red due to the fact sunlight would
have traveled through so much more atmosphere. However, the ozone
absorption prevents this reddening effect, and you still get a blue. It is
said that those who look for the slight blue change in color overhead can
see the difference take place at the appropriate time during sunrise and
sunset, but I haven't tried to do this.

An enjoyable read is "Why the Sky is Blue" - Gotz Hoeppe (2007) , which
addressed many interesting atmospheric phenomenon and with quite lucid
explanations that went into considerable depth (given the simplistic
sounding title). [I had hoped to find his view regarding the Sun's true
color (my pet research project), but he, like most others, ignored or
avoided it.]

"Coope"

From: asa-owner@lists.calvin.edu [mailto:asa-owner@lists.calvin.edu] On
Behalf Of Randy Isaac
Sent: Friday, August 22, 2008 6:35 AM
To: asa@calvin.edu
Subject: Re: [asa] Rudwick does it again (back to Adam)

George Cooper wrote:

Other interesting evidence that supports a round Earth include the shadow of
the Earth as seen from a mountain. This is noted by the colorful Belt of
Venus: http://apod.nasa.gov/apod/ap060723.html

Many thanks for the info about the Belt of Venus. I have oftened observed it
in the mornings here in Massachusetts but never understood what it was or
what it was called. I'm still not sure I understand it. How does it support
a round earth and what does it have to do with a shadow of the earth? A
mountain doesn't seem to be necessary to see it, either.

Randy

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Received on Fri Aug 22 14:47:33 2008