Christoph Hormann wrote:

> Dan P wrote:
> 
>>> Earth is neither 21% brighter nor 21% more bright than moon assuming 
>>> the given albedo values.  If you want to give percentual values you 
>>> have to do it correctly, otherwise simply say the albedo of earth is 
>>> 0.21 higher than that of earth (which of course isn't a very useful 
>>> information).
>>
>> The values are between 0 and 1. If they were between 0 and two, the 
>> percentage would be half of what I said and so forth.
> 
> That's nonsense - in fact the albedo isn't even limited to [0..1] by 
> definition.  Anyway i am not really interested in convincing you from 
> using correct formulations if you don't want to, my arguments were clear 
> and easy to understand IMO, the rest is up to you.

You may be right. This is what I understand about albedo values:

0 = Reflects no light.
1 = Reflects all light.

An surface cannot reflect a negative amount of light.
An surface cannot reflect more light than is provided to it.

This understanding may be incorrect; I'm wrong a lot!
My ego is not invested in being right here -- for the good of the group, 
please set us straight, Christoph! (The last sentence is actually not 
being sarcastic).
-- 
Respectfully,
Dan P
http://<broken link>

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Dan P wrote:

> Christoph Hormann wrote:
> 
>> Dan P wrote:

<snip />

> This understanding may be incorrect; I'm wrong a lot!
> My ego is not invested in being right here -- for the good of the group, 
> please set us straight, Christoph! (The last sentence is actually not 
> being sarcastic).

Another thing; when I speak of an albedo value, I only speak of the 
value itself, not the application of the value. As you have mentioned, 
the albedo value depends on the angle the viewer observes it. If you 
bring in time, a body like Venus rotates over time and, thus, the albedo 
value changes because the surface the light is exposed to changes. So, 
an albedo value like the ones the web site provided are averages at best 
and are only useful for quantifying, somehow, the relative albedos of a 
set of objects; for example, how much more bright the object appears 
from earth compared to some other body in the set. It may also be useful 
for detecting changes on a surface. There are all kinds of APPLICATIONS 
for an albedo value, but this does not change the NATURE of an albedo value.

If an albedo value is not from 0 to 1, then it is bound within some 
arbitrary range described by the object with the highest albedo value we 
have observed at some moment in time. The definition, "the fraction of 
light that is reflected by a body or surface." can support this: a 
fraction can be 2+1/2, for example, or 250%. So, if we were to observe 
an albedo value of Venus at some moment in time, then we could say at 
some future moment in time that the albedo value is 250% higher than before.

I thought this was interesting:

"At BRW and MLO, albedo values above 1.0 are due to instrument noise at
low aerosol concentration. These high albedo values are not present in 
daily averaged data. Furthermore, these high albedo values are not 
present if data are excluded where ?sp is below 1 Mm-1. Hence, the high
albedo values result from an instrument detection limitation problem."
http://www.cmdl.noaa.gov/publications/annrpt26/3_1_3.pdf

So, an albedo value can be above 1, but is it valid?

I'm finding it hard to find albedo values above 1. Take the following 
example of snow albedos:

http://www.agu.org/pubs/sample_articles/cr/2001JD001160/figures.shtml#fig15

They assume from 0 to 1 on their graphs. Same for this site:

http://cires.colorado.edu/~maurerj/albedo/albedo.htm


So, the crux: the definition itself does not specify that an albedo must 
be from 0 to 1, but I believe that to be axiomatic (self-evident). I 
will attempt to create a proof:

Truth: A surface may not reflect less light than is applied to it.
Truth: A surface may not reflect more light than is applied to it.
Truth: A body cannot reflect less light than is applied to it.
Truth: A body cannot reflect more light than is applied to it.

Therefore

A minimum albedo means a surface reflects no light.
A minimum albedo means a body reflects no light.

A maximum albedo means a surface reflects all light.
A maximum albedo means a body reflects all light.

0% of light means no light.
100% of light means all light.

0% may be represented as 0.
100% may be represetnted as 1.

Therefore

A minimum albedo is 0
A maximum albedo is 1


If you have some further truths to add to this proof to disprove my 
claim, please add them; it will help mine and other's understanding of 
this interesting subject!

Some more web-sites of interest:
http://www.experimentarium.dk/uk/naturvidenskab_og_teknik/artikler/artikel.118.html
http://en.wikipedia.org/wiki/Albedo


(See, Mom, I can sound smart too!)
-- 
Respectfully,
Dan P
http://<broken link>

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"Christopher James Huff" <cja### [at] earthlinknet> wrote in message
news:cjameshuff-253827.19080523042004@news.povray.org...
> In article <40889a54@news.povray.org>, "St." <dot### [at] dotcom> wrote:
>
> > > We've been there, remember? ;-)
> >
> >     Heh, yes, I think I was about 6-7yo, cross-legged, wearing
grey
> > shorts, grey socks, in our sports-hall, watching a black and white
TV
> > when they touched down. A nice memory.  :)
>
> Er...well, I didn't mean personally remembering the landing (I
wasn't
> even alive yet), but remembering that we have landed.

    Ah yes, but it was still a nice memory...  :)

>

> Damn...try this:
> http://tinyurl.com/34weg

   Thanks! Amazing images. Almost PoV-like.

>
>... I don't know if
> > it was magma, it could have been anything native I guess, but I
> > wouldn't know what.
>
> Technically, magma is the still molten stuff under the crust. It
becomes
> lava when it leaks out onto the surface, and some form of igneous
rock
> when it solidifies. Solidified lava is often called just "lava
rock".
> What you describe sounds like some kind of obsidian (volcanic glass)
> rich lava rock.

> >    I found many shell fossils too, if that's a clue?
>
> Shells are found in sedimentary rock, lava rock is igneous. Though
it
> could be from lava that flowed into sedimentary rock, or sedimentary
> rock formed from ash and volcanic debris falling on a body of water.
> Perhaps beach sand partially melted by lava? (That would explain the
> glassiness and the shells.)

Lava rock is what someone else called it back then too - I'd forgotten
that. Yes, it was very 'glassy' and would break cleanly, so I also
like the melted beach sand theory too - I guess that it actually
happens somewhere on earth.

    ~Steve~


>
> -- 
> Christopher James Huff <cja### [at] earthlinknet>
> http://home.earthlink.net/~cjameshuff/
> POV-Ray TAG: <chr### [at] tagpovrayorg>
> http://tag.povray.org/

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"Dan P" <dan### [at] yahoocom> wrote in message
news:4089a790@news.povray.org...
> St. wrote:

> >  Yes, I used to walk our defunct railway lines when I was younger,
and
> > believe that I found samples of magma - I'm not sure, but that was
> > what I was told. It was a dark substance with a golden
> > sheen/iridescence to it, but it definitely looked molten and was
round
> > in shape when solidified. It was very 'glassy', and would break in
> > half very cleanly, leaving a very reflective surface. I don't know
if
> > it was magma, it could have been anything native I guess, but I
> > wouldn't know what.
> >
> >    I found many shell fossils too, if that's a clue?
>
> Interesting! Where did you live when you were younger? Was it near
any
> volcanic activity?

    Near Swindon, Wiltshire? No, I don't think so. ;)

   The aggregate was probably imported, but from where, I wouldn't
know. One of the other 'stones' that was common, was a very 'spongy'
looking mineral, very light in weight with zillions of holes and
bubbles in it - I've no idea what that was, but probably an indication
of intense heat too.

   Another one was a beige coloured solid stone that smelled of
sulphur - I really liked this one, but it was the wildlife that I was
looking for that really interested me, lizards, mice, slow-worms, etc.

  The fossils were great too. There were plenty of "Devils Toenails",
with which I would scare the life out of my sister...

     ;)

 ~Steve~



> -- 
> Respectfully,
> Dan P
> http://<broken link>

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Among other things, Alain wrote:

> Albedo is the fraction of the total light, from low infrared to the
> hardest UVs, faling onto a celestial body that is sent back into space.
> Depending on the caracteristics of the surface, the body can be more
> reflective along prevelieged directions, but the amount of incident
> light won't chage it. Taking the moon closer to the sun, like at the
> same distance as Mercury, will have no effect on it's albedo, altough it
> will looks much brighter if you are to look at it from the same distance
> as you look at it now.

OK, If you say so I believe you.

BUT, from other scientific courses and past experience I've learnt that 
things are often not so simple. Sometimes it is convenient to express 
quantities as quotients between other quantities, so that you have a number 
which will, hopefully, remain constant. Sometimes this number is constant 
only in a certain range of conditions (for example, the "rate constant" of 
chemical reactions is not constant, it changes with the Temperature; the 
"absorption coefficient" of Lambert-Beer's Law is constant in a range of 
concentrations, but not outside it).

So, I believe this could well be the case of albedo. Maybe, given the set of 
conditions usually found in the solar system, we can take the fraction of 
the incident radiation being reflected as constant for a given body. But 
this doesn't mean it *has* to be constant, it could change with extremely 
high or low light intensities. I believe this is a valid concern, given the 
way the real world behaves.

-- 
light_source{9+9*x,1}camera{orthographic look_at(1-y)/4angle 30location
9/4-z*4}light_source{-9*z,1}union{box{.9-z.1+x clipped_by{plane{2+y-4*x
0}}}box{z-y-.1.1+z}box{-.1.1+x}box{.1z-.1}pigment{rgb<.8.2,1>}}//Jellby

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Among other things, Dan P wrote:

> Truth: A body cannot reflect less light than is applied to it.

Huh?

-- 
light_source{9+9*x,1}camera{orthographic look_at(1-y)/4angle 30location
9/4-z*4}light_source{-9*z,1}union{box{.9-z.1+x clipped_by{plane{2+y-4*x
0}}}box{z-y-.1.1+z}box{-.1.1+x}box{.1z-.1}pigment{rgb<.8.2,1>}}//Jellby

Post a reply to this message

In article <408b492d@news.povray.org>, "St." <dot### [at] dotcom> wrote:

> > Damn...try this:
> > http://tinyurl.com/34weg
> 
>    Thanks! Amazing images. Almost PoV-like.

Pretty realistic, huh?
Oh, wait... ;-)


> Lava rock is what someone else called it back then too - I'd forgotten
> that. Yes, it was very 'glassy' and would break cleanly, so I also
> like the melted beach sand theory too - I guess that it actually
> happens somewhere on earth.

Well, obsidian is usually formed from the lava itself, from flows rich 
in silica, not from sand melted by it (though underground quartz may be 
the actual source). It's just the combination of glassy rock and shells 
that made me think of that possibility...shells aren't likely to stand 
immersion in lava for long, but a lava flow onto a beach may cool 
quickly enough to preserve them.

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: <chr### [at] tagpovrayorg>
http://tag.povray.org/

Post a reply to this message

In article <408b492e@news.povray.org>, "St." <dot### [at] dotcom> wrote:

>    The aggregate was probably imported, but from where, I wouldn't
> know. One of the other 'stones' that was common, was a very 'spongy'
> looking mineral, very light in weight with zillions of holes and
> bubbles in it - I've no idea what that was, but probably an indication
> of intense heat too.

That was probably pumice, formed from lava containing a lot of dissolved 
gasses. It can often float due to its low density. It's often used as an 
abrasive, either ground into a powder or as solid blocks.


>    Another one was a beige coloured solid stone that smelled of
> sulphur - I really liked this one, but it was the wildlife that I was
> looking for that really interested me, lizards, mice, slow-worms, etc.

Don't know what that was...but native sulfur is often yellow-amber in 
color, that might have been an impure form.


>   The fossils were great too. There were plenty of "Devils Toenails",
> with which I would scare the life out of my sister...

Devil's Toenails...trilobites?

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: <chr### [at] tagpovrayorg>
http://tag.povray.org/

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Jellby wrote:
> Among other things, Dan P wrote:
> 
>>Truth: A body cannot reflect less light than is applied to it.
> 
> Huh?

Yep; strange to think that way, I know, but that is a truth!
(Please don't bring up black holes either -- we aren't modelling those)
-- 
Respectfully,
Dan P
http://<broken link>

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Dan P nous apporta ses lumieres ainsi en ce 2004/04/25 13:04... :

> Jellby wrote:
>
>> Among other things, Dan P wrote:
>>
>>> Truth: A body cannot reflect less light than is applied to it.
>>
>>
>> Huh?
>
>
> Yep; strange to think that way, I know, but that is a truth!
> (Please don't bring up black holes either -- we aren't modelling those)

Receive visible light, absorbs it and transform it to heat, re-emit it 
as thermal infrared light. Energy in = energy out.

Alain

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