Dan P wrote:
> [...]
> 
> Try substituting the word "brighter" with "more bright" and it may sound 
> more clear.

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 whole thing is probably a bit confusing because the unit of albedo 
is 1 (it is a ratio between two values of the same unit.  If you have 
two people, one 1,60m tall and one 1,90m you also would not say one is 
30% taller/more tall than the other...

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 21 Mar. 2004 _____./\/^>_*_<^\/\.______

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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.


> <http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/moon_rocks/7422
> 0.h
> > tm>
> 
>     Aargh! 404 even when pasting the URL directly.

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


>  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.

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.)

-- 
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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Christoph Hormann wrote:

> Dan P wrote:
> 
>> [...]
>>
>> Try substituting the word "brighter" with "more bright" and it may 
>> sound more clear.
> 
> 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.

> The whole thing is probably a bit confusing because the unit of albedo 
> is 1 (it is a ratio between two values of the same unit.  If you have 
> two people, one 1,60m tall and one 1,90m you also would not say one is 
> 30% taller/more tall than the other...

True; but that is a different set that does not map to a range between 
zero and one because it constantly changes. If you took a snapshot in 
time, say the shortest person is A and the tallest person is B, then you 
could normalize the number and, indeed, use percentages.

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

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St. wrote:

> "Christopher James Huff" <cja### [at] earthlinknet> wrote in message
> news:cjameshuff-4EC0B0.15182322042004@news.povray.org...
> 
<snip />

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

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Dan P wrote:

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

<snip />

> 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.
> 
>> The whole thing is probably a bit confusing because the unit of albedo 
>> is 1 (it is a ratio between two values of the same unit.  If you have 
>> two people, one 1,60m tall and one 1,90m you also would not say one is 
>> 30% taller/more tall than the other...
> 
> True; but that is a different set that does not map to a range between 
> zero and one because it constantly changes. If you took a snapshot in 
> time, say the shortest person is A and the tallest person is B, then you 
> could normalize the number and, indeed, use percentages.

I just had a thought: the real question here is, "Is the function of an 
albedo value linear?" If we were to plot it on a graph from zero to one, 
would the line be straight or curved? If straight, you we can relate the 
values to each other using percentages. If not, we cannot.
-- 
Respectfully,
Dan P
http://<broken link>

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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.

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 21 Mar. 2004 _____./\/^>_*_<^\/\.______

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Dan P nous apporta ses lumieres ainsi en ce 2004/04/23 19:46... :

> <snip>
> I just had a thought: the real question here is, "Is the function of 
> an albedo value linear?" If we were to plot it on a graph from zero to 
> one, would the line be straight or curved? If straight, you we can 
> relate the values to each other using percentages. If not, we cannot.

Albedo is linear. A planet with an albedo of 0.50 reflect 50% (2/4) of 
the incident lignt, albedo 0.25 mean 25% (1/4) of incident light, albedo 
0.75 = 3/4.

Alain

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Among other things, Alain tuvo wrote:

> Albedo is linear. A planet with an albedo of 0.50 reflect 50% (2/4) of
> the incident lignt, albedo 0.25 mean 25% (1/4) of incident light, albedo
> 0.75 = 3/4.

But it's not necessarily constant, is it?

A planet's albedo could be 0.25 with a certain amount of incident light, but 
maybe, with twice as much incident light, its albedo would be only 0.21 
(not talking about changes in the planet's surface, angle of incidence, 
etc.)

-- 
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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Jellby nous apporta ses lumieres ainsi en ce 2004/04/24 09:47... :

>Among other things, Alain tuvo wrote:
>
>  
>
>>Albedo is linear. A planet with an albedo of 0.50 reflect 50% (2/4) of
>>the incident lignt, albedo 0.25 mean 25% (1/4) of incident light, albedo
>>0.75 = 3/4.
>>    
>>
>
>But it's not necessarily constant, is it?
>
>A planet's albedo could be 0.25 with a certain amount of incident light, but 
>maybe, with twice as much incident light, its albedo would be only 0.21 
>(not talking about changes in the planet's surface, angle of incidence, 
>etc.)
>
>  
>
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.
The albedo depend entirely and only on the nature of the surface. Take 
Venus and bring it on earth orbit, then most of it's clouds will rain 
down, giving a lower albedo. Bring it down to Mercury orbit and it's 
clouds will completely vaporize, maybe escaping into space, also 
lowering it's albedo.
 From my past time in amateur astronomy, my physics cources and tons of 
reading on the subject...

Alain

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Alain wrote:
> Jellby nous apporta ses lumieres ainsi en ce 2004/04/24 09:47... :
> 
>> Among other things, Alain tuvo wrote:
>>
>>> Albedo is linear. A planet with an albedo of 0.50 reflect 50% (2/4) of
>>> the incident lignt, albedo 0.25 mean 25% (1/4) of incident light, albedo
>>> 0.75 = 3/4.
>>
>> But it's not necessarily constant, is it?
>>
>> A planet's albedo could be 0.25 with a certain amount of incident 
>> light, but maybe, with twice as much incident light, its albedo would 
>> be only 0.21 (not talking about changes in the planet's surface, angle 
>> of incidence, etc.)
>>
> 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.
> The albedo depend entirely and only on the nature of the surface. Take 
> Venus and bring it on earth orbit, then most of it's clouds will rain 
> down, giving a lower albedo. Bring it down to Mercury orbit and it's 
> clouds will completely vaporize, maybe escaping into space, also 
> lowering it's albedo.
>  From my past time in amateur astronomy, my physics cources and tons of 
> reading on the subject...

Then it's settled! Because the albedo of a surface is linear, then it 
can be expressed in terms of percentages :-)
-- 
Respectfully,
Dan P
http://<broken link>

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