Rosetta's comet sounds like a candy bar: crunchy outside, marshmallow inside


#1

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#2

Armadillos!


#3

So the moon, ‘properly exposed,’ is a 12% gray card?


#4

Well, a gray card reflects about 18% of the light. That moon is at 12%. Plus is a little on the low-contrast side… Maybe it was shot in Beijing?

(Note: the exposure time/f-stop, when you’re photographing the moon, is exactly the same at sun-lit objects on the surface of Earth.)


#5


#6

I’d rather have 12% than 18%.

http://www.bythom.com/graycards.htm


#7

12% better than 18%? Maybe not…what are you drinking? Beer or wine?

Seriously, however: it’s pretty easy to settle. Measure the incident light with a light meter to get the exposure time, then measure the gray card by reflection and see which, 12%, 18%, or the back of your hand gives you (closer) to same exposure time. Or use the handy info inside the box your film came in. ;->


#8

Nevertheless, I still don’t understand why a moon with a 12% albedo doesn’t photograph darker when illuminated by sunlight and exposed using the ‘sunny 16’ rule.


#9

Sort of a pillow mint for this guy:

Image by: Mitch Breitweiser


#10

Oh, that rules applies here. (I have to start by saying that I’ve never heard of this rule-name until now.)

The sun-lit moon falls into the “Sunny/cloudless, Distinct Shadows” category. So f/16, 1/ISO applies here. As for the moon albedo, it’s worse than you think: the moon’s is 7%, not 12%.

My guess is that because the moon’s reflected luminance range (histogram?) is so narrow (compared to typical Earth scenery) that our brain interprets it so as to seem to have a wider/brighter range. The lighter/whiter areas are flagged by our, um, qualia has being ‘white’. Paging Daniel Dennett, white courtesy telephone please!


#11

The overall albedo of the Moon is frequently quoted as being about 7%. This is actually the so-called Bond albedo
at visible wavelengths, which refers to the fraction of the total
energy impinging on a surface that is reflected in all directions. It is
a concept which is useful in studies of planetary enegy balance, but
has little relevance to perceived brightness, which depend entirely on
the intensity reflected in a specific direction. The NASA Moon Fact Sheet
gives the Bond albedo of the Moon (presumably averaged over the entire
solar spectrum, including non-visible wavelengths) as 0.11.

http://the-moon.wikispaces.com/Albedo


#12

But if you’re photographing the Moon using the visible light, then the 7% figure is the one to use, no?


#13

No, because the moon isn’t perfectly diffuse, so even thought reflected light rays are largely randomized off of the incident rays, they are not completely randomized, so there is still a little bit of “angle of incidence = angle of reflection” specularity, just enough to produce an average 0.11 albedo as seen from Earth. I guess.


#14

Well, yeah. Any one who as watched dirty snow piles melt in the spring could have predicted this. The dirt gets concentrated on the surface as the pile shrinks.


#15

In Larry Nivin’s seminal work Lucifer’s Hammer, I recall the JPL scientists modeling Hamner-Brown as a Hot Fudge Sundae. That falls on Tuesdae.


#16

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#17

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