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




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


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.)



I’d rather have 12% than 18%.

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. ;->

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.

Sort of a pillow mint for this guy:

Image by: Mitch Breitweiser

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!

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.

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

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.

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.

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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.

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