Monday, July 20, 2009

Cast Shadows, Part 2

As the Apollo astronauts observed 40 years ago today, cast shadows are nearly black on the moon, because the sky above the moon’s surface is nearly black.

I say “nearly” because there’s a little bit of starlight and there’s a little bit of reflected light trickling into moonshadows.

On Earth cast shadows are flooded by various sources.

To understand those sources, try to imagine yourself as a little eyeball mounted on an the back of an ant. As you walk across the shadow, imagine yourself looking around at all the bright patches of light shining down on you, not just the blue sky, but also white clouds, buildings, or other bright objects. Those patches of light determine the brightness and color temperature of your shadow.

Here’s a shadow cast across a rooftop by a dormer. An ant walking across the shingles would look up and see a sky with high clouds. But he would also see a large white wall just off to the right, the illuminated side of the second dormer. That white patch is brighter than the sky, and it pours light into the right half of the shadow.

Beneath the photo are samples of two areas of the shadow. You can see how much the cast shadow changes as the sources of infilling light change in relative intensity.

On Earth, cast shadows tend to be blue only because they’re normally thrown across surfaces that look up to the blue of the sky. But be aware the ant doesn’t always see blue patches. On overcast days, the fill light is white. And sometimes the sky patch is small and other patches are bigger and stronger.


René PleinAir said...

You forget the light bouncing off the Earth on to the Moon, ... you can see that when the Moon is almost new they call it "Gegenschein" in good old German, a very weak glow on the dark side of the Moon.

Anonymous said...

I love the green shadows in forests. :)

Steve said...

Helpful post, thanks.

This summer I've been trying to capture (in watercolor) shadows of clouds on fields. Something about them strikes me as unlike any other shadow. The other aspect to this is the shadows within the clouds themselves.

~ Rebecca Harbison said...

All of the Apollo landings were done at local morning, so the scenes were shot with a nearly-full Earth in the sky. Much brighter than a moonlit night. (Actually, I came across this trivia at Phil Plait's site,, taking to task the Moon Hoax folks, who claim that the shadows should be pitch black. There's a lot of discussion of lunar shadows on that page.)

jeff jordan said...

What would Howard Pyle say NOW?

Andrew said...

So would it be correct to say that the color of cast shadows are determined mostly by the color of the sky, or, if the sky is heavily blocked, by the color of whatever is blocking it (such as a dense forest canopy.)

I know that seems like a gross oversimplification, but I figured that'd be a good step to understanding it all (not that your post didn't clear up more than a few things!)

Charley Parker said...

Also shadows here on Earth have an element of indirect light reflected and refracted by molecules of air, the same effect that causes the sky to be blue.

That condition doesn't exist in orbit or on the Moon, so only directly reflected light from objects (including the Earth) or perhaps shining from stars (I'm not sure how visible that would actually be) fills shadows with illumination.

BTW, Pyle would be envious of Alan Bean.

Jared said...

Even under dense canopy (coniferous forests in California Sierras) I found to get "neutral" colors of stuff on the ground in the shadows (flowers, mushrooms) I was using Shadow white balance (9000K-10000K) on my DSLR, which indicates to me how strong the blues are in the shadows, even though the canopy is "greenish" (that seems to have no effect on the shadows).

"White" is a loaded word in photography! On cloudy days, the light in shadowy areas is indeed less "blue" (usually 9000-10000K on clear days) but the "white" in and out of the shadows on overcast days is a much cooler (bluer) white (7000-8000K) than the "white" of direct sunlight (5000-6000K):

(Although in terms of blackbody radiation, higher color temperature is hotter and more blue, which is the opposite of the way the terms are used in art)

White balance explained well, at least in digital photography; should apply to art as well:

Steve said...

Charley Parker mentions Alan Bean -- the Apollo astronaut who does paintings recalling scenes on the moon. Going to Charley's blog, Lines and Colors, connects you to an excellent recent post of his about the paintings Bean has been doing. The New York Times website also had a good article about Bean within the last couple weeks.

Roberto said...

That “white” light is bouncing all over the place!
Look at the photo of the dormers- the under side of the eves of the dormer on the right are flooded with light that is bouncing off the shingles of the roof and the side of the dormer. If the roof had red(orange) tiles, that light would be a warm orange (tint).
I love this stuff! -RQ