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The sky is blue, and everyone knows why (There is a physical phenomenon called Rayleigh scattering that causes light to scatter when it passes through particles that have a diameter one-tenth that of the wavelength/color of the light. Sunlight is made up of all colors of light, but because of the elements in the atmosphere, mostly nitrogen, the color blue is scattered much more efficiently than the other colors).

What people don’t realize is that all that diffraction between air molecules and dust bounces light every which way. If you’re out under the afternoon sun, and you look at the shadow at your feet, it’s a dim outline, but it obscures nothing. Everything that was visible in the light is there in the shadow, and we take it for granted. But on the surface of the moon, where there is not atmosphere to play havoc with the photons, shadows are stark. Broad daylight is completely blanked out by the body in its path.

The Apollo Chronicles from Science@NASA relates the tales of the Apollo astronauts dealing with the tricks of light and shadow. Most are simple things about being unable to read a recessed gauge or the like, but one recollection stood out to me:

Apollo 12 astronauts Pete Conrad and Al Bean landed in the Ocean of Storms only about 600 yards from Surveyor 3, a robotic spacecraft sent by NASA to the moon three years earlier. A key goal of the Apollo 12 mission was to visit Surveyor 3, to retrieve its TV camera, and to see how well the craft had endured the harsh lunar environment. Surveyor 3 sat in a shallow crater where Conrad and Bean could easily get at it⁠—or so mission planners thought.

The astronauts could see Surveyor 3 from their lunar module Intrepid. “I remember the first time I looked at it,” recalls Bean. “I thought it was on a slope of 40 degrees. How are we going to get down there? I remember us talking about it in the cabin, about having to use ropes.”

But “it turned out [the ground] was real flat,” rejoined Conrad.

What happened? When Conrad and Bean landed, the sun was low in the sky. The top of Surveyor 3 was sunlit, while the bottom was in deep darkness. “I was fooled,” says Bean, “because, on Earth, if something is sunny on one side and very dark on the other, it has to be on a tremendous slope.” In the end, they walked down a gentle 10 degree incline to Surveyor 3⁠—no ropes required.

Travel to the moon to be befuddled by a shadow. The things we take for granted …

Related links:
RedOrbit Article

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