Post by Chicago Astronomer - Astro Joe on Jan 5, 2006 2:43:40 GMT -6
Shadows on the Moon
(Jan 4, 2006) When the Apollo astronauts stepped out onto the lunar surface, they were the first human eyes to see this alien landscape. And one of the strangest things that they saw were the shadows. On Earth, our shadows aren't black, but blue, thanks to the scatter light passing through our atmosphere. But on the Moon, which has no atmosphere, the shadows are utterly black. The darkness of the shadows was one of the first things that Neil Armstrong noticed as he stepped off the lunar module and onto the surface of the Moon.
Consider the experience of Apollo 14 astronauts Al Shepard and Ed Mitchell:
They had just landed at Fra Mauro and were busily unloading the lunar module. Out came the ALSEP, a group of experiments bolted to a pallet. Items on the pallet were held down by "Boyd bolts," each bolt recessed in a sleeve used to guide the Universal Handling Tool, a sort of astronaut's wrench. Shepard would insert the tool and give it a twist to release the bolt--simple, except that the sleeves quickly filled with moondust. The tool wouldn't go all the way in.
The sleeve made its own little shadow, so "Al was looking at it, trying to see inside. And he couldn't get the tool in and couldn't get it released--and he couldn't see it," recalls Mitchell. "Remember," adds Mitchell, "on the lunar surface there's no air to refract light--so unless you've got direct sunlight, there's no way in hell you can see anything. It was just pitch black. That's an amazing phenomenon on an airless planet." (Eventually they solved the problem by turning the entire pallet upside down and shaking loose the moondust. Some of the Boyd bolts, loosened better than they thought, rained down as well.)
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.
The full facinating story here at Universe Today: www.universetoday.com/am/publish/apollo_chronicles_moon.html?412006
===========
This was a very intriguing story of lunar shadows and how dark is really dark.