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die Nullte · Feb 17, 2004, 04:26 pm

</span><blockquote><span class="smallfont">Quote:</span><hr size="1" />Originally Posted by
There are shadows going in different angles in the photos. However, that is because of sunlight reflecting off of the moon itself. <hr size="1" /></blockquote><span class='postcolor'>

Two separate phenomena have been confused here. Some authors have claimed that, because the Sun is the only source of light on the Moon, all shadows should be absolutely black. Not true. The Moon itself is reflective and casts light into the shadows. A different claim has been made that, because the Sun is the only source of light on the Moon, all shadows should be parallel. They are plainly not parallel in the photos taken on the lunar surface by the astronauts, and this has been taken as evidence that there were multiple sources of light, as, say, in a movie studio. Well, for one thing, the Sun's rays are not truly parallel. However, for the sake of argument, we'll concede that, on a local basis and to the perception of nearby observers, the Sun's rays are essentially parallel. But you still have perspective effects -- shadows tapering from nearby to a vanishing point in the distance. This effect has been recognized by artists for centuries, and photographers routinely emphasize this effect for artistic purposes. Anyone can step outside and see that shadows are not parallel when viewed from one location. Also, if there were multiple studio lights, then we'd see multiple shadows. It's unavoidable.

Now, we can't see stars in the photos taken from the Moon because the stars are too faint for snapshot exposures. Most of the astronauts' photos used shutter speeds of 1/125 and 1/250 and apertures from f/5.6 to f/11 -- consistent with daylight photos on the ASA 64 film they were using. Even a photo shot at 1/125 at f/5.6 isn't going to capture stars, even if they are in a black sky. Drop the shutter speed to 1/60 or 1/30 and the aperture to f/2.8 for, maybe, some photos taken in shadows -- still not enough to get stars. I say to anyone who questions this, "Try it!" "Well," someone says, "Aren't the stars much brighter on the Moon than on Earth?" No, they are not significantly brighter than they are from an excellent dark-sky location on the Earth. "Well, then, what's the big deal about the Hubble Space Telescope?" The Earth's atmosphere is turbulent, and this limits the resolution you can achieve with long-exposure images. Hubble gets those amazing images because it rides above this turbulence. As to why the astronauts didn't repeatedly exclaim, "Holy @#$%, look at those stars!" -- well, their eyes were stopped down (to use the photographic term) by all of that bright light off the Moon, so it would have actually been hard to see stars most of the time. I suppose if they had found a spot shaded on all sides they could have seen an impressive view, provided there was no attenuation from their visors.

Feb 17, 2004, 04:26 pm	#18 (permalink) (top)
die Nullte Sedimentary Rock Posts: 9	</span><blockquote><span class="smallfont">Quote:</span><hr size="1" />Originally Posted by There are shadows going in different angles in the photos. However, that is because of sunlight reflecting off of the moon itself. <hr size="1" /></blockquote><span class='postcolor'> Two separate phenomena have been confused here. Some authors have claimed that, because the Sun is the only source of light on the Moon, all shadows should be absolutely black. Not true. The Moon itself is reflective and casts light into the shadows. A different claim has been made that, because the Sun is the only source of light on the Moon, all shadows should be parallel. They are plainly not parallel in the photos taken on the lunar surface by the astronauts, and this has been taken as evidence that there were multiple sources of light, as, say, in a movie studio. Well, for one thing, the Sun's rays are not truly parallel. However, for the sake of argument, we'll concede that, on a local basis and to the perception of nearby observers, the Sun's rays are essentially parallel. But you still have perspective effects -- shadows tapering from nearby to a vanishing point in the distance. This effect has been recognized by artists for centuries, and photographers routinely emphasize this effect for artistic purposes. Anyone can step outside and see that shadows are not parallel when viewed from one location. Also, if there were multiple studio lights, then we'd see multiple shadows. It's unavoidable. Now, we can't see stars in the photos taken from the Moon because the stars are too faint for snapshot exposures. Most of the astronauts' photos used shutter speeds of 1/125 and 1/250 and apertures from f/5.6 to f/11 -- consistent with daylight photos on the ASA 64 film they were using. Even a photo shot at 1/125 at f/5.6 isn't going to capture stars, even if they are in a black sky. Drop the shutter speed to 1/60 or 1/30 and the aperture to f/2.8 for, maybe, some photos taken in shadows -- still not enough to get stars. I say to anyone who questions this, "Try it!" "Well," someone says, "Aren't the stars much brighter on the Moon than on Earth?" No, they are not significantly brighter than they are from an excellent dark-sky location on the Earth. "Well, then, what's the big deal about the Hubble Space Telescope?" The Earth's atmosphere is turbulent, and this limits the resolution you can achieve with long-exposure images. Hubble gets those amazing images because it rides above this turbulence. As to why the astronauts didn't repeatedly exclaim, "Holy @#$%, look at those stars!" -- well, their eyes were stopped down (to use the photographic term) by all of that bright light off the Moon, so it would have actually been hard to see stars most of the time. I suppose if they had found a spot shaded on all sides they could have seen an impressive view, provided there was no attenuation from their visors.