I was reading Scott Lowther's "Origins and Evolution: Lockheed SR-71
Blackbird" (highly recommended, by the way) and came across this
statement referring to the SR-71's astro-inertial navigation system:
"For an aircraft operating about 80,000ft, stars would be visible at all
hours of the day. At that altitude the sky is black, not blue; the
aircraft appears to be in space."
My first thought was that this couldn't be right; I was reminded of
claims by moon hoaxers of why no stars were visible in photos taken on
the moon. The lunar surface is so bright during the lunar day that it
drowns out any starlight.
Thinking on this further I began to have doubts. At 80,000 ft there is
no dust to scatter sunlight and perhaps the aircraft would be far enough
above the earth that the reflected light from the surface wouldn't be
that bright.
Can stars be tracked at that altitude during the daylight hours? If so,
at what altitude does this become practical?
Jim Davis
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