Stars are not visible on photos taken on the moon because the photos
didn't have enough exposure. On earth, if you take a picture of the
night sky with a shutter speed of 1/60 second you will not see stars in
the picture, at least not with the kind of camera they had on the moon.
The Apollo astronauts didn't take photos with a shutter speed of several
seconds to see the stars because what they wanted to capture would have
been over-exposed. I see no reason why you couldn't use stars for a
navigation system at 80,000 feet during daylight hours. I don't know at
which altitude that becomes practical.
Alain Fournier
On Apr/1/2023 at 13:40, Jim Davis wrote :
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