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Pilots are taught that one's dark adaptation begins to fall off above 5000 foot altitude. Yet, skies should grow darker with increasing altitude, since there is less absorption of star light by the atmosphere and less atmosphere to scatter light pollution. What is the ideal altitude for visual observing without supplemental oxygen? It may be as low as 7000 feet. No doubt the skies grow darker with increasing altitude, but your ability to perceive fainter objects may be significantly diminished by relative oxygen deprivation to your brain and retina. This will vary from person to person, depending on one's age, general state of health, and the specific health of your eyes.

One factor not to use in estimating sky darkness is how "black" the sky appears. A pitch black sky does not translate into a dark sky. In fact, the darkest skies often have a faint greenish background. How do we know this to be true? One of us (JM) once was on Wake Island in the middle of the Pacific Ocean. There was no light pollution and an incredibly black sky free of clouds. Yet, the limiting magnitude was about 4.5. Why? Because of heavy humidity. The Grasslands Observatory lies at a very dark site with a 5000 foot altitude. When we get out of the car for an evening's observing, the sky seems inky black with the Milky Way boldly standing out. However, a couple hours later when we are thoroughly dark adapted, the sky has a faint greenish glow. The Milky Way blends imperceptibly with the rest of the sky. The Gengenschein is readily discernible, and our visual stellar limiting magnitude estimates are higher, but the sky does not seem as dark.

What is taking place is the Contrast Effect or Contrast Illusion. When you are not dark adapted, your pupil is more contracted and your physiologic mechanisms (eye/brain system) perceive more contrast. This is easy to demonstrate. Notice how difficult it is to walk into a movie theater after the lights have been turned off and the film started. The screen is brilliant, almost blinding; you can not see the aisle to find your seat. After a few minutes, the screen is no longer painful to look at, and you can easily look around the theater and recognize your friends a few rows away.
 

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