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Try an experiment. When you are at a dark site and ready to quit
observing for the evening, notice how much contrast the Milky
Way has and how the sky seems to have a slight greenish glow.
You are seeing the natural airglow, a fairly faint phenomenon.
Now walk over and open your car door and flood the area with
some white light. Notice how the sky background immediately
turns black, and the Milky Way suddenly stands out against the
black sky. You obviously can't be seeing as faintly, but the sky
seems darker. In a similar fashion, notice how black the sky
seems when you are driving along and looking at constellations
through the car window. These examples show the Contrast
Illusion at work. To avoid the insidious nature of this effect,
you have to be very careful in describing a dark sky. To prove
one site is darker than another, you must objectively compare
the two by carefully observing selected objects and judging them
on reproducible criteria, such as the faintest stars visible
with the naked eye. A site at 5000 feet with a 20 degree arc of
urban sky glow from a nearby city may be a darker site for the
unpolluted part of the sky than a site at 2000 feet altitude and
no visible sky glow. The first site may not be aesthetically
pleasing because of the visible light pollution, but it may be
the better site overall. Be aware of the Contrast Illusion and
don't let it fool you. |
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January 1, 1999 |
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