Sky Darkness and the Contrast Illusion
How dark is your sky? This is not an easy question to answer
with exactitude. You first have to decide what you mean by
"dark" and then provide a definitive way of measuring darkness.
It's fairly easy to say the sky in downtown New York City is not
very dark, while the sky atop Kitt Peak is fairly dark, and the
sky atop Mauna Kea is very dark.
It is not known if there is a direct relationship between the stellar limiting magnitude for a given night and site and the limiting magnitude for faint, extended deep-sky objects, such as nebulas and galaxies. Some persons have had the experience of being able to see very faint stars on a given evening but not seeing deep sky objects as well as might be expected. The "seeing" quality or steadiness of the atmosphere may be quite important. A site with steady seeing may allow better visualization of faint objects than one that is intrinsically better (higher altitude and less light pollution) but with poorer seeing.
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
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.
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.
First posted January 1, 1999