Sky Darkness and the Contrast Illusion

by Tim Hunter and James McGaha

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.

One convenient way to judge sky darkness is to visually estimate the stellar limiting magnitude overhead and at 45 degree angles above the horizon in various directions away from obvious areas of light pollution. The limiting magnitude can be judged with the naked eye or with optical instruments.

It is most important when comparing one site with another or one night with another to be meticulous and consistent in your visual estimates. You should be fully dark adapted and refrain from smoking or drinking for several hours beforehand. It is best to use good star charts that will allow estimates accurate to about 0.1 to 0.2 of a magnitude. You will be very inaccurate if you just look up and say " it seems like a 6.0 magnitude sky tonight."

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 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.

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