"True Dark" Skies
Consider the following incident, related by Michael Purcell:
"One afternoon during our trip to New Mexico Skies (in April 2002), I was chatting with a couple of astronomers from New Jersey. The previous evening we had had good observing from before midnight until dawn. They wanted to know what I thought of the previous evening's sky. They had been expecting 'magnitude seven' seeing, and seemed disappointed. One of them had a farm in upstate New York, and thought the skies there were darker than in New Mexico."
This poses a question: what does a really dark sky look like? Amateur astronomers are often happy just to find a site from which to see the Milky Way, especially if that site is within a reasonable driving distance from our light-polluted back yards. Professional astronomers also search for the Holy Grail of dark sky, but their search is a lot more methodical than ours. In an article on sky darkness, Brian Skiff of Lowell Observatory coined the term "true dark" to distinguish night skies that are as unaffected by manmade light pollution as is possible from planet Earth and pass the maximum amount of light from objects in space.
First of all, we have to distinguish "true dark" from a sky that merely appears dark. I've heard people assert that the skies in places like Michigan's Upper Peninsula are darker than they've experienced anywhere else. If that's true, then why aren't any major observatories being built in this part of the country? The fact is that a "true dark" site does not always appear as dark as you might expect.
Due to naturally occurring skyglow, there is an upper limit to how dark the sky can be. According to Skiff, "The main contributions to the natural skyglow are: the zodiacal light, the night-airglow ("permanent aurora"), and scattered starlight in the atmosphere." This is why the night sky far removed from population centers such as in the desert Southwest doesn't seem totally dark. Skiff pointed out that even a site without light-pollution can appear relatively bright to the eye depending on the state of the Earth's geomagnetic field, which affects how the charged particles from the sun interact with the atmosphere. To completely escape these effects you'd have to go into space well out of the plane of the solar system, because even in Earth orbit you'd still have to contend with the zodiacal light. For more details on the Earth's natural airglow and an interesting photo of it taken from the International Space Station, see:
Places that appear really dark to our eyes have enough particulates (aerosols) in the air to filter out the natural airglow. But in doing so, they also filter out some of the photons coming from things like distant galaxies. This is due to "extinction" - the amount of light absorbed by the atmosphere, which is usually measured during photometric observations.
A sky that has the minimum amount of aerosols is said to have good "transparency". This is almost solely a function of altitude; the higher the better. However, for visual observing, if you go too high you actually lose sensitivity because not enough oxygen is getting to your brain. The optimum altitude range seems to be from about 5000 up to 9000 feet. Below 5000 feet, the amount of aerosols in the atmosphere increases dramatically, and above 9000 feet most people have at least mild visual effects from a lack of oxygen.
Skiff explains that extinction arises in the visible part of the spectrum from three main components. First there's "Rayleigh scattering", which happens because the sizes of air molecules are not a lot different from the wavelengths of visible light. The scattering is much higher in the blue than in the red end of the spectrum. This is why the sky is blue: the blue part of sunlight is scattered much more than the redder wavelengths. Rayleigh scattering also depends on altitude: higher places have less air to cause the scattering.
Next is absorption of light caused by the ozone layer, which is concentrated at the base of the stratosphere up about 20km. The main effect here is a small extinction in the yellow-green band. Since this is so high in the atmosphere, it is a nearly fixed extinction for any site on Earth regardless of altitude.
Finally (and probably most important), there are the aerosols, which include dust, humidity, and stuff that humans, volcanoes, vegetation, and other things dump into the atmosphere. Here's where things are variable from site to site and in different seasons of the year. The Southern Hemisphere appears to be cleaner in terms of aerosols, because there is both less human activity and less land from which to generate dust. At the European Southern Observatory at La Silla, Chile, the extinction values are slightly lower than in the U.S. Southwest at the same altitude and they show a smaller inter-seasonal variation. But Skiff adds that the actual difference for visual and photographic observing is very small.
You can get a rough measure of extinction using the "Flagstaff thumbnail test". Examine the sky close to the sun by holding your thumb at arm's length to block the sun. You can also use the edge of a building, tree branch, etc. Note the relative amount and brightness of the scattered light close to the Sun when there are no clouds interfering. Compare the scattered light level with weather patterns (wind direction, humidity, etc.), time of year, and with nighttime sky quality. If this is done at the same time each day, you'll eventually determine the range of variation in extinction and be able to predict the sky quality on the night following. You can also do this with a bright Moon at night.
We've often done something similar to the "thumbnail test" by casually estimating how the night sky will look based on whether the daytime sky is a clear blue or whether it tends toward a sort of milky white. I have often noted the prominent azure blue of the sky in the Southwest, particularly in the mountains of Colorado where there is less windborne dust. This is the sort of true dark sky to which Skiff is referring. Locally, we simply have to find as dark a sky as possible, realizing that it'll take at least a couple day's drive to get anywhere near a "true dark" sky. And then that sky won't really look as pitch black as we might expect.
Our atmosphere protects us from excessive solar radiation and the bombardment of meteoroids, but unfortunately it also results in less than perfect observing conditions. As Michael Purcell suggests, "The only way to get the perfect combination of black skies and dim object observing is to pay a million dollars and visit the International Space Station. Lottery tickets, anyone?"
Brian Skiff's "true dark" article is on the Internet at: