A Dark Sky

What is the value of a dark Sky? What is a dark sky?

Most of us live surrounded by sources of man made light. At night rather then helping us to see, these lights actually limit our ability to see by destroying our night vision. Our eyes make a chemical, lets call it visual purple, which allow us to detect light. This chemical is broken down by the light, which allows us to see. If there is a lot of light, a lot of the chemical is broken down. Sure, we can see where the light falls but not beyond that.

However, if we turn off the lights or go to where we cannot see the lights an amazing thing happens. The amount of visual purple in our eyes builds up and we can see fainter and fainter light. I believe that it takes an hour to fully dark adapt. But it is this process of dark adaptation that allows us to see fainter and fainter objects. But if we look a close by light (car, neighbor’s porch light) the visual purple will break down and we will loose our dark adaptation and our ability to see faint light.

When looking at the sky it is good to try and avoid or hide from near by light sources (local light) so as to be able to see more in the sky.

However, the closer that you live to a developed area there is also light reflected by the atmosphere to deal with. This seems to be more of a problem when it is humid. But look back at any city from a rural site and you’ll see a light dome projecting above the city from all of the lights down below. This reflected light competes with "star light" and limits our ability to see fainter objects. Diffuse or "spread out" objects, such as nebulae and galaxies, are affected more then "point sources" such as stars. The more lights a city has, the more the sources of light in the sky are hidden by the city’s light being reflected off of the atmosphere above it. In the largest cities only the moon, a few planets, and the brightest stars can be seen

Star brightness is listed on a scale of magnitude. As the magnitude number increases, the brightness on the star decreases. Each magnitude is 2.5 times dimmer then the preceding magnitude. So a magnitude 6 star is 5 times fainter then a magnitude 4 star. Many feel that to begin to really see the objects that lie beyond our solar system and those beyond our galaxy you should be able to see stars in the sky that are of magnitude 5 or brighter (like those shown on Orion’s deep map 600). But the further that you move from man made light, the better. Magnitude 6 sky (i.e. a sky where you can see a magnitude 6 star after you have dark adapted) will show more then a magnitude 5 sky. Astronomers long for magnitude 7 or better sky. For when the sky is truly dark, the only light that you see is star light and your eyes can fully adapt to the darkness. In this setting, the objects beyond our solar system reveal detail hidden under city skies. Objects unseen in the city are visible to the naked eye. Galaxies, nebulae and star clusters are seen with sharp contrast and can be savored – allowing the heart to soar.

But we Americans leave our lights on:

So we must drive far from the maddening crowd to find peace, solitude, and dark sky.

Want to learn more about light pollution? Visit the International Dark-Sky Association (IDA) page