Light Pollution

Almost everyone observing the night sky in developed countries has to deal with light pollution. Light pollution pretty much scales with electric lighting, which scales as population times wealth. Skies in Colorado are on average much brighter than those in Wyoming, and parts of India have dark skies even though lots of people live there. In the US, there are hardly any dark skies left east of the Mississippi, and of course the west coast is also pretty well lit.

There are fantastic maps of the brightness of the night sky produced at the University of Padua, Italy using satellite (DMSP) measurements of upwelling visible light, which is then used as input to a radiative transfer code that integrates illumination scattered from all sky directions back to the surface at each point on Earth.

The resulting World Atlas of Artificial Night Sky Brightness predicts the downwelling visible radiation of the clear moonless night sky everywhere. You can read about the details of the calculation in this article, obtain detailed maps of your area from cleardarksky.com, or download data layers to use in Google Maps.

The sky brightness is expressed in apparent magnitude per square arc second, but almost all amateur astronomers use a number and color scale attributed to John Bortle. I’m fortunate to live in a place where I can get to very dark skies in a couple of hours drive, but that’s pretty unusual. The Colorado Front Range is an island of bright lights in the sea of High Plains and Intermountain darkness.

Even though I have this nice cabin two miles above sea level 150 miles from Denver, I only get to observe and image under those pristine skies (Bortle 2) maybe 20 nights a year due to work, family, weather, and the Moon. I look at the sky pretty much every night from my back yard, and on clear moonless nights I can (barely!) see all the stars in the Little Dipper, so my limiting magnitude in the north is about 5. It’s much worse in the south because the whole Denver urban corridor shines down there. I’ve never seen the Milky Way from my house (Bortle 7). Using binoculars, I can find most Messier objects on clear nights. It’s fun to keep track of the stars and hang out in the yard with the dog before going to bed, but it’s not usually worth spending more than a few minutes minutes each night.

My observatory faces the back of a school across a concrete parking lot. This means I have a big problem with very local light pollution. Using a rotating dome helps a lot, by obstructing lights from shining directly into my telescope. As you can see from the galleries here, it’s still possible to image the cosmos from this very urban setting, but the signal-to-noise ratio is weak. A lot of faint details are lost, and the post-processing is all about subtracting gradients in background light pollution. The images from the cabin are way easier to process, and the results are way nicer. Sigh.

On a moonless summer night at the cabin (Bortle 2), the Milky Way just dominates the sky. The central disk of the Galaxy fills the south, and clouds of interstellar dust 25,000 light years deep color it a distinct ruddy brown with smaller dust lanes snaking out into spiral arms glittering with steely-blue clusters of bright young stars. The whole structure of the Galaxy is right there, in your face, filling half the sky. On autumn nights the Andromeda Galaxy hangs in the eastern sky, five times the size of the full Moon, 2.5 million light years away. I can see the intersecting planes of the Solar System and the Galaxy. Sometimes it seems I can feel the spin of the Earth, the orbits of the Moon and planets, and the slow turn of the Galaxy itself.