Saturday, August 23, 2014

Are Your Clouds at Night Bright or Dark?

My day job is working for the International Dark-Sky Association, where it is our mission to fight light pollution. I didn't need it, but I had a pretty vivid reminder of what we are fighting for on my recent vacation in Hawai'i.

My wife and I rented a place in the Puna District of the Big Island, right on the ocean, where there is very little artificial light. The first night there I hadn't yet adjusted to the time change and woke up hours before sunrise. When I realized that I wasn't going to be able to get back to sleep, I grabbed my binoculars and stepped in the dark to the beach-facing balcony. There the glittering majesty of the night sky was an awesome thing to behold. Ocean waves crashed below but my attention was on the sky. The stars of late fall and early winter filled the sky, all the way down to the horizon.
Venus rises, with Orion at right
It has been quite some time since I experienced skies so dark. When Venus finally poked up above the distant clouds its light quite noticeably brightened the deck.
In the evening skies the summer Milky Way (above) was brilliant with its bright 'clouds' of stars and dark lanes of dust instantly visible - no dark adaptation was needed.

Occasionally clouds drifted past and I was reminded of a paper published two years ago, Red is the new black, how the color of urban sky glow varies with cloud cover (pdf). The paper points how how in cities with sky glow clouds actually magnify the problem, but in times before the advent of artificial light skies actually got darker when it was cloudy. There are a lot of implications from this (read the paper), but let me illustrate the phenomenon.
Sagittarius and Scorpius with dark clouds in silhouette
Compare the view (above) that I took last month in Hawai'i with dark clouds silhouetted against the star-filled sky, with this shot (below) that I took last night from my home near Tucson, Arizona.
Sagittarius in cloudy, light polluted skies
The difference is shocking. The two photos were taken on moonless nights, just about 1 month apart with the same camera and settings looking at the same part of the sky.

Even though there are relatively strict outdoor lighting ordinances in Tucson/Pima County, the skies here are awash with light pollution and sky glow. Sadly, many communities have little or no controls of outdoor lighting and there skies there are even worse. The clouds in the photo above are white, almost as if illuminated by moonlight. The likely culprit illuminating the clouds with white light is a sports complex to my south. Clouds in other directions had a more orange hue, as in the photo below:
Cygnus in light polluted, party cloudy skies
The photos show that there is much work to be done in controlling light pollution, even in communities where the effort is underway. Learn more about the problem and what you can do about it at

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