See Light Pollution in Action

by Markus Pössel on March 7, 2014

Like anyone else who’s ever looked up at the night sky in any but the smallest cities, I’ve seen light pollution first-hand. Like anyone else even marginally involved in amateur astronomy, I know about the fight against light pollution. And I know that, what with new LED lights and everything, it’s not going to be easy.

When, the other day, I was looking around for images demonstrating the effects of light pollution, it didn’t take me long to find some scary examples – the satellite images tracing human presence on Earth by its light pollution are rather unequivocal, and on Wikimedia Commons, there was an impressive image showing the same region of the night sky when viewed from a dark and from a lighter location:


The images were taken by Jeremy Stanley and are available via Wikimedia Commons under the CC BY 2.0 license. According to the author’s comment, he tried to match the two images’ sky brightness to his memory of how bright the sky appeared to his eyes.

What I didn’t find was an image showing a comparison of two images with the same specs (same camera and lens, same ISO, aperture and exposure time) under different viewing conditions. In the end, I found that I could produce such an example myself, using images I had taken during a trip to South Africa last spring.

During the first leg of our trip, we had visited South Africa’s national science festival, SciFest Africa, which is held annually in Grahamstown in the Eastern Cape Province. Grahamstown has a population of 70.000, and there is some visible light pollution. I took an image of the Milky Way, including the Southern Cross, from the reasonably well-lit courtyard of our hotel:


Some days later, we visited the Sutherland site of South Africa’s National Observatory SAAO, home, among other things, to the 10 m South African Large Telescope (SALT). In the small city of Sutherland, with a population of only about 3000, the observatory a mere 7 miles away and a spirit of cooperation with the astronomers’ needs, light pollution levels are low.

When we took some images of the sky from the backyard of our hotel, the biggest light pollution problem was the moon. Here’s an image that shows, among other objects, the Southern Cross, Alpha Centauri and Carina:


It was only much later that I realized that these images could be used for the light pollution comparison I was looking for. They were both taken with the same camera (Canon EOS 450D = EOS Rebel XSi), the same lens (Tokina 11-16 mm at 11 mm) with the same settings (ISO 1600, aperture 2.8, exposure time 10 seconds). Whatever difference you see is really due to the viewing conditions. To show what you can do with a dark, high-contrast sky, I added a third image. Its only difference to the second image is the exposure time (20 seconds to 10 seconds), which brings out the Milky Way much more strongly.

I combined the images, used GIMP to increase the contrast and saturation on the combined image (to make sure I treated all three images the same), and separated the images again. Here is the result:




The difference between the first two images is fairly drastic. And keep in mind that, as far as light pollution goes, Grahamstown is likely to be fairly harmless, compared with a big, brightly-lit city. (And yes, if I should get the chance, I’ll try to take an image with the same set-up in a larger city!)

This is just one of all too many examples. Through careless lighting, many of us are missing out on one of humanity’s most fundamental experiences: an unobstructed view of the enormity of what’s out there, far beyond space-ship Earth.


Markus Pössel is a theoretical physicist turned astronomical outreach scientist. He is the managing scientist at the Centre for Astronomy Education and Outreach a Haus der Astronomie in Heidelberg, Germany.

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