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1. Why should you care?

   The Sky is dark at night, right? Well, yes – but just how dark should it be and how dare are we making it?

   When you look up at the sky at night, you may find yourself thinking –  Wow, there are/aren’t a lot of stars out tonight. The truth is – there are always a lot of stars “out” at night. However, depending on your location the problem is not that there aren’t a lot of stars out, but rather what is preventing you from being able to see them?

   Not only you, but think about the many nocturnal animals and ecosystems that depend on a darkly moon-lit environment to hunt for food, hide from predators, or to search for a new habitat in (click the link for more info). For example, think of a rabbit attempting to hide from a predator in the cover of night. This rabbit is running around just outside of a well-lit city in the night time hours. What would normally be a pitch dark sky is now brightened from city lights, preventing the rabbit from finding the coverage needed to quickly escape from predators. Along comes a coyote and well… you can choose how the story ends.

   Or take the coyote for example – under the same brightened night sky (that should otherwise be completely dark) it attempts to hide in order to hunt the rabbit. However, because the rabbit is able to see more clearly than it naturally should – running away from the coyote is easier than normal. Thus, the coyote must exert more energy for hunting which believe it or not – can have other impacts to the surrounding ecosystem.

   While either of these situations could work in favor of/against either the rabbit or the coyote, the point is that unnaturally bright night skies are capable of having unintended consequences on our surrounding ecosystems.

   Too many rabbit-stuffed coyotes means more coyotes around your house. Too few coyotes who are not as able to catch rabbits efficiently means other possible trophic cascades to the area. Take a look at this video to learn about a really interesting example of what a trophic cascades is through an example of one taking place in Yellowstone.

So, now that you hopefully have a better idea of just one small example of how man-made lights in a city can impact more than just our ability to see the stars, check out this study to get an even better idea of what is going on…

2. Why was this study carried out?

The light emitted by cars, buildings, street lights and other man-made sources can do more than just obscure your ability to see the stars. Depending on where you live, what scientists are now calling artificial sky glow can disrupt the livelihood of many nocturnal creatures by reducing the natural darkness of our night time hours. Sky glow occurs when artificial light is reflected into our atmosphere and back down to earth.  To find out just how much artificial light can brighten the naturally dark skies, the authors of the study measured how city lights impacted sky brightness (or sky glow) when compared to the natural light already provided by the moon.

3. How was the study carried out?

Scientists compared the artificial brightness of the skies in three different areas to what they would naturally be like. (Naturally refers to the natural light that is already provided by the different phases of the moon each evening.)

Each location was at a different distance from the first location – Plymouth city centre in the Uk (one was .5 miles away and another at 1.2 miles away). These different areas were chosen to account for possible differences in man-made light being emitted at each area.

The sky glow at the three locations was monitored for ten months. At the same time, the brightness of the moon at each location was also recorded. (This allowed scientists to adjust for any changes in the sky glow due to natural changes in the brightness of the moon.)

In the end, scientists compared the number of “full moon” brightness hours created at each site throughout the ten months. (“Full moon” hours are the hours that the sky would naturally be lit by a full moon. However, for this study, full moon hours were considered based on how bright the skies were at each location to account for sky glow – regardless of wether it was actually a full moon or not.)

4. What were the results of the study?

Scientists found that night time sky brightness was eight times greater in the city centre than at any of the other locations. They were also able to tell differences in sky brightness depending on natural changes in the moon at the two locations further outside of the city centre.

However, it was not possible to tell any differences in changes in sky glow at the city centre. This means that the artificial lights in the city centre increased sky brightness enough to prevent scientists from accounting for natural changes in light created by the moon (so basically, regardless of the time of month – the night skies were so bright in the city centre from man-made mechanisms that it really didn’t matter what phase the moon was in).

Additionally, there were more “full moon” hours recorded in the city center than in the two other locations. The two other locations had fewer “full moon” hours attributed to changes in natural light provided by the moon.

5. Want to know more details?

What do you think? Any questions? Please send them my way in the comment box below and I will do my best to find out the answers for you or to just respond back.

Also, check out the original article, Artificial Light Alters Night Time Regimes of Night Time Sky Brightness if you would like to view a more detailed version of the information summarized in sections 2-4 above.

Authors: Thomas W. Davies, Jonathan Bennie, Richard Inger and Kevin J. Gaston.

Published: April 2013 by Scientific Reports