A Sky with Fewer and Fewer Stars

By Agence Science-Presse

The nighttime sky is less and less starry: urban lighting has reduced the number of stars we can see at night by 7 to 10% each year between 2011 and 2022, according to data from over 51,000 amateur astronomers around the globe.

These “citizen-scientists” are among those who have participated over the past 14 years in the international Globe at Night project. It aims to raise awareness of the impact of light pollution. Participants submit their observations of which stars are visible to the naked eye in the night sky from where they are. Four researchers from Germany and the United States had the idea to create this data base to see if a trend could be detected. Their analysis was published in January in the journal Science.  

The authors write that by comparison, someone who could see 250 stars in their night sky in the year 2000 can only see a hundred from the same spot today.

The only real, hard data we have about light pollution comes from certain satellites. The estimates by satellites are more optimistic than those from the Globe at Night study: they indicate an increase in light pollution of 2.2% per year between 2012 and 2016. The researchers explain this difference in part by pointing out that satellites can’t detect blue light from LEDs (light emitting diodes) that have begun to be used outside in the last decade or so.

It must be noted that this estimate of a reduction of 7 to 10% in visibility is an international average, but one that applies mostly to Europe and North America, where most of the 51,000 astronomy enthusiasts hail from. Over the past 15 years, some countries have introduced rules to reduce nocturnal lighting. Some have even created regions called “dark sky preserves,” characterized by a severe limitation of light pollution. However, these efforts have mostly been in Europe or North America. It hasn’t stopped observers from reporting a deterioration in the star viewing situation.

Light pollution isn’t just a problem for stargazers. It’s also an environmental problem, two Spanish physicists write in a commentary accompanying the article. A growth in artificial light at night causes changes in an ecosystem – predators can take advantage, and prey can become endangered species. It also reduces the production of melatonin, a hormone that controls biological clocks, ours as well as those of animals. They argue that there should be limits placed on light pollution equivalent to those placed on air pollution as of the 1970s.