Thursday, December 07, 2023

Is Light Pollution Getting Better?

David J. Lorenz's Light Pollution Atlas 2006, 2016, 2020 includes global light pollution layers for three different years. It also includes a layer which shows where light pollution around the world has become better or worse during 2014-2020.

This 2014-2020 light pollution trend layer shows that light pollution in most of the UK and France and in the eastern U.S. significantly reduced from 2014-2020. This surprised me a little. I live in the UK and anecdotally I haven't noticed street lights being turned off at night or any huge reduction in home lighting. I also don't believe that I can see any more stars at night from my London home, despite the map telling me that the light pollution has been reduced in southern England. 

Intrigued as to why France, the UK and some areas of the U.S. are showing reduced levels of light pollution I asked ChatGPT 'Why has light pollution got better in the France, UK and eastern U.S.?' ChatGPT claims that the main reason for the fall in light pollution is:
Adoption of LED lighting: Many cities and municipalities have been replacing traditional lighting fixtures with energy-efficient LED lights. LED lights can be directed more effectively, resulting in reduced light spillage and wastage. They also typically emit a ‘cooler’ light that is less likely to scatter in the atmosphere, further reducing light pollution.

However most of the respondents on this British Astronomical Association hosted discussion on the Light Pollution Trends map layer seem to believe that light pollution has actually become worse not better in the period measured by the map. Intriguingly a few of the posts on the tread claim that these trends may be more to do with how light pollution is measured than to any real decrease in the actual levels of pollution. 

The light pollution assessments used by the map rely on satellite measurements. In other words they measure light levels from a vantage point looking down on the Earth. Unfortunately amateur astronomers experience light pollution looking in the opposite direction, from the ground up. According to the replies on the BAA chat board it is true that LED lights scatter more in the atmosphere than conventional lights. However this only makes the light less visible for satellites. If anything it actually increases the levels of light pollution for star gazers staring into space from the ground. 

There is one other factor that may have contributed to the measured drop in light pollution in 2014-2020. In the first few months of 2020 countries around the world started imposing Covid lock-downs. A national star count carried out in February 2021 by the Campaign to Protect Rural England (CPRE) revealed a significant drop in light pollution levels across the United Kingdom over the previous year. The CPRE states that the Covid lockdown was the most likely reason for the reduction of light pollution measured in Feb 2021.

It is possible that the reduced industrial and human activity from lock-downs in 2020 did lead to a real fall in light pollution and those results appear in the 2014-2020 light pollution trend map. However if this was true I would expect the map to show light pollution going down around the world rather than in the few pockets that it actually reveals. I guess we will have to wait for a few post-lockdown light pollution measurements to see if the light pollution reductions in France, the UK and the eastern U.S. were just a temporary trend.

The Light Pollution Map is another interactive map which uses satellite measurements to show how people around the world suffer from light pollution. If you want to know what the night sky would like from your home without all this night pollution then you should check out Clear Night Sky.

Clear Night Sky does a very good job at visualizing what urban citizens around the world are missing because of light pollution. In Clear Night Sky the star mapping website Under Lucky Stars has taken 27 night-time photos of cities around the world and 'reimagined' them to show you how they would look if they were free from light pollution. 

On each of these 27 city views you can drag a slider to compare how each city's skyline looks at night (with the effects of light pollution) with how each city would look without the pollution blocking your view of the stars. I think you will agree that all these cities look so much more beautiful when you can actually see the stars shining above them. 

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