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Light pollution – are we losing the night sky or is there still hope?

I guess it was inevitable that I would eventually write a post about light pollution – the modern day scourge which reduces the visibility of celestial objects and forces astronomers to travel hundreds or sometimes thousands of miles in order to avoid it. There’s even a saying that an astronomers most useful piece of equipment is a car! Probably the most damaging effect of light pollution is not that it makes faint galaxies and nebulae difficult to spot and photograph (there are ways of overcoming this), but that whole generations of children grow up not knowing what a truly dark sky looks like!

Figure 1. The effect of light pollution on the night sky. This split image shows how artificial light washes out most of the faint detail in the constellation Orion.

Figure 1. The effect of light pollution on the night sky. This split image shows how artificial light washes out most of the faint detail in the constellation Orion.

 

I am one of those children. I grew up in suburban England (about 60 miles north west of London) where the night sky had a beige/orange tinge, the constellations were difficult to spot and the Milky Way was something you either looked up in a book or ate. I was about 14 when first I saw a proper night sky; on holiday in North West Scotland. I was so fascinated with the sight that an interest in astronomy embedded itself in me and never left! I was lucky, I was still quite young and my interest could be nurtured before the realities of life (exams, chores, jobs…) stepped in. Many aren’t so lucky. I always wonder, how many inquisitive people never experience the joy of observing the universe because of that orange glowing veil of light pollution (LP). It is the barrier that light pollution creates that prompted me to write this post.

I will now concentrate on the issues LP poses to astronomy. Before I do so, I should say that good evidence exists showing that LP can negatively affect human health (such as disrupting sleep cycles) and the natural environment (changing bird migration patterns etc), detailed discussions can be found here. Regarding astronomy, light pollution is

Figure 2. Direct light pollution. These street lights in Atlanta radiate light across a wide area, stargazing near these will be very difficult. Image taken from http://www.darkskiesawareness.org

Figure 2. Direct light pollution. These street lights in Atlanta radiate light across a wide area, stargazing near these will be very difficult. Image taken from http://www.darkskiesawareness.org

problematic for two main reasons. (1) Unwanted light can travel directly into your eyes ruining the dark adaption they need to observe faint celestial objects. It can also invade telescopes causing washed out images and unwanted glare. This form of light pollution involves light traveling directly from an unwanted light source (such as a street lamp) to your eye/telescope.

The second source of LP comes from the combined effect of thousands of artificial lights, known as sky glow. Sky glow is form of LP most people are familiar with; the orange tinge that

Figure 3. Skyglow in Manchester. This light is scattering off the atmosphere and falling back to the ground. As a result, the sky looks bright orange. Image taken from https://commons.wikimedia.org/

Figure 3. Skyglow in Manchester. This light is scattering off the atmosphere and falling back to the ground. As a result, the sky looks bright orange. Image taken from https://commons.wikimedia.org/

in some places can be bright enough to read by! Sky glow exists because the Earth’s atmosphere is not completely transparent, it contains dust, water droplets and other contaminants that scatter man made light moving through it. Some of this light is scattered back down towards the Earth, it is this scattered light that drowns out the distant stars and galaxies. It is a visual reflection of the amount of wasted light energy we throw up into the sky.

You may be thinking that LP spells the end for astronomy in urban areas. Well luckily there are ways around the problem. One way is to  filter it out. The good thing about skyglow is that it is produced mainly by street lamps that use low pressure sodium bulbs. The light from these bulbs  is almost exclusively  orange with 589nm wavelength. Figure 4 shows a spectrum of the light given out by one of the lamps.

Figure 4 - Different colours of light produced by a typical low pressure Sodium street light. The vast majority of the light is orange (589nm) as shown by the bright orange bar. Image taken from: https://commons.wikimedia.org/

Figure 4 – Different colours of light produced by a typical low pressure Sodium street light. The vast majority of the light is orange (589nm) as shown by the bright orange bar. Image taken from: https://commons.wikimedia.org/

Since this light is comprised of essentially one colour, we can use a simple filter to cut out this wavelength whilst leaving other wavelengths unaffected. In addition, the wavelength of the sodium lights is quite different from the colours produced by many nebulae. Therefore when we filter out the orange light, we don’t also block the light coming from astronomical objects.

So…what am I worrying about then? If light pollution can be overcome by filtering out certain wavelengths of light then astronomy should be possible from anywhere. Well, not quite. Filters are not perfect, even the best filters will block other colours and dim our view of the stars. There is also another reason to worry – street lights are changing. As you may

Figure 5 - LED and sodium streetlights outside my house. LEDs produce light that is harder to block using conventional filters, Sodium lights (seen here as orange) shine lots of light into the sky contributing to sky glow. (Image is my own)

Figure 5 – LED and sodium streetlights outside my house. LEDs produce light that is harder to block using conventional filters, Sodium lights (seen here as orange) shine lots of light into the sky contributing to sky glow. (Image is my own)

already know, street lights are being altered from the sodium bulbs to LEDs. These LEDs are more energy efficient and produce a more natural white light. However, this white light is harder for astronomers to filter out without also blocking light coming from deep space. Luckily, these newer lights are better at directing their glow downwards towards the ground rather than allowing it to leak up into the sky. Figure 5 shows the LED and Sodium lights outside my house. The LED lights appear darker because most of their light is directed towards the ground.

There is still debate in the astronomy community about whether the new street lighting will be beneficial for astronomy. At the moment, LEDs are being introduced slowly so it is difficult to make a clear comparison. My hunch is that when Sodium lights are replaced completely, there will be an improvement in our night skies and finally young people will grow up seeing more of the night sky.

Post by: Daniel Elijah

 

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4 Responses to Light pollution – are we losing the night sky or is there still hope?

  1. LeDoyle Pingel says:

    One advantage to living in rural areas is low LP. In fact many areas have declining population leading to almost no LP. Bitter cold winters in the Midwest leads to spectacular night sky’s. I’m building a remote operated telescope to take advantage of the cold without having fight frostbite. I have read of remote telescopes placed on high mountains for the same reason. With modern equipment the human eye is far too insensitive and crude anyhow.

  2. LeDoyle Pingel says:

    I should have stated that it is Midwest area in USA. Rural America is really quite good for optical telescopes. I suspect the dry air in winter is good for radio telescopes as well.

  3. P Edward Murray says:

    Thank you Mr Daniel Elijah for writing about LP!

  4. G Ortega IV says:

    I agree , people should be aware of the reality of their surroundings, and question the status quo within the education system. However, I stumbled upon this post with a similar question; what is the average nautical distance from a coast to see the night sky without artificial interference (I.e. Sky glow)?

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