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!
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
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
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.
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
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