“All humans, everywhere in the world and throughout history, have looked up at the sky and wondered at it. This experience is now denied to most people, because of the background light in towns and cities. It is important to ensure that there will be somewhere in England where young people can fully enjoy a cosmic panorama.” – Martin Rees, the British Astronomer Royal, at the opening of the Northumberland Dark Sky Park in 2013.
Kielder observatory- image from https://kielderobservatory.org/
What is light pollution?
Light pollution is the unnecessary and excessive use of light at night. It includes such things as over-illuminating the outside of buildings with bright floodlights, excessive use of bright security lights, lights on in offices at night long after all the occupants have left, brightly lit shop windows still blazing when there are no passers-by doing window shopping, and the use of bright streetlights on quiet rural roads where there is little need for this from a health and safety point of view. All these things contribute to sky glow, when all the light from the various sources passes upwards into the sky, interacting with dust particles and air molecules and is scattered in all directions causing the sky to glow faintly. As a result the sky is never properly dark. .
An observer at ground level (A) sees a dull glow in the sky due to this scattered light.
The image below is a long exposure photograph of New York City and the orange colour of the sky is due to sky glow. Not all sky glow is man made, there is a small natural component to sky glow, caused by effects such as scattered starlight, scattered moonlight and also reflection of sunlight by dust in the Solar System (a phenomenon known as zodiacal light). However all these natural effects added together only give rise to a small fraction of the total sky glow over cities, where the vast majority of sky glow comes from human activity.
Image from Wikimedia Commons
One of the effects of sky glow is that it greatly reduces the number of stars which can been seen on a clear night. On a perfectly clear moonless night well away from light pollution, it is possible for someone with good eyesight to see up to 4,500 stars (King 2014). However, in the centre of a large city such as New York or London, it will only be possible to see around 50 stars – the very brightest ones. The night sky is just too bright to allow fainter stars to stand out and it is impossible to make out the constellations.
Even in smaller towns and villages there is still sky glow from brightly lit roads and buildings. The result of this is that the vast majority of people – probably at least 95% of the population in densely populated countries like the UK – live in a place where they will never have the chance to see the beauty of the Milky Way, the dim band of light arching across the night sky, whose individual stars cannot be distinguished by the naked eye.
This image was produced by NASA from a number of night time images and shows the extent to which the US is lit up at night.
In 1988 International Dark-Sky Association (www.darksky.org) was set up in an attempt to remedy this situation. It is active in a number of areas, including the education of communities and public officials about light pollution and the suggestion of practical solutions which balance the human need for lighting against the human desire to see what exists beyond our lives on the Earth. For example, it both promotes and gives a seal of approval to outdoor lighting fixtures that minimise glare and provide light only where it is needed. Another very important activity of the IDA is its certification of dark sky parks and reserves in fairly remote places, where the public can see the stars free of light pollution.
The Northumberland Dark Sky Park
The Northumberland Dark Sky Park is a 1,500 square kilometre area in one of the remotest parts of England, near the border with Scotland. Compared to England as a whole the area is relatively unpopulated (a mere 2,200 people live in the area) and is home to Kielder Forest, the largest man-made woodland in England, and Kielder Water the largest artificial lake by volume in the whole of the UK. The park is has some of the darkest skies in England and is rated as 3 on the Bortle Scale. Land developers who wish to be active in the area are only given permission to do so if they follow regulations limiting the use of lighting. At the centre of the park is the Kielder Observatory (http://www.kielderobservatory.org/) This houses a collection of small telescopes including two reflecting telescopes with mirrors 16 inches and 20 inches in diameter for use by the public in outreach events.
My wife and I attended such an event a couple of weeks ago on 10 October 2015. I’d chosen this date very carefully in advance for two reasons. Firstly, there would be no Moon visible at night, so the sky would be as dark as possible and we should be able to clearly see the Milky Way. Secondly, on this date it gets dark early, sunset was just after 6 pm, but temperatures are much milder than in winter . We stayed in the small village of Kielder near the observatory and set off for our evening of observing at 7:30 pm. It was interesting to see how incredibly dark it was compared to Manchester as we drove from our bed and breakfast accommodation through the tiny village. The main road was unlit and the few residential side streets were barely illuminated up by a small number of dim streetlights. I doubt if the light levels in these side streets were more than 1% of what they are in our street in Manchester, which is brightly lit by the bright LED lamps which are common in the UK. We left the village on the unlit road, and then turned off to drive for miles on a narrow track through dense pitch black forest. When we arrived at the observatory we were guided to the entrance by volunteers with torches.
I’m very sad to say that this story does not have a happy ending. We didn’t see a thing!! We were at the observatory for three hours and the sky remained covered in thick clouds all this time and there was not a single star to be seen. We had chosen a bad weekend!
So, you don’t just need a lack of light pollution – you also need clear skies, and the northern half of the UK is not renowned for its glorious weather. This is why professional astronomical observatories are situated on high mountains, to get well above the clouds! In my student days as a PhD researcher, I was lucky enough to use the Isaac Newton telescope in an observatory on a mountain in La Palma in the Canary Islands.
Some telescopes at the Roque de los Muchachos Observatory on La Palma in the Canary Island. This observatory at 2,400 m (nearly 8,000 feet) above sea level is above the clouds.
Dark skies are not only good for humans
This post has so far focused only on the impact of light pollution on the human occupants of our planet. However, it also has a negative effect on the behaviour of nocturnal animals. For example, glare from artificial lights can confuse amphibians such as frogs and toads in their wetland habitats. Their nighttime croaking is part of the breeding ritual, and if artificial light means that they do not think it is nighttime, they may fail to perform the necessary courtship rituals to attract a mate, interfering with reproduction and leading to a reduction in populations. You can read more about this here: http://darksky.org/light-pollution/wildlife/.
Reviewed and updated 8 January 2020
King, B (2014) 9,096 Stars in the Sky — Is That All?, Available at:http://www.skyandtelescope.com/astronomy-resources/how-many-stars-night-sky-09172014/ (Accessed: 24 October 2015)