Issues Magazine

Light Pollution

By Nick Lomb

What are the effects of too much light in our towns and cities, and how can that light be reduced? As well as increasing the visibility of the stars and planets, improving lighting has many other benefits.

It is indeed a feeble light that reaches us from the starry sky. But what would human thought have achieved if we could not see the stars …? Jean Perrin, French physicist and Nobel Prize winner

If you have been in the country away from lights on a clear moonless night, you would have seen the full magnificence of the night sky: thousands of bright and faint stars and the white streak of the Milky Way arching overhead. Sadly, most of us live in cities where bright lights blot out all but a sprinkling of stars. It is fortunate that this was not the case in the past: as the French physicist and Nobel Prize winner Jean Perrin says in the quotation above, without the inspiration provided by the sky, humanity may not have reached our present state of scientific, technological and medical advancement.

Let us quantify the effect of light pollution on the night sky. Astronomers indicate the brightness of stars by a quantity called magnitude. The brightest stars are first magnitude or less while the faintest stars visible with the unaided eye under good conditions are sixth magnitude. It is an unusual scale in that the fainter an object, the larger is its magnitude.

The brightest star in the sky is Sirius in the constellation of Canis Major the Great Dog, and that has a magnitude of –1.4. Some of the planets can be even brighter: the cloud-covered planet Venus reached a magnitude of –4.7 in December 2013, triggering numerous reports of strange sightings in the sky.

Too much upward light in cities brightens the night sky and reduces the contrast between stars and the sky. This means that the faintest stars can no longer be seen. Sky brightness and contrast vary depending on the size of the city and the distance of the location from the city centre, but at a typical suburban location in Australia there is no longer any trace of the Milky Way and the faintest star that can be seen is around magnitude 3.5. As the magnitude of the fifth star of the Southern Cross, Epsilon Crucis, is 3.6, from many places only four stars are seen in the Cross, while from others the fifth star can just be glimpsed using averted vision (our eyes are more sensitive to low light when we do not look directly).

Now we can consider how many stars we can see. Catalogues list the number of stars visible to each magnitude limit, and the graph above plots these numbers for stars that can be potentially seen by the unaided eye. From the graph we can estimate that there are 250 stars up to magnitude 3.5. As half the stars will be below the horizon, we get an estimate of about 125 stars visible from a light-polluted spot near the centre of a major city such as Sydney or Melbourne.

Similarly, for a country site with a limiting magnitude of 6.0 we can estimate that about 2400 stars are visible. This is not innumerable, but is still about 20 times more than can be seen from the centre of a city. Of course, a pair of binoculars will dramatically increase the number of stars that we can see even further.

Australia is still one of the better countries in the world as far as light pollution is concerned. Although being able to see only 100 or so stars from the suburbs of Sydney or Melbourne is disappointing, that number is still better than the handful that the people in large overseas cities such as New York, London, Paris or Beijing can see. Moreover, unlike many countries overseas, Australia has vast tracts of land away from towns and cities where the sky is dark, and locals and visitors can experience the full impact of the night sky.

The problem of wasted light and energy is not trivial. The International Dark-Sky Association estimates that there are about 750 million outdoor lighting fixtures worldwide. Many of these fixtures waste energy to the extent that 1.1 petawatt hours of electricity are wasted each year. That energy could power almost eight million homes, but instead puts an unnecessary 680 million tonnes of carbon dioxide into the atmosphere at a cost of about US$110 billion.

Perhaps the only benefit of light pollution hiding all but the few brightest stars from view is that it is easier at a light-polluted location to become familiar with the sky, with its main stars and with its most important constellations such as the Southern Cross, Orion the Hunter and Scorpius the Scorpion. Then when we see the real sky with its 2000-plus stars from a dark site we are not overwhelmed and can find our way among the stars using familiar signposts.

Of course, astronomers do not expect cities to be completely dark. Cities need street lighting, traffic lights, security lights, advertising signs and decorative lighting. However, lighting is often poorly controlled and illuminates more than what it is meant to illuminate. We can see this at any city centre on nights with low clouds as the coloured lights reflect off the clouds, or on plane trips at night we can see the lights of the city below. Why? There is no benefit in illuminating the sky as no one lives there. All that upwards light is wasted light and wasted energy that creates the sky glow above cities associated with light pollution.

Wasted upwards light is not enough to cause light pollution by itself – without a scattering agent it would just travel upwards and out into space. The scattering is provided by particles of pollutants in the atmosphere together with air molecules. These scatter the upwards-directed light in all directions. The colour of the light is important: in a phenomenon known as Rayleigh scattering, short-wavelength blue light is scattered more than longer wavelength red light. Thus upwards-directed blue light has worse consequences for light pollution than red light. The original direction of the light matters as well, with light coming only just above the horizontal scattered more than light going vertically upwards.

In the past there was little artificial illumination. The only night-time illumination came from the occasional candle in the richest homes so that most people adapted their lives to the pattern of day and night. They went to bed when it became dark and rose as the Sun brightened the sky in the morning. They were familiar with the cycles in the sky such as the patterns of the seasons and the phases of the Moon. Anyone planning any night-time activity would schedule it for the time that the Moon was full, since without street lights that was the only time there was light at night. Sadly, people today have lost such an affinity with nature.

Other losers due to night-time lighting are animals that evolved to fit in with the pattern of the motion and visibility of the Sun. Some creatures lose their sense of direction when surrounded by artificial lighting, while some nocturnal animals become increasingly vulnerable to predators as they lose the cloak of darkness. Turtle hatchlings, for example, need to reach the sea as soon as possible to minimise their vulnerability to land-based predators so that they instinctively crawl towards the glittering reflections of stars and moonlight in the sea. With brightly lit developments now common along beachfronts, they crawl in the opposite direction with disastrous consequences.

Scientific studies show that people are also affected by night-time lighting. We all have an internal 24-hour biological cycle called the circadian rhythm. Quickly travelling from one time zone to another leads to jet lag due to the short-term disruption of that rhythm. There are other more subtle ways in which the circadian rhythm can be disrupted. The rhythm is controlled by the hormone melatonin, which is secreted by the pineal gland in the brain. The amount of melatonin is controlled by light, especially blue light. If we have sufficient melatonin we can fall asleep at night, while morning light turns off the supply of melatonin, helping us to wake up.

Studies suggest that disruption of this natural rhythm can have adverse health consequences. Shift workers working at night under artificial illumination are especially at risk, but it is likely that even small amounts of light, especially blue light, could cause problems. The light could come internally from a computer screen or a television set or from outside due to street lights.

Once people are aware of the problem they can easily control internal lighting, but external lighting from street lighting or a neighbour’s poorly installed security light is much harder. External light installations should follow the principles of good lighting:

  • Light should always be below the horizontal to avoid contributing to sky glow.
  • The light source should never be visible to drivers and pedestrians as that causes glare, reducing visibility, especially for older people.
  • Security lights should be controlled by sensors so that they do not waste electricity and disturb people in surrounding properties.
  • Night-time decorative lights should be of low intensity, close to the surface that is to be illuminated, have warm colours (to reduce scattering and sleep disturbance) and be turned off late at night when there are few, if any, people around to appreciate them.

Light-emitting diodes are increasingly replacing older light sources. These are becoming inexpensive, are easily available and use much less electricity than previous sources of light. They provide opportunities to reduce light pollution as they are small and compact, so that it is possible to direct their light only where needed and to avoid glare and wasted upwards light. However, their use is becoming so widespread that you may see your neighbours lighting up their properties without realising that they are disturbing nearby residents and degrading the night-time environment.

Light pollution is gradually reaching distant sites used by astronomers. The main site in Australia for optical telescopes is on Siding Spring Mountain near Coonabarabran in NSW. Even from there, astronomers with their sensitive instruments can detect the lights of Sydney 500 km away, as well as the lights of nearby towns. The local shire council tries to protect the observatory by implementing a lighting code for the surrounding district, requiring that the principles of good lighting be followed throughout the district, especially by people living close to Siding Spring Mountain.

The Australia-wide standard would make a large contribution to improving the night-time environment if it were widely followed. Unfortunately, its use is not mandatory and it is now somewhat out of date as new developments such as LEDs are not included.

Unlike other forms of pollution, light pollution is not permanent. With public education and the widespread adoption of good lighting standards light pollution could be reduced in a way that would be a win–win for all. We could keep our cities and towns as bright and exciting places, yet by improving the quality of lighting increase public safety, security and health as well as darkening the night sky. Maybe in the future we will again be able to see the Milky Way from the suburbs, the way our grandparents did when they were young.

Sydney Observatory is part of the Powerhouse Museum. School groups can come in the day for an educational program that usually involves a digital sky show and, if clear, a safe look at the Sun. Although the observatory is at the centre of a large city, evening visitors can still see the Moon, the planets and the main features of the southern sky through the telescopes.