28 January 2013

Light Pollution. Do we have to be concerned?

Artificial lighting is one of the most dramatic effects of mankind on the face of the Earth (to understand the scale this timelapse taken on the ISS shows the human settlements on the face of the night-time Earth as eerie filaments dwarfing thunderstorms and comparable to auroras in size). Main concern with astronomical observations is the type of light pollution called “skyglow”, which happens when light from cities is scattered by the atmosphere and redirected towards a distant observer. But the problem and concept of light pollution is much more complex than just an irritation to skywatchers, it seriously affects the natural patterns of humans and animals around the world. (See for example Wikipedia article for more details). Also the amount of energy (and therefore money) wasted to inefficient lighting is tremendous. It is estimated that 1/3 of all the lighting in the USA is wasted, amounting to $2 Billion in electricity cost.


Common measure for skyglow is Bortle Dark-Sky Scale which is a nine-level numeric scale with associated color-coding, 1 (black) being the best and 9 (white) being the worst observing conditions. Shortly put, with level 1 sky it is possible to see faint (up to magnitude 8) objects with naked-eye (e.g. M33 Triangulum Galaxy), and with level 9 sky practically only Moon, planets and few star clusters are visible. Other way to evaluate it is with the limiting magnitude, which tells the magnitude of the faintest star visible by naked eye.
 Many effects increase the amount of perceived skyglow. The cloud cover may cause up to 10 times increase, as was observed in a study (this does not affect astronomical observations, though, since observing is quite difficult under significant cloud-cover). The wavelength-dependent sensitivity of human eye (Purkinje effect) together with wavelength-dependent Rayleigh scattering of the skyglow light cause white or blue-rich light to contribute significantly more to skyglow than an equal amount of yellow light.

The sky isn’t absolutely dark without the artificial light, though. There is always some background light eg. from ionization of solar radiation in high altitudes, as well as scattering of solar light from interplanetary dust particles (“airglow” and zodiacal light). These amount to approx. minimum sky brightness of 22 mag/arcsec.
Some natural phenomena can also cause skyglow, the light from Tunguska event in Siberia 1908 was seen as far as in England for weeks.

Light pollution now and in the future

An Italian research group at the University of Padova has extensively studied the amount of light pollution around the world. Below is a map of Europe. Unfortunately the map data is quite old, from the latter half of 1990’s. And since they estimate that the artificial light amount is increasing in Europe at a average rate of 10% / year, it would imply that all these figures are about 1.5 mag too dim! By this rate clear skies will really become a rarity in Europe in a couple of decades. From the picture it can be seen that the only places in continental Europe where the sky can attain its natural darkness is in northern Scandinavia and in islands far from the continent.

Light pollution in Europe (1997)
In our visit to Sahara Sky hotel in south Morocco, I interviewed the owner about the development of light pollution in the area over past 15 years.

Interestingly, as developed countries are starting to understand and in some cases even decrease the negative effects of artificial lighting, in developing countries the opposite is often the case. The increased lighting is observed as a sign of prosperity and a measure of civilization, and therefore something to aim for. And who actually builds the lighting systems are mostly the organizations/companies from developed countries. And they often make the systems overpowered and with old technology, causing unnecessary light pollution and energy consumption.
For example there was a school built within sight of the hotel with 120(!) bright street lamps lit 24/7 and causing harm to hotel’s star observing. Luckily this was solved by just pointing out how much the lighting will cost to the community as electric bills, as the surroundings is quite poor area in Moroccan backcountry. But in the case of eg. Dubai, there is neither lack of money, nor lack of will to “show to the world what we’re capable of”.
As with many other aspects when bringing the “western” type of culture to developing countries, it’s not necessarily the best way to try to copy everything, sometimes it might be more fruitful to try to preserve something they still have, something that is already lost in the “developed” world.

In this case the unpolluted skies.

Light pollution and physiological effects in humans and animals

One interesting, but unfortunate aspect of increased artificial sky brightness is human adaptation to it. During a large-scale power outage following 1994 Los Angeles earthquake there were numerous phone calls to local observatories from people wondering why the sky suddenly featured “A strange giant, silvery cloud” (aka Milky Way). A quote from FAU webpage : “Like when astrology first started with a mythology about objects in the sky, a modern version of it is the true form of the night sky. “Since so many of us never see a non-light-polluted night sky from one year to the next, a mythology about what the people think a true star-filled sky looks like has emerged.”

Artificial lighting affects animals and plants in various, often fatal, ways. Two examples:
 -- A moth flying into a lamp. It is suggested, that the lamp resembles the Moon to the moth, which it uses as a distant reference point for flying. And since this point should never be reached, the moth will freak out when it suddenly hits the “Moon”.
 -- Lights on tall structures can disorient migrating birds, killing estimated at least from 4 to 5 million birds per year are in US only.

More about light pollution and wildlife in this National Geographic article and Wikipedia.

Efforts to decrease light pollution

Direct skyglow can easily be reduced by selecting lighting fixtures which does not emit light to directions above horizon.
With modern LED technology the direction and intensity of light can more easily be controlled than with older HID (High Intensity Discharge) lamps commonly used with commercial lighting. Problem with LEDs though, might be that since they possess bluer spectrum than incandescent lamps, and since it was mentioned that blue light is more pollutant than yellow, the total pollution may not decrease.

Since the 1980’s there has been a coordinated effort to fight against the light pollution called International Dark-Sky Association, or IDA. In their own words: “The mission of the International Dark-Sky Association (IDA) is to preserve and protect the nighttime environment and our heritage of dark skies through environmentally responsible outdoor lighting.”

In Finnish Lapland there is a convention in popular tourist resort Ylläs to turn off all public lighting from 22-08 to allow people see the Northern lights better. (from an article in finnish about light pollution). With observatories it is a common practise that surrounding public lighting is possible to turn off from observatory or is turned off at a specific time every night, which, unfortunately, is not possible with most amateur observatories.

Even in Sahara desert one cannot escape light pollution: The light in left is probably the city of Zagora (~40 000 inhabitants), about 50 km to north-northwest from the picture location. Small clouds are amplifying the glow effect. The light source in right might be the village of Tamegroute about 40 km to north. The red light is a single faint red LED, for aesthetic and reference purposes.
Exposure time 20s, ISO 12800, f/4.

The developed countries should take light pollution in consideration when building lighting systems in developing countries. The skylight is a precious, beautiful sight and the availability for starry sky should rather be encouraged than discouraged. In the not-so-distant future the only way to see the stars might be above the Earth’s atmosphere, when from the ground the sky would look like the one in Highlander II (a really bad situation to be in a situation of a bad movie).

But, there might be one thing where light pollution would actually be beneficial, (though not necessarily on the face of the Earth); It might be quite effective way to find extra-terrestrial civilizations, and let us transmit our point of origin to other civilizations. Light pollution is, after all, one sign of a (infant) Type I civilization…

Article written by Janne U. Leppäkoski

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