Istituto di Scienza e Tecnologia dell'Inquinamento Luminoso

Light Pollution Science and Technology Institute

Frequently Asked Questions

We collected here the most frequently asked questions. So far almost all received questions could have been answered with an accurate reading of our last three papers on Monthly Notices of the Royal Astronomical Society downloadable from www.lightpollution.it/cinzano/papers.html. 

What is the difference between the Atlas and a night-time satellite image of the Earth?

Satellite night-time images of the Earth show the light emitted upward, toward the satellite. Our maps show the effects on the night sky due to the propagation of the upward light flux in the atmosphere and its scattering by molecules and particles. Differences are easily recognizable e.g. in lakes, seas or desertic lands near populated areas, where there are no sources. They are black in satellite images but the sky brightness in our maps is not zero due to light pollution propagation.

What's all that light near the Falkland islands? For a small set of islands with more sheep than people, how can they possibly generate so much light?

Satellite data also record the offshore lights where oil and gas production is active (visible e.g. in the North Sea, Chinese Sea and Arabic Gulf), other natural gas flares (visible e.g. in Nigeria) and the fishing fleets (visible e.g. near the coast of Argentina, in Japan Sea and near Malacca). Note that their upward emission functions likely differ from the average emission function of the urban night-time lighting that we use so that the predictions of their effects have some uncertainty. 

It would have been interesting to have a complete map. Alaska isn't even there!

The presence of snow could make the upward emission function differ from the average emission function of the urban night-time lighting and so it could add some uncertainty (see the discussion in our first paper on Monthly Notices). For this reason, we neglected territories near the poles. In few years we hope to be able to update our method in order to take into account the effects of the snow, so that we will be able to make computations for the missed territories too.

Why should we all stop using any and all outdoor lights immediately, to make things easier for astronomers living in densely-populated areas?

Light pollution is a problem for the population rather than for astronomers. The loss of the perception of the "world" where we live, i.e. the Universe, is a worrying problem. The Milky Way is nothing strange: it is simply the galaxy where we live. So we cannot be surprised if people are impressed when they realize that they cannot see it anymore. Moreover, we do not need to stop all outdoor lighting immediately. Surely it is the most effective measure, but it cannot be proposed in a modern society. An effective measure applied in many countries is the use of fully shielded fixtures taking care to avoid overlighting and to light only where it is necessary.

Why didn't you prepared an atlas of the worldwide pollution?

We are working both to a world atlas of the total light flux emitted upward and to an atlas of the growth rates of of light pollution in the World. Please be patient some years.

I notice that there is a small area of the middle east that has been erased. Political lines are even gone. Is this a mistake?

The computation of the world atlas was extremely time-consuming, so when we divided the World in sub-maps we has been forced to optimize as much as possible their size. This is the annoying reason of the missed area between two sub-images. Fig. 11 of our paper shows the areas covered by the Atlas.

You present in your web site JPEG images. How can images obtained with destructive compression be scientifically correct?

Our scientific result are the original high-resolution maps which can be downloaded. The images on the pages of the web site have been obtained reducing and automatically optimizing them as much as possible with a web-mastering imaging program, Adobe ImageReady, in order to favour the fastest opening of the pages (e.g. Oceania passed from 285MB to 23KB). Before to put the images on line they have been checked for recognizable differences at their scale. However do not expect a scientific rigourous detail at nearly pixel level. If you need to enlarge them and to look in detail, please use the original science images and not the web-optimized images.

I heard that the pictures of Europe were taken at a later date, and they subtracted 20% to compensate for that. It seems to me too much large.

The DMSP data of Europe were taken in the same period than the other data (1996-97). We do not subtracted anything. When comparing our map of Europe with available photometric measurements of sky brightness taken from sites in Europe in 1998-1999, we had to scale back these measurements to 1996-97. We approximately based the rescaling on available mesurements of the growth rate of light pollution in that continent (10% per year, see e.g.  www.lightpollution.it/cinzano/en/page94en.html ). If the growth rate in Europe was smaller, the rescaled measurements would be larger than our predictions and this means that our maps would underestimate the brightness in Europe.

I am an amateur astronomer and last evening I measured a smaller brightness than you predict on the mountain where I usually observe the heavens, at 2000 meters over the sea level. 

Please remember that the maps of the World Atlas are computed for sea level and standard clean atmosphere. They are intended to show the levels of pollution in the atmosphere rather than the stellar visibility. The assumption of sea level allows to compare pollution of different areas, to recognize more polluting sources or darker areas (areas with less light in the atmosphere and not areas where you see better the stars) without be confused by the altitude effects. However you would not see a big difference in the maps accounting for altitude (e.g. for Mount Ekar, near the padana plane, we computed that going from sea level to 1350 m of elevation the artificial sky brightness diminishes less than about 20% whereas color levels in our maps shows each one three times larger brightness than the previous one.)

Why do you used a latitude/longitude projection?

We used latitude/longitude projection because it is a standard in our field. Both radiance measurements, digital population data and digital elevation data use this projection.

You say that you used models that take into account the scattering of light by molecules and particules. Do you mean natural dust or particles of pollution?

I mean the atmospheric gaseous molecules (Rayleigh scattering, the scattering that makes the sky blue during the day) and haze aerosols like dust, vulcanic dust, humidity and similar (Mie scattering, the scattering producing the white-gray colour that you can see looking to the horizon). Even particles of pollution play a role but more likely only at low elevation near the cities.

Colours are representing ratios between artificial brightness and natural brightness. What is natural brightness exactly? The brightness of the Moon?

The natural night sky brightness is the background brightness of the sky where the eye do not detect stars or planets. It is produced by a number of natural processes: light emission in the high atmosphere (due to chemical reactions powered by the sunlight during the previous day), sunlight reflected over interplanetary particles, light from unresolved stars, starlight scattered by intergalactic dust, etc. 

Are there some interesting studies on the consequences of light pollution on man's vision, animal behavior or reproduction ?

The study of the environmental effects of light pollution is a very young branch of science. Look at www.lightpollution.it/cinzano/refer/node8.html for a very preliminar and incomplete reference list of published papers.

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