Artificial Night Sky Brightness in Europe

Total night sky brightness in Europe accounting for altitude (in V mag/arcsec^2)

Naked eye star visibility in Europe in V mag. 

Maps of the number of visible stars

Colours correspond to ratios between the artificial sky brightness and the natural sky brightness of:

<0.11 (black), 0.11-0.33 (blue), 0.33-1 (green),

1-3 (yellow),

3-9 (orange),

>9 (red). 

These maps do not give information on the star visibility, however mainly polluted areas usually are at sea level, so very approximately we can say that the orange level in our standard scale indicates areas where the milky way is invisible or quite difficult to see by an average observer in normal clear nights. The red areas indicates zones where very approximately one hundredth of stars, or few more, is visible over 30 degrees of elevation. Blue border indicates artificial sky brightness over 10% than the natural brightness which is the definition of "light polluted sky". Yellow indicates an artificial sky brightness equal to the natural so that the total sky brightness is doubled.

Maps show the artificial sky brightness at the zenith in clean nights in V band, obtained by integration of the contributions produced by every surface area in the surroundings of the site. Each contribution is computed taking into account based on Garstang models the propagation in the atmosphere of the upward light flux emitted by the area and measured by the Operational Linescan System of US Air Force DMSP satellites. We account for extinction along light paths, double scattering of light from atmospheric molecules and aerosols, Earth curvature and aerosol content of the atmosphere.

The maps of the the total night sky brightness show the quality of the night sky in the territory. They usually are computed at zenith, accounting for the elevation and the natural sky brightness. In smaller-size maps we also account for screening by mountain and terrain elevation. 

The elevation has effect on the natural sky brightness, on the artificial sky brightness and on the stellar extinction and is obtained from a digital elevation map (DEM). The natural sky brightness depends on the chosen direction of view and on the altitude and it is obtained with Garstang (1989) models which account for the light coming from the entire sky and scattered along the line of sight of the observer and for the given atmospherical conditions. The mountain screening is obtained evaluating the elevation of each pixel along the line which connect each site with each source and then computing the maximum screening angle from which we determine the fraction of the line of sight shielded. This is very time consuming, in particular if the line of sight is not vertical and requires computation for each of its points.  

Darker areas (white color) looks slightly larger in these maps than in maps of artificial night sky brightness. This is an apparent effect due to the large interval of our colour levels (0.5 mag/arcsec^2) which do not show where the artificial sky brightness is a fraction of the natural one.

Levels correspond to total sky brightness  of V mag/arcsec2:

>21.5 white

21-21.5 green

20.5-21 dark green

20-20.5 kaki

19.5-20 yellow

19-19.5 dark yellow

18.5-19 pink

18-18.5 orange

17.5-18 maroon

<17.5 dark red

The maps of the stellar visibility (naked eye limiting magnitude) show the capability of the population to see the stars. They usually are computed at zenith, accounting for the extinction of the star light in the atmosphere from the top of the atmosphere to the observer and the eye capability to detect point sources over a light background. They are unsuitable to evaluate the light pollution of the atmosphere because the elevation and extinction of the light confuse the behaviour.

As an example, the mountains near the top of the image below could appear unpolluted because they show the same limiting magnitude than unpolluted zones of the sea at lover left corner. However the stellar extinction from an elevated site is less than from sea level and the number of particles and molecules which can scatter the artificial light is less too, so limiting magnitude and stellar visibility increase with elevation. In conclusion the similar limiting magnitude in the mountains and in the unpolluted areas of the sea means that the mountains are polluted SO MUCH that the stellar visibility there is comparable with the visibility from sea level.

Levels correspond to V magnitudes:

>6.0 black

5.75-6.0 grey

5.5-5.75 blue

5.25-5.5 light lue

5.0-5.25 azure

4.75-5.0 yellow

4.5-4.75 golden yellow

4.25-4.5 orange

4.0-4.25 dark orange

3.75-4.0 red 

<3.75 violet 

The maps of the number of visible stars show how many stars are visible in the sky. They are obtained from the maps of the naked eye limiting magnitude. The limiting magnitude can be related to the number of star which are visible in the sky but a detailed computation requires an evaluation of the stellar visibility in each direction of the sky and not only at the zenith, accounting for the increase of the stellar extinction with the zenith distance. This requires an incredible amount of computational time. This kind of maps are still in progress. Here below a preliminary version.

The maps of the loss of visible stars are simply obtained by difference between the map of the number of visible stars and a map of the same quantity evaluated assuming no light pollution in the area. Like the maps of the loss of naked eye limiting magnitude, these maps shows better the effects of the light pollution but are less useful to evaluate the capability of the population to see the stars than the maps of the number of visible stars.

Levels correspond to magnitude loss (in V mag.):

<0.1 black

0.1-0.2 purple

0.2-0.4 orchid

0.4-0.6 blue

0.6-0.8 light blue

0.8-1.0 green

1.0-1.2 golden yellow

1.2-1.4 yellow

1.4-1.6 orange

1.6-1.8 red 

1.8-2.0 hot pink 

>2.0 pink

Maps of the loss of naked eye limiting magnitude 

The maps of the loss of naked eye limiting magnitude show the loss of the capability of the population to see the stars. They are obtained simply by the difference between the map of the star visibility and a map of the same quantity evaluated assuming no light pollution in the area. The difference with the previous map is that in this case the effects of the light pollution are clearly visible but these maps are less useful to find the best observative sites.

With many thanks to Istituto di Scienza e Tecnologia dell'Inquinamento Luminoso

Light Pollution Science and Technology Institute for the use of the charts an technical data