Short text alternative

[Jan Hollan <jhollan@amper....muni.cz>]

Wim, an attempt for an alternative to your proposal. Unfortunately, not 
quite short:

'All the upward light is contributing to the increase of sky glow.

Under overcast sky, the total amount of light coming back down again is 
little dependent on the direction of upward going emissions, just the 
geographic distribution of the light returning to the landscape is, as 
well as the sky luminance distribution as observed from any site.

Under cloudless sky, however, the direction of upward going emissions is 
of paramount importance.

Vast majority of light emitted steep upwards escapes to the Universe. (It 
is an exact analogy of summer noon sunlight going undispersed to the 
ground by vast majority, so that shadows, illuminated by dispersed light 
only, are much darker.) From steep-upward emitted light, just about 8 per 
cent return back to the ground due to dispersion by cloudless air.

Light going upwards almost horizontally, however, is dispersed almost 
completely on its path. (It is an exact analogy of Sun being faint when 
setting down, almost all horizontal illuminance of the ground being due to 
its dispersed light.) Half of that light disperses upwards, half of it 
downwards, as dispersion is symmetric.

So, such slowly climbing light contributes to skyglow six times more (50 ~ 
8x6). Where the skyglow from that light appears, depends on air turbidity. 
With very clear air, it may be as far as 300 km, with very turbid air, 
most of that light becomes dispersed within ten kilometers from the 
source. In any case, skyglow at sites located many tens of kilometres away 
from the light sources depends almost solely on light emitted just several 
degrees above horizon. Reducing this component as much as possible is 
therefore the most important measure to protect the night view of the 
Universe and natural night environment in general.

(Many papers and computer programmes have shown it quantitatively, for 
various atmospheric conditions, within the last ten years -- for those who 
are interested E.g., for lambertian emissions, the total contribution to 
skyglow is thrice lower than for almost horizontal emissions.)'


Jenik