Sky maps for Apr 6, 2011, 21 h local mean time 
(i.e., ‘winter’ time at the proper meridian). 

<p>
Files *A.pdf are sets of all-sky maps for various latitudes. 
Each set is a sequence of five maps showing the sky 
for growing detection limits. The detection limit depends on the sky luminance,
which is much larger in cities than in true nature. Star areas are proportional
to their brightness. File allpdf.zip contains all of them. 

<p>The maps are computed assuming a chosen (quite common) 
attenuation of starlight by the atmosphere. The attenuation depends on the
air transparency. The parameter of the chosen attenuation is such, that
the stars are assumed to be attenuated by 0.25&nbsp;mag when in zenith 
(so called zenith extinction). Then 
at 30&deg; they are attenuated by 0.5&nbsp;mag etc., depending on the amount
of air between the observer and the star. With excellent transparency 
(zenith extinction of just 0.18&nbsp;mag), starlight would be a lot stronger 
near horizon. 

<p>The limit given in the right bottom corner is for the faintness 
of the starlight as attenuated by the atmosphere, for the stars as they appear.