> of course if anyone has some knowledge of the "real" mathematical model for a
> twilight sky sphere (and an approximative function) , i ll be glad to hear it !

I looked into this a couple of years ago.  If I recall correctly, a good
approximation can be made by combining a ground fog (Copper can work well) with
a regular gradient on the sky_sphere oriented in the same direction as the sun.
 The gradient's scale and wave form pick the first color in the color_map for
the sky right behind the sun, while the last color shows up on the opposite side
of the sky_sphere (usually below the horizon).  The color_map for the gradient
will vary with the time of day.

The sky sphere simulates the molecular (Raleigh) scattering, while the ground
fog approximates lower-atmosphere aerosol (water and solid particle) scattering.

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