I am not sure, but I guess this is the reason:

The sky sphere has an infinite distance from the origin. So no matter how
thin the atmosphere is, over this inifinite distance it still accumulates to
block all light.

Try using a real (hollow) sphere instead of the sky sphere. Make it very
large and center it around the origin, so that it contains your scene. Use
your sky texture for it. Then you can vary it's radius until it shines
through the atmosphere just the way you want it.

Johannes.

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