Sphere A:

 

Contrast set to 56%.

Beyond the angle where the light reaches it`s maximum you get

an unnaturally overexposed texture.

This is best visible when using a simple color with no bumps.

Sphere B:
 

Here the contrast is driven by "Angle of Incidence". What it does is provide you with a relatively sharp contrast around the terminator while getting smoother near the 

edge of the sphere.

Of course additional nodes can be added to the function for finetuning.

This function is not needed in any case but it`s useful for moons/planets with a sharp terminator.

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