Hi Pilou,
> I want click annywhere on the Sphere and draw a tangent circle* in one click (the best)! or 2 (challenger) or 3 (last chance) ;)
With just one click you would be only specifying the tangent constraint, there are an infinite number of circles of different radius values that could satisfy this description so you would at least need to specify that.
You can use the Draw curve > Circles > Center command to draw a circle tangent to a sphere, if you place the center point of the circle on to the sphere, you will then be able to pick a radius value and the circle that is created will be on a plane tangent to the sphere at that point.
It's not totally clear to me whether you actually want a circle tangent to a sphere like you're writing (which means on a _plane_ tangent to the sphere, that's what circle from center point will give you) or whether you're looking for a circular segment that is on the surface of the sphere but with the center of the circle actually submerged a distance below the sphere...
Probably the easiest way at any rate is to draw a circle with circle from center point, and then optionally project it on to the surface of the sphere if you need that instead.
 Michael
