>A circle is defined as all the points in the same distance from a certain center point. I think they also need to be on the same plane, otherwise it's a hollo

>A circle is defined as all the points in the same distance from a certain center point. I think they also need to be on the same plane, otherwise it's a hollow sphere. Also, by that definition, there's an infinity of centers. There can be a line that passes through the center and perpendicular to the plane on which the circle lies. Every point of that line is equally distant from the points of the circle. And for an infinity of planes parallel to our plane of interest, the intersection of the line and that plane gives the center of the projection of our first circle onto that new plane.