Think of it similar to an impostor: in world space, a sphere is always a sphere, and you should do all your lighting in world-space, so you don't need to account for the distortion. You do need to project into the camera's viewing, typically typically the projection matrix. But since a sphere under rotation has the same silhouette, you can also rotate the quad to view different parts of the sphere, and use a circle inside, though the rotation math would have to view a different part of the sphere. To integrate with other objects, you likely would turn on depth writing, and use gl_FragDepth to write the front depth to the buffer, and discard when outside of the silhouette.