Hey everyone, I would like to find out equation of circle in 3D space. My problem's details are: I have a sphere, and I need to calculate circle equation which its center is on sphere's center. To be clear, think about a sphere in x,y,z coordinate system where it's origin is at 0,0,0. And select a dot on the sphere surface. Now draw a circle which it's origin is at 0,0,0 (same as sphere) and also selected dot proves the circle (circle's at selected dot)... Well how can I calculate this circle's x,y,z values. It would be better if think this sphere at spherical system which is represented by radius r, teta angle, and phi angle. After finding these values I can calculate x,y,z values. I hope it was clear.. Thank you everybody, if you cannot understand let me explain a lil bit more..