Illustration for projecting a circle

I'm a teacher on Descriptive geometry. Now, our topic is how to draw a circle to a given plane. The first step is to rotate the plane to horizontal position, where the circle will look in real size. Parallel projection of a circle is an ellipse in which the main axis equal to circle's diameter. This main axis is derived from the horizontal diameter of the circle and the minor axis is come from the diameter, which is perpendicular to the horizontal one. Ratio of main and minor axes depends on an