A parabolic mirror brings parallel light incident along its axis to a single focus. The Sun appears to us as a disc of apparent angular diameter α = 0.5°. Thus every element of surface of a parabolic mirror illuminated by the Sun acts as a source of a diverging cone of rays. A parabolic mirror will therefore produce an image of the Sun of finite size at its focus. If the atmosphere is even slightly dusty this image will appear as a small sphere of light hovering in space at the focus.