The term limb darkening refers to the outer edge of a body (such an outer edge is referred to as a limb) as seen from a distance appearing darker than the area between. An example of such a body is the Sun. A key to the phenomenon is a somewhat fuzzy outline, where light is generated by or passing through a medium that is not totally transparent. By studying the apparent brightness across the visible surface, conclusions can be drawn about the atmosphere. Less brightness at the limbs suggests the light from inside has to pass through more of this not-quite-transparent atmosphere, so the amount and position of the darkening gives clues to the atmosphere's thickness, makeup, and layering.