limb

The term limb refers to the edge of an astronomical object as viewed, i.e., the ring-shaped region consisting of the outer edge of the visible disk for spherical objects such as the Sun or Earth. Often, the term horizon can be equally-well used. Analysis of the limb can yield information about the object, particularly its atmosphere, if the limb is generating or transmitting EMR, and the term limb is used in discussions of such analysis. Limb darkening refers to a characteristic of an object (such as the Sun) in which the intensity from the limb is less than from the viewed center of the object, and is of interest because it can reveal the depth within the object from which various amounts of its EMR emission is produced.