Limb, Astronomical

Definition

In astronomy, the limb of an object such as the Sun, a planet, or a satellite is the object’s apparent edge as seen against the dark sky background. The solar “limb darkening” refers to the fact that the Sun is less bright (because of radiative transfer effects) toward its edge than toward its center. The limb may be compared to the “terminator.” For a planet or satellite that is not observed at zero phase angle (i.e., at full phase), the terminator is the locus of points on the object where the line to the Sun (or other source of illumination) is tangent; it thus separates the dayside from the nightside: e.g., the moon’s crescent is bounded by the limb on one side and by the terminator on the other.