Limb (Astronomical)

Synonyms

Terminator

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. Thus, solar “limb darkening” refers to the fact that the Sun is less bright (because of radiative transfer effects) toward its visible 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 day side from the night side.