From AMS Glossary
A blue or bluish gray wedge seen in the antisolar sky between the astronomical horizon and the pink, orange, or purple antitwilight arch.
During clear twilights, the dark segment is visible (assuming a flat horizon) at sun elevations ∼ -0.5° > h0 > -7°. The dark segment is sometimes called the earth's shadow since one of its causes is the sunlight shadow that the earth casts on the nearby atmosphere. However, atmospheric scattering and extinction make the dark segment's luminance and color distribution more complicated than that for simple shadows. For example, the dark segment's vertical width above the antisolar point is consistently greater than
, contrary to the behavior of a simple geometric shadow.
term edited 5 Oct 2015
Reference: R. L. Lee, Jr., "Measuring and modeling twilight’s Belt of Venus," Applied Optics, v. 54 (2015), pp. B194-B203.