Dusk &
Dawn: The time required by the intersection of the sun hand and the
ecliptic to pass through the light grey zone is the twilight time. Dawn is
given on the left (East), dusk on the right (West). As soon as the
intersection point leaves the twilight zone, it becomes completely dark. The
example shows the difference between December (when the duration of dawn and
dusk are long) and June (when the duration of dawn and dusk are long).
