TERMINATOR

The line along a planetary body that divides the daytime side from the nighttime side. An observer along the terminator would view either a sunrise or a sunset. Due to the angle of illumination shadows cast by mountainous terrain or crater rims will be elongated and more visible giving a more 3D visual effect to an external observer. The full Moon has, in effect, no terminator which is why lunar imagery is rarely taken at that time. The lack of shadows due to the Sun’s light hitting the Moon from directly above leaves the Moon looking flat and featureless.