A composite image (simulated earth) showing the terminator dividing night from day, running across Europe and Africa. Observers on the surface of the earth along this terminator will see a sunset.
Photograph of part of the terminator crossing the surface of the Earth, as seen from the ISS. The terminator is diffuse and shows the gradual transition to darkness that is experienced as twilight on the surface.
The transition from light to dark takes place on two fronts in this image of Mimas. The two terminators that stretch across the moon are created by sunshine across the north and Saturn-shine in the east.
The orientation of the terminator between the northern and southern hemispheres depends on the season. On the spring and fall equinoxes (around March and September 21), there is no tilt of the earth with respect to the sun so the terminator line is parallel with the axis of the earth and with the lines of longitude.
The terminator on January 3. Note the extreme difference in the amount of light in the Northern Hemisphere compared to the Southern Hemisphere.