terminator

Definition: terminator: The dividing line between the illuminated and the unilluminated part of the moon's or a planet's disk. The line of sunrise or sunset on a planet or its satellite. At dawn and dusk when the sun is lowest in the sky (low sun), topographic features cast their longest shadows. This reveals information about the size and shape of the objects casting the shadows. Therefore, features near the terminator are imaged in order to obtain morphologic information.