|This article is written from the|
|Real World point of view|
The terminator or grey line or twilight zone is the line between day and night, where light and shadow meet.
This apparent line is caused by the meeting of the far dark side of the Earth that lies obscured by the sun's rays and the area of the Earth currently illuminated by them. This line is not straight, because of the sun's rays bending in the Earth's atmosphere. This means that, contrary to popular belief, there is never a 50/50 split between the area covered in night and the area in day. Instead, the illuminated area possesses greater area because of this bending light. Its position does change shape, however, through the passage of the seasons and even the course of the day. 
Earth is not the only place a terminator can be found. Any planetary object with an obscured side from its local star will have such a line, the same as any object with a light source on one side and a resulting shadow will.