Terminator (solar)
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The terminator is the line between the illuminated, day side and dark, night side of a planetary body (also known as the "grey line"). It is defined as the locus of points on a moon or planet where the line through the sun is tangent.
In the frame of reference of a rotating planet, the terminator sweeps across the surface of the planet bringing sunrise on one side of the planet and sunset on the opposite side.
Examination of the terminator can yield information about the surface of the body; for example, a fuzzy terminator indicates the presence of an atmosphere.