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terminator
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moving line that separates the illuminated day side and the dark night side of a planetary body
~7 min read
Encyclopedic overview
Earth's terminator as seen from space A terminator or twilight zone is a moving line that divides the daylit side and the dark night side of a planetary body. The terminator is defined as the locus of points on a planet or moon where the line through the center of its parent star is tangent. An observer on the terminator of such an orbiting body with an atmosphere would experience twilight due to light scattering by particles in the gaseous layer.
Earth's terminator
Excerpted from Wikipedia’s “terminator” article, available under the CC BY-SA 4.0 licence.