terminator

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 On Earth, the terminator is a circle with a diameter that is approximately that of Earth. The terminator passes through any point on Earth's surface twice a day, at sunrise and at sunset, apart from polar regions where this only occurs when the point is not experiencing midnight sun or polar night. The circle separates the portion of Earth experiencing daylight from that experiencing darkness (night). While a little over one half of Earth is illuminated at any point in time (with exceptions during eclipses), the terminator path varies by time of day due to Earth's rotation on its axis. The terminator path also varies by time of year due to Earth's orbital revolution around the Sun; thus, the plane of the terminator is nearly parallel to planes created by lines of longitude during the equinoxes, and its maximum angle is approximately 23.5° to the pole during the solstices. Surface transit speed