Terminator (Solar)

A terminator, also called a twilight zone, is the moving line on a planet or moon that separates its sunlit day side from its dark night side, defined as the set of points where a line to the center of the parent star is tangent to the surface. An observer standing on the terminator of a body with an atmosphere experiences twilight rather than an abrupt change from day to night, because particles in the atmosphere scatter light and smooth the transition. On Earth the terminator circles the planet roughly in half and sweeps across any given point twice a day, at sunrise and at sunset, except in polar regions experiencing midnight sun or polar night; its tilt shifts with the seasons, running nearly along lines of longitude at the equinoxes and tilting about 23.5 degrees at the solstices. The terminator has practical uses too: amateur radio operators exploit grey-line propagation along it for long-distance contacts, and some satellites are placed in orbits that keep them on the terminator for continuous sunlight.