- published: 09 Jul 2011
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A terminator, twilight zone or "grey line" is a moving line that separates the illuminated day side and the dark night side of a planetary body. A terminator is defined as the locus of points on a Moon or planet where the line through a Sun is tangent.
On Earth, the terminator is a circle with a diameter that is approximately that of the Earth. The terminator passes through any point on the Earth's surface twice a day, once at sunrise and once 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 the Earth experiencing daylight from that experiencing darkness (night). While a little over one half of the Earth is illuminated at any point in time (with exceptions during eclipses), the terminator path varies by time of day due to the rotation of the Earth on its axis. The terminator path also varies by time of year due to the revolution of the Earth around the sun: the plane of the terminator is nearly parallel to planes created by lines of longitude during the equinoxes, and at its maximum angle of approximately 23.5 degrees to the equator during the solstices.
Terminator may refer to:
Solar may refer to: