Lunar terminator

A terminator or twilight zone 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 its 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, 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 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 pole during the solstices.

At the equator, under flat conditions (no obstructions such as mountains; or at a height above any such obstructions), the terminator moves at approximately 1668 kilometers per hour (1036 miles per hour). This speed can appear to increase when near obstructions—such as the height of a mountain, for example—as the shadow of the obstruction will be cast over the ground in advance of the terminator along a flat landscape. The speed of the terminator decreases as it approaches the poles, where it can reach a speed of zero (full-day sunlight or darkness).

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