Aterminator ortwilight zone is a moving line that divides thedaylit side and the darknight side of aplanetary body. The terminator is defined as thelocus of points on aplanet ormoon where the line through the center of its parentstar istangent. An observer on the terminator of such an orbiting body with anatmosphere would experiencetwilight due tolight scattering by particles in the gaseous layer.

OnEarth, the terminator is a circle with a diameter that is approximately that of Earth.^[1] The terminator passes through any point on Earth's surface twice a day, atsunrise and atsunset, apart frompolar regions where this only occurs when the point is not experiencingmidnight sun orpolar 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 duringeclipses), the terminator path varies by time of day due toEarth's rotation on its axis. The terminator path also varies by time of year due toEarth's orbital revolution around the Sun; thus, the plane of the terminator is nearly parallel to planes created by lines oflongitude during theequinoxes, and its maximum angle is approximately 23.5° to thepole during thesolstices.^[2]

At theequator, under flat conditions (without obstructions like mountains or at a height above any such obstructions), the terminator moves at approximately 463 metres per second (1,040 mph). This speed can appear to increase when near obstructions, such as the height of a mountain, 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).^[3]

Supersonic aircraft likejet fighters orConcorde andTupolev Tu-144supersonic transports are the only aircraft able to overtake the maximum speed of the terminator at the equator. However, slower vehicles can overtake the terminator at higherlatitudes, and it is possible to walk faster than the terminator at the poles, near to theequinoxes. The visual effect is that of seeing the sun rise in the west, or set in the east.

Strength ofradio propagation changes between day- and night-side of theionosphere. This is primarily because theD layer, which absorbshigh frequency signals, disappears rapidly on the dark side of the terminator, whereasthe E andF layers above theD layer take longer to form.^[4] This time-difference puts theionosphere into a unique intermediate state along the terminator, called the "grey line".^[5]

Amateur radio operators take advantage of conditions along the terminator to perform long-distance communications. Called "gray-line" or "grey-line"propagation, this signal path is a type ofskywave propagation. Under good conditions, radio waves can travel along the terminator toantipodal points.^[5]

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  A seasonal animation of the terminator line at sunset over central Europe.
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  The transition from light to dark takes place on two fronts in this image ofMimas.
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  A photograph of part of the terminator crossing the surface of the Earth, as seen from theISS. The terminator is diffuse and shows the gradual transition to darkness that is experienced astwilight on the surface.
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  Boznańska crater near the terminator onMercury

Thelunar terminator is the division between the illuminated and dark hemispheres of theMoon.^[6] It is the lunar equivalent of the division betweennight andday on the Earthspheroid, although the Moon's much lower rate of rotation^[7] means it takes longer for it to pass across the surface. At the equator, it moves at 15.4 kilometres per hour (9.6 mph), as fast as an athletic human can run on earth.

Due to the angle at whichsunlight strikes this portion of the Moon, shadows cast bycraters and other geological features are elongated, thereby making such features more apparent to the observer. This phenomenon is similar to the lengthening of shadows on Earth when the Sun is low in the sky. For this reason, much lunar photographic study centers on the illuminated area near the lunar terminator, and the resulting shadows provide accurate descriptions of thelunar terrain.

The lunar terminator (or tilt) illusion is anoptical illusion arising from the expectation of an observer on Earth that the direction of sunlight illuminating the Moon (i.e. a lineperpendicular to the terminator) should correspond with theposition of the Sun, but does not appear to do so. The illusion results from misinterpreting the arrangement of objects in the sky according to intuition based onplanar geometry.^[8][9]

Examination of a terminator can yield information about the surface of a planetary body; for example, the presence of anatmosphere can create a fuzzier terminator. As the particles within an atmosphere are at a higher elevation, the light source can remain visible even after it has set at ground level. These particles scatter the light, reflecting some of it to the ground. Hence, the sky can remain illuminated even after the sun has set. Images showing a planetary terminator can be used to map topography: the position of the tip of a mountain behind the terminator line is measured when the Sun still or already illuminates it while the base of the mountain remains in shadow.^[10] 

Low Earth orbit satellites take advantage of the fact that certain polar orbits set near the terminator do not suffer fromeclipse, therefore theirsolar cells are continuously lit by sunlight. Such orbits are called dawn-dusk orbits, a type ofSun-synchronous orbit. This prolongs the operational life of a LEO satellite, as onboard battery life is prolonged. It also enables specific experiments that require minimum interference from the Sun, as the designers can opt to install the relevant sensors on the dark side of the satellite.

• Subsolar point
• Ground track
• Colongitude
• Lunar grazing occultation
• Lunar phase

• Current terminator
• aa.usno.navy.milArchived 2006-12-05 at theWayback Machine – Website calculating synthetic images (B&W or color) representing the terminator for a given time (date & hour)
• The Moon Terminator Illusion (video)