A terminator or twilight zone is a moving line that divides the daylit side and the dark night side of a planetary body. A 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.
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.
At the Equator, under flat conditions (without obstructions like mountains or at a height above any such obstructions), the terminator moves at approximately 463 m/s. 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).
Supersonic aircraft like jet fighters or Concorde and Tupolev Tu-144 supersonic 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 higher latitudes, and it is possible to walk faster than the terminator at the poles, near to the equinoxes. The visual effect is that of seeing the sun rise in the west, or set in the east.
Strength of radio propagation changes between day- and night-side of the ionosphere. This is primarily because the D layer, which absorbs high frequency signals, disappears rapidly on the dark side of the terminator, whereas the E and F layers above the D layer take longer to form. This time-difference puts the ionosphere into a unique intermediate state along the terminator, called the "grey line".
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 of skywave propagation. Under good conditions, radio waves can travel along the terminator to antipodal points.
The transition from light to dark takes place on two fronts in this image of Mimas.
The orientation of the terminator between the northern and southern hemispheres depends on the season. On the spring and fall equinoxes (around March and September 21), there is no tilt of the Earth with respect to the Sun so the terminator line is parallel with the axis of the Earth and with the lines of longitude.
The lunar terminator is the division between the illuminated and dark hemispheres of the Moon. It is the lunar equivalent of the division between night and day on the Earth spheroid, although the Moon's much lower rate of rotation means it takes longer for it to pass across the surface.
Due to the angle at which sunlight strikes this portion of the Moon, shadows cast by craters 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 the terrain.
The lunar terminator (or tilt) illusion is an optical illusion arising from the erroneous expectation of an observer on Earth that the direction of sunlight illuminating the Moon (i.e. a line perpendicular to the terminator) should correspond with the position of the Sun, but does not appear to do so. The cause of the illusion is simply the observer not taking into account that the observed slope of a light ray will change across the sky because of the lack of visual clues to establish 3D perspective.
Examination of a terminator can yield information about the surface of a planetary body; for example, the presence of an atmosphere 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.
Low Earth orbit satellites take advantage of the fact that certain polar orbits set near the terminator do not suffer from eclipse, therefore their solar cells are continuously lit by sunlight. Such orbits are called dawn-dusk orbits, a type of Sun-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.