The terminator or twilight zone is a fictive line that delimits the illuminated day side and the dark night side of a planetary body (also known as the "grey line"). On Earth, the terminator is a circular line 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 terminator is defined as the locus of points on a moon or planet where the line through the Sun is tangent. The line separates the portions of the earth experiencing daylight from the portion of the planet experiencing darkness. While one half of the Earth is illuminated at any point in time (with exceptions during eclipses), the location of the terminator line varies by time of day due to the rotation of the earth on its axis as well as the revolution of the earth around the sun. The terminator line also varies by time of year, with the angle of the line being almost parallel to lines of longitude during the equinoxes, and at its maximum angle of approximately 23.5 degrees during the solstices.
At the equator, under flat conditions (no obstructions such as mountains; or at a height above any such obstructions), the terminator line moves at approximately 1600 kilometers per hour (1000 miles per hour). This speed can appear to be increased when near obstructions—such as the height of a mountain, for example—as the shadow of the obstruction will be broadcast over the ground in advance of the terminator line along a flat landscape. The speed of the terminator line decreases as one approaches the poles, where it can reach a speed of zero (full-day sunlight or darkness).
Concorde and the Tupolev Tu-144 were the only passenger airplanes able to overtake the maximum speed of the terminator. However, slower vehicles can overtake the terminator at higher latitudes, with it 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.
Examination of the terminator can yield information about the surface of the 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.
Amateur radio operators take advantage of conditions at the terminator (so-called "grey line" conditions) to perform long distance communications. Under good conditions, radio waves can travel along the terminator to antipodal points. This is primarily because the D layer, which absorbs High frequency signals, disappears rapidly on the dark side of the terminator line. This process is known as skywave propagation.
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 the minimum influence of the sun, as the designers can opt to install the relevant sensors on the dark side of the satellite.
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