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. Apart from polar regions, the terminator passes through any point on the Earth's surface twice a day: at sunrise and sunset.
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 solstices. 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.
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 solstices. 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.
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