The
terminator or
twilight zone is a fictive line that delimits the illuminated
dayOn Earth, daytime is roughly the period on any given point of the planet's surface during which it experiences natural illumination from indirect or direct sunlight....
side and the dark
nightNight or nighttime is the period of time when the sun is below the horizon. The opposite of night is day . Time of day varies based on factors such as season, latitude, longitude and timezone.-Duration and geography:Nights are shorter than days on average due to two factors...
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 regionsEarth's polar regions are the areas of the globe surrounding the poles also known as frigid zones. The North Pole and South Pole being the centers, these regions are dominated by the polar ice caps, resting respectively on the Arctic Ocean and the continent of Antarctica...
, the terminator passes through any point on the Earth's surface twice a day: at
sunriseSunrise is the instant at which the upper edge of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the point at which the sky begins to lighten, some time before the sun itself appears, ending twilight...
and
sunsetSunset is the daily disappearance of the sun below the horizon as a result of the Earth's rotation. The atmospheric conditions created by the setting of the sun, occurring before and after it disappears below the horizon, are also commonly referred to as "sunset".In astronomy the time of sunset is...
.
Now the
locusIn mathematics, a locus is a collection of points which share a property. The term locus is typically used of a condition which defines a continuous figure or figures, that is, a curve...
of points on a
moonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
or
planetA planet , is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
where the line through the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
is
tangentIn geometry, the tangent line to a curve at a given point is the straight line that "just touches" the curve at that point...
.
The
terminator or
twilight zone is a fictive line that delimits the illuminated
dayOn Earth, daytime is roughly the period on any given point of the planet's surface during which it experiences natural illumination from indirect or direct sunlight....
side and the dark
nightNight or nighttime is the period of time when the sun is below the horizon. The opposite of night is day . Time of day varies based on factors such as season, latitude, longitude and timezone.-Duration and geography:Nights are shorter than days on average due to two factors...
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 regionsEarth's polar regions are the areas of the globe surrounding the poles also known as frigid zones. The North Pole and South Pole being the centers, these regions are dominated by the polar ice caps, resting respectively on the Arctic Ocean and the continent of Antarctica...
, the terminator passes through any point on the Earth's surface twice a day: at
sunriseSunrise is the instant at which the upper edge of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the point at which the sky begins to lighten, some time before the sun itself appears, ending twilight...
and
sunsetSunset is the daily disappearance of the sun below the horizon as a result of the Earth's rotation. The atmospheric conditions created by the setting of the sun, occurring before and after it disappears below the horizon, are also commonly referred to as "sunset".In astronomy the time of sunset is...
.
Now the
locusIn mathematics, a locus is a collection of points which share a property. The term locus is typically used of a condition which defines a continuous figure or figures, that is, a curve...
of points on a
moonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
or
planetA planet , is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
where the line through the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
is
tangentIn geometry, the tangent line to a curve at a given point is the straight line that "just touches" the curve at that point...
. 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
eclipseAn eclipse is an astronomical event that occurs when one celestial object moves into the shadow of another. The term is derived from the ancient Greek noun , which is derived from the verb , "to cease to exist," a combination of prefix , from preposition , "out," and of verb , "to be absent"...
s), 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 longitudeLongitude , identified by the Greek letter lambda , is the geographic coordinate most commonly used in cartography and global navigation for east-west measurement...
during the
equinoxAn equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the Sun being vertically above a point on the Equator...
es, and at its maximum angle of approximately 23.5 degrees during the
solsticeA solstice is an astronomical event that happens twice each year, when the tilt of the Earth's axis is most inclined toward or away from the Sun, causing the Sun's apparent position in the sky to reach its northernmost or southernmost extreme...
s.
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).
ConcordeThe Aérospatiale-BAC Concorde aircraft was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...
and the
Tupolev Tu-144The Tupolev Tu-144 was the world's first supersonic transport aircraft , constructed under the direction of the Soviet Tupolev design bureau headed by Alexei Tupolev....
were the only passenger airplanes able to overtake the maximum speed of the terminator. However, slower vehicles can overtake the terminator at higher
latitudeLatitude, usually denoted by the Greek letter phi gives the location of a place on Earth north or south of the equator. Lines of Latitude are the imaginary horizontal lines shown running east-to-west on maps that run either north or south of the equator...
s, 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
atmosphereAn atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
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 radioAmateur radio, often called ham radio, is both a hobby and a service in which participants, called "hams," use various types of radio communications equipment to communicate with other radio amateurs for public service, recreation and self-training....
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 layerThe D region is the portion of the ionosphere that exists approximately 50 to 95 km above the surface of the Earth.Note: Attenuation of radio waves, caused by ionospheric free-electron density generated by solar radiation, is pronounced during daylight hours. Because solar radiation is not...
, which absorbs
High frequency signalsHigh frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
, disappears rapidly on the dark side of the terminator line. This process is known as
skywave propagationSkywave is the propagation of electromagnetic waves bent back to the Earth's surface by the ionosphere. As a result of skywave propagation, a broadcast signal from a distant AM broadcasting station at night, or from a shortwave radio station can sometimes be heard as clearly as local...
.
Low earth orbitA low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
satellites take advantage of the fact that certain polar orbits set near the terminator do not suffer from
eclipseAn eclipse is an astronomical event that occurs when one celestial object moves into the shadow of another. The term is derived from the ancient Greek noun , which is derived from the verb , "to cease to exist," a combination of prefix , from preposition , "out," and of verb , "to be absent"...
, therefore their
solar cellsA solar cell is a device that converts the energy of sunlight directly into electricity by the photovoltaic effect. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the light source is...
are continously lit by sunlight. Such orbits are called dawn-dusk orbits, a type of
sun-synchronousA sun-synchronous orbit is a geocentric orbit which combines altitude and inclination in such a way that an object on that orbit ascends or descends over any given point of the Earth's surface at the same local mean solar time. The surface illumination angle will be nearly the same every time...
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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