Terminator (solar)

# Terminator (solar)

Discussion

Encyclopedia
A terminator, twilight zone or "grey line" is a moving line that separates the illuminated day
Daytime (astronomy)
On 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 night
Night
Night or nighttime is the period of time when the sun is below the horizon. This occurs after dusk. The opposite of night is day...

side of a planetary body. A terminator is defined as the locus
Locus (mathematics)
In geometry, a locus is a collection of points which share a property. For example a circle may be defined as the locus of points in a plane at a fixed distance from a given point....

of points on a Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

or planet
Planet
A 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 a Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

is tangent
Tangent
In geometry, the tangent line to a plane curve at a given point is the straight line that "just touches" the curve at that point. More precisely, a straight line is said to be a tangent of a curve at a point on the curve if the line passes through the point on the curve and has slope where f...

.

## The Earth's terminator

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
Sunrise
Sunrise 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 once at sunset
Sunset
Sunset or sundown is the daily disappearance of the Sun below the horizon in the west as a result of Earth's rotation.The time of sunset is defined in astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west...

, apart from polar region
Polar region
Earth'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...

s where this only occurs when the point is not experiencing midnight sun
Midnight sun
The midnight sun is a natural phenomenon occurring in summer months at latitudes north and nearby to the south of the Arctic Circle, and south and nearby to the north of the Antarctic Circle where the sun remains visible at the local midnight. Given fair weather, the sun is visible for a continuous...

or polar night
Polar night
The polar night occurs when the night lasts for more than 24 hours. This occurs only inside the polar circles. The opposite phenomenon, the polar day, or midnight sun, occurs when the sun stays above the horizon for more than 24 hours.-Description:...

. The two terminators that stretch across the Moon are created by sunshine across the north and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

-shine in the east.
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 eclipse
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...

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: on Earth the angle of the line is almost parallel to lines of longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....

during the equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

es, and at its maximum angle of approximately 23.5 degrees during the solstice
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...

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).

Supersonic aircraft
Supersonic aircraft
A supersonic aircraft is designed to exceed the speed of sound in at least some of its normal flight configurations.-Overview:The great majority of supersonic aircraft today are military or experimental aircraft...

like jet fighters or Concorde
Concorde
Aérospatiale-BAC Concorde 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 Tupolev Tu-144
Tupolev Tu-144
The Tupolev Tu-144 was a Soviet supersonic transport aircraft and remains one of only two SSTs to enter commercial service, the other being the Concorde...

supersonic transport
Supersonic transport
A supersonic transport is a civilian supersonic aircraft designed to transport passengers at speeds greater than the speed of sound. The only SSTs to see regular service to date have been Concorde and the Tupolev Tu-144. The last passenger flight of the Tu-144 was in June 1978 with its last ever...

s are the only aircraft able to overtake the maximum speed of the terminator. However, slower vehicles can overtake the terminator at higher latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

s and it is possible to walk faster than the terminator at the poles, near to the equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

es. The visual effect is that of seeing the Sun rise in the west.

Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...

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
Antipodes
In geography, the antipodes of any place on Earth is the point on the Earth's surface which is diametrically opposite to it. Two points that are antipodal to one another are connected by a straight line running through the centre of the Earth....

points. This is primarily because the D layer
D region
The 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 signals
High frequency
High 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 propagation
Skywave
Skywave 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...

.

## Scientific significance

Examination of a terminator can yield information about the surface of a planetary body; for example, the presence of an atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the 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.

Low Earth orbit
Low Earth orbit
A 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 eclipse
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...

, therefore their solar cell
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....

s are continuously lit by sunlight. Such orbits are called dawn-dusk orbits, a type of Sun-synchronous orbit
Sun-synchronous orbit
A 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...

. 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.