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Terminator (solar)

A terminator or twilight zone is a moving line that divides the daylit side and the dark night side of a planetary body. The 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.

For other meanings, see Terminator.
A seasonal animation of the terminator line at sunset over central Europe.

A seasonal animation of the terminator line at sunset over central Europe.

The transition from light to dark takes place on two fronts in this image of Mimas.

The transition from light to dark takes place on two fronts in this image of Mimas.

A photograph of part of the terminator crossing the surface of the Earth, as seen from the ISS. The terminator is diffuse and shows the gradual transition to darkness that is experienced as twilight on the surface.

A photograph of part of the terminator crossing the surface of the Earth, as seen from the ISS. The terminator is diffuse and shows the gradual transition to darkness that is experienced as twilight on the surface.

Boznańska crater near the terminator on Mercury

Boznańska crater near the terminator on Mercury

Scientific significance[edit]

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.[10] 


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.

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Subsolar point

Ground track

Colongitude

Lunar grazing occultation

Lunar phase

Current terminator

Archived 2006-12-05 at the Wayback Machine – Website calculating synthetic images (B&W or color) representing the terminator for a given time (date & hour)

aa.usno.navy.mil

The Moon Terminator Illusion (video)

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