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.
Earth's terminator
On
Surface transit speed
At the equator, under flat conditions (without obstructions like mountains or at a height above any such obstructions), the terminator moves at approximately 463 metres per second (1,040 mph). This speed can appear to increase when near obstructions, such as the height of a mountain, as the shadow of the obstruction will be cast over the ground in advance of the terminator along a flat landscape. The speed of the terminator decreases as it approaches the poles, where it can reach a speed of zero (full-day sunlight or darkness).[3]
Grey-line radio propagation
Strength of
Amateur radio operators take advantage of conditions along the terminator to perform long-distance communications. Called "gray-line" or "grey-line" propagation, this signal path is a type of skywave propagation. Under good conditions, radio waves can travel along the terminator to antipodal points.[5]
Gallery
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A seasonal animation of the terminator line at sunset over central Europe.
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The transition from light to dark takes place on two fronts in this image ofMimas.
Lunar terminator
The lunar terminator is the division between the illuminated and dark hemispheres of the Moon.[6] It is the lunar equivalent of the division between night and day on the Earth spheroid, although the Moon's much lower rate of rotation[7] means it takes longer for it to pass across the surface. At the equator, it moves at 15.4 kilometres per hour (9.6 mph), as fast as an athletic human can run on earth.
Due to the angle at which
Lunar terminator illusion
The lunar terminator (or tilt) illusion is an
Scientific significance
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.
See also
- Ground track
- Colongitude
- Lunar grazing occultation
- Lunar phase
References
- ISBN 9781402042386.
- ^ "SOS:Day Night Terminator". Archived from the original on 2009-02-06. Retrieved 2009-02-06.
- ^ Venus Revealed by David Harry Grinspoon, page 329
- ^ Adrian Weiss. (2011). Ionospheric Propagation, Transmission Lines, and Antennas for the QRP DXer, pp. 1–16, 1-22–1-24. Green Valley, AZ: Milliwatt QRP Books.
- ^ a b "Propagation".
- ^ "List of basic lunar features". Archived from the original on 2006-02-03.
- ^ The lunar day cycle is 29.53 Earth days in length (see [1]), so the terminator moves across the lunar surface at 15.4 kilometers per hour.
- ^ Jones, Christopher B. (January 2014). "Lunar Terminator Illusion". Ellipsis: unfinished thought... Retrieved 21 May 2016.
- ^ Myers-Beaghton, Andrea K.; Myers, Alan L. "The Moon Tilt Illusion" (PDF).
- S2CID 120609206.
External links
- Current terminator
- aa.usno.navy.mil 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)
- The Moon Terminator Illusion (video)