Editing Terminator (solar)

{{Short description|Line dividing day and night on a celestial body}}
{{For|other meanings|Terminator (disambiguation){{!}}Terminator}}
[[File:Apollo 11 Mission Image - View of Earth terminator (5052744816).jpg|upright=1.35|thumb|Earth's terminator as seen from space]]
A '''terminator''' or '''twilight zone''' is a moving line that divides the [[daylight|daylit]] side and the dark [[night]] side of a [[astronomical object|planetary body]]. The terminator is defined as the [[locus (mathematics)|locus]] of points on a [[planet]] or [[natural satellite|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==
{{see also|Earth's shadow}}

On [[Earth]], the terminator is a circle with a diameter that is approximately that of Earth.<ref>{{Cite book|url=https://books.google.com/books?id=TiGgJip0l64C&q=earth%27s+terminator+is+circle+size+of+earth&pg=PA67|title=Carbon in the Geobiosphere: – Earth's Outer Shell –|last1=Mackenzie|first1=Fred T.|last2=Lerman|first2=Abraham|date=2006-12-29|publisher=Springer Science & Business Media|isbn=9781402042386|language=en}}</ref> The terminator passes through any point on Earth's surface twice a day, at [[sunrise]] and at [[sunset]], apart from [[polar region]]s where this only occurs when the point is not experiencing [[midnight sun]] or [[polar night]]. The circle separates the portion of Earth experiencing daylight from that experiencing darkness (night). While a little over one half of Earth is illuminated at any point in time (with exceptions during [[solar eclipse|eclipse]]s), the terminator path varies by time of day due to [[Earth's rotation]] on its axis. The terminator path also varies by time of year due to [[Earth's orbit]]al revolution around the Sun; thus, the plane of the terminator is nearly parallel to planes created by lines of [[longitude]] during the [[equinox]]es, and its maximum angle is approximately 23.5° to the [[celestial pole|pole]] during the [[solstice]]s.<ref>{{cite web |url=http://sos.noaa.gov/datasets/Land/day_night.html |title=SOS:Day Night Terminator |access-date=2009-02-06 |url-status=dead |archive-url=https://web.archive.org/web/20090206103757/http://sos.noaa.gov/datasets/Land/day_night.html |archive-date=2009-02-06 }}</ref>

===Surface transit speed===
[[File:ISS flies over Africa, the Mideast, and the Terminator line.ogv|thumb|Video of the Earth from the [[International Space Station|ISS]] as it approaches the terminator.]]
At the [[equator]], under flat conditions (without obstructions like mountains or at a height above any such obstructions), the terminator moves at approximately {{convert|463|m/s|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).<ref>''Venus Revealed'' by David Harry Grinspoon, page 329</ref>

[[Supersonic aircraft]] like [[Jet fighter#Second generation jet fighters (mid-1950s to early 1960s)|jet fighter]]s or [[Concorde]] and [[Tupolev Tu-144]] [[supersonic transport]]s are the only aircraft able to overtake the maximum speed of the terminator at the equator. However, slower vehicles can overtake the terminator at higher [[latitude]]s, and it is possible to walk faster than the terminator at the poles, near to the [[equinox]]es. The visual effect is that of seeing the sun rise in the west, or set in the east.

===Grey-line radio propagation===
Strength of [[radio propagation]] changes between day- and night-side of the [[ionosphere]]. This is primarily because the [[D region|D layer]], which absorbs [[High frequency|high frequency signals]], disappears rapidly on the dark side of the terminator, whereas [[E region|the E]] and [[F region|F layer]]s above the [[D region|D layer]] take longer to form.<ref name=Weiss_2011>Adrian Weiss. (2011). ''Ionospheric Propagation, Transmission Lines, and Antennas for the QRP DXer'', pp.&nbsp;1–16, 1-22&ndash;1-24. Green Valley, AZ: Milliwatt QRP Books.</ref> This time-difference puts the [[ionosphere]] into a unique intermediate state along the terminator, called the "grey line".<ref name=dx_qsl_net>{{Cite web|url=http://dx.qsl.net/propagation/|title=Propagation}}</ref>

[[Amateur radio]] operators take advantage of conditions along the terminator to perform long-distance communications. Called "gray-line" or "grey-line" [[Radio propagation|propagation]], this signal path is a type of [[Skywave|skywave propagation]]. Under good conditions, radio waves can travel along the terminator to [[Antipodes|antipodal]] points.<ref name=dx_qsl_net/>

==Gallery==
<gallery class="center">
File:XEphem-sunset-animation.gif|A seasonal animation of the terminator line at sunset over central Europe.
File:Mimas double terminator.jpg|The transition from light to dark takes place on two fronts in this image of [[Mimas (moon)|Mimas]].
File:Earthterminator iss002 full.jpg|A photograph of part of the terminator crossing the surface of the Earth, as seen from the [[International Space Station|ISS]]. The terminator is diffuse and shows the gradual transition to darkness that is experienced as [[twilight]] on the surface.
File:Boznańska crater EW1023346761G.jpg|[[Boznańska (crater)|Boznańska]] crater near the terminator on [[Mercury (planet)|Mercury]] 
</gallery>

==Lunar terminator==
[[File:Keeler crater AS13-60-8635.jpg|thumb|right|An oblique view of the large lunar crater [[Keeler (lunar crater)|Keeler]] at the terminator (from [[Apollo 13]])]]
[[File:Timocharis crater terminator AS15-P-9423.jpg|thumb|right|The east side of [[Timocharis (crater)|Timocharis]] crater while at the terminator (from [[Apollo 15]])]]
[[File:AS16-124-19897-19901 mosaic.jpg|thumb|right|Mosaic of [[Apollo 16]] photos along the terminator showing [[Darney (crater)|Darney]], [[Lubiniezky (crater)|Lubiniezky]], and [[Bullialdus (crater)|Bullialdus]] craters]]

The '''lunar terminator''' is the division between the illuminated and dark hemispheres of the [[Moon]].<ref>{{cite web |url=http://brahms.phy.vanderbilt.edu/a103/labs/tl_moon.shtml |title=List of basic lunar features |url-status=dead |archive-url=https://web.archive.org/web/20060203074721/http://brahms.phy.vanderbilt.edu/a103/labs/tl_moon.shtml |archive-date=2006-02-03 }}</ref> 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<ref>The [[lunar day]] cycle is 29.53 Earth days in length (see [http://cseligman.com/text/sky/rotationvsday.htm]), so the terminator moves across the lunar surface at 15.4 kilometers per hour.</ref> means it takes longer for it to pass across the surface. At the equator, it moves at {{convert|15.4|kph|mph}}, as fast as an athletic human can run on earth.

Due to the angle at which [[sunlight]] strikes this portion of the Moon, shadows cast by [[impact crater|craters]] and other geological features are elongated, thereby making such features more apparent to the observer. This phenomenon is similar to the lengthening of shadows on Earth when the Sun is low in the sky. For this reason, much lunar photographic study centers on the illuminated area near the lunar terminator, and the resulting shadows provide accurate descriptions of the [[lunar terrain]].

===Lunar terminator illusion{{anchor|Lunar terminator}}===
The lunar terminator (or tilt) illusion is an [[optical illusion]] arising from the expectation of an observer on Earth that the direction of sunlight illuminating the Moon (i.e. a line [[perpendicular]] to the terminator) should correspond with the [[position of the Sun]], but does not appear to do so. The illusion results from misinterpreting the arrangement of objects in the sky according to intuition based on [[planar geometry]].<ref>{{cite web |last=Jones |first=Christopher B. |title=Lunar Terminator Illusion |url=http://chrisjones.id.au/MoonIllusion/|website=Ellipsis: unfinished thought... |access-date=21 May 2016|date=January 2014}}</ref><ref>{{cite web |first1=Andrea K. |last1=Myers-Beaghton |first2=Alan L. |last2=Myers |title=The Moon Tilt Illusion |url =http://www.seas.upenn.edu/~amyers/MoonPaper20June.pdf}}</ref>

==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.<ref>{{Cite journal|last=Furger|first=Markus|date=February 2009|title=Cloud-base or mountain shadow?|journal=Weather|volume=64|issue=2|pages=53|doi=10.1002/wea.352|bibcode=2009Wthr...64...53F|s2cid=120609206 |issn=0043-1656|doi-access=free}}</ref> 

[[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 cell]]s 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==
*[[Subsolar point]]
*[[Ground track]]
*[[Colongitude]]
*[[Lunar grazing occultation]]
*[[Lunar phase]]

==References==
{{reflist}}

==External links==
{{Commons category|Terminator}}
* [http://www.fourmilab.ch/cgi-bin/uncgi/Earth/action?opt=-p Current terminator]
* [http://aa.usno.navy.mil/data/docs/earthview.html aa.usno.navy.mil] {{Webarchive|url=https://web.archive.org/web/20061205203123/http://aa.usno.navy.mil/data/docs/earthview.html |date=2006-12-05 }} – Website calculating synthetic images (B&W or color) representing the terminator for a given time (date & hour)
*[https://www.youtube.com/watch?v=Y2gTSjoEExc The Moon Terminator Illusion (video)]

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[[Category:Earth phenomena]]
[[Category:Light]]
[[Category:Solar phenomena]]
[[Category:Parts of a day]]
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[[Category:Lunar science]]