Editing Terminator (solar) (section)

==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/>