Editing Solstice (section)

== Definitions and frames of reference ==
[[File:Seasons1.svg|thumb|The [[seasons]] (with the transition points of the [[June solstice]], [[September equinox]], [[December solstice]], and [[March equinox]]) and [[Earth's orbit]] characteristics.]]
For an observer at the [[North Pole]], the Sun reaches the highest position in the sky once a year in June. The day this occurs is called the June solstice day. Similarly, for an observer on the [[South Pole]], the Sun reaches the highest position on the December solstice day. When it is the [[summer solstice]] at one Pole, it is the [[winter solstice]] on the other. The Sun's westerly motion never ceases as Earth is continually in rotation. However, the Sun's motion in declination (i.e. vertically) comes to a stop, before reversing, at the moment of solstice. In that sense, solstice means "sun-standing".

This modern scientific word descends from a [[Latin]] scientific word in use in the late [[Roman Republic]] of the 1st century BC: ''solstitium''. [[Pliny the Elder|Pliny]] uses it a number of times in his ''[[Natural History (Pliny)|Natural History]]'' with a similar meaning that it has today. It contains two Latin-language morphemes, ''sol'', "sun", and ''-stitium'', "stoppage".<ref>{{cite book
|title=[[The American Heritage Dictionary of the English Language]]
|contribution= solstice
|publisher=[[Houghton Mifflin Harcourt]]
|edition=Fifth
|date=2015
|contribution-url=https://ahdictionary.com/word/search.html?q=solstice
|access-date=December 8, 2015}}</ref> The Romans used "standing" to refer to a component of the [[relative velocity]] of the Sun as it is observed in the sky. Relative velocity is the motion of an object from the point of view of an observer in a [[frame of reference]]. From a fixed position on the ground, the Sun appears to orbit around Earth.<ref>The [[Principle of relativity]] was first applied to inertial frames of reference by [[Albert Einstein]]. Before then, the concepts of [[absolute space and time]] applied by [[Isaac Newton]] prevailed. The motion of the Sun across the sky is still called "apparent motion" in [[celestial navigation]] in deference to the Newtonian view, but the reality of the supposed "real motion" has no special laws to commend it, both are visually verifiable and both follow the same laws of physics.</ref>

To an observer in an [[inertial frame of reference]], [[Earth's rotation|planet Earth is seen to rotate]] about an [[rotation|axis]] and [[orbit]] around the Sun in an [[ellipse|elliptical]] path with the Sun at one [[focus (geometry)|focus]]. Earth's axis is [[axial tilt|tilted]] with respect to the [[orbital plane (astronomy)|plane of Earth's orbit]] and this axis maintains a position that changes little with respect to the background of [[star]]s. An observer on Earth therefore sees a solar path that is the result of both rotation and revolution.

[[File:Solargraph APEX.tif|thumb|left|A [[solarigraphy|solargraph]] taken from the [[Atacama Pathfinder Experiment]] at the [[Llano de Chajnantor Observatory]] in the southern hemisphere. This is a long-exposure photograph, with the image exposed for six months in a direction facing east of north, from mid-December 2009 until the southern [[winter solstice]] in June 2010.<ref>{{cite web
|url=http://eso.org/public/images/potw1039a/
|publisher=[[European Southern Observatory]]
|title=A Solargraph taken from APEX at Chajnantor
|access-date=December 9, 2015}}</ref> The Sun's path each day can be seen from right to left in this image across the sky; the path of the following day runs slightly lower until the day of the winter solstice, whose path is the lowest one in the image.]]

The component of the Sun's motion seen by an earthbound observer caused by the revolution of the tilted axis—which, keeping the same angle in space, is oriented toward or away from the Sun—is an observed daily increment (and lateral offset) of the [[horizontal coordinate system|elevation]] of the Sun at noon for approximately six months and observed daily decrement for the remaining six months. At maximum or minimum elevation, the relative yearly motion of the Sun perpendicular to the [[horizon]] stops and reverses direction.

Outside of the tropics, the maximum elevation occurs at the summer solstice and the minimum at the winter solstice. The path of the Sun, or [[ecliptic]], sweeps north and south between the northern and southern hemispheres. The lengths of time when the sun is up are longer around the summer solstice and shorter around the winter solstice, except near the equator. When the Sun's path crosses the [[equator]], the length of the nights at latitudes +L° and −L° are of equal length. This is known as an [[equinox]]. There are two solstices and two equinoxes in a tropical year.<ref>For an introduction to these topics of astronomy refer to {{cite book
|title=The American Practical Navigator: an Epitome of Navigation
|first=Nathaniel
|last=Bowditch
|author-link=Nathaniel Bowditch
|publisher=National Geospatial-Intelligence Agency
|location=Bethesda, Maryland
|date=2002
|url=http://msi.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/Chapt-15.pdf
|access-date=December 9, 2015
|at=Chapter 15 "Navigational Astronomy"
|archive-url=https://web.archive.org/web/20160208021807/http://msi.nga.mil/MSISiteContent/StaticFiles/NAV_PUBS/APN/Chapt-15.pdf
|archive-date=February 8, 2016
|url-status=dead
}}</ref>

[[File:Length of solar day.png|thumb|450px|Derivative of −Δt, the so-called [[Equation of time]]. The axis on the right shows the length of the [[solar day]], also called the synodic day.]]
Because of the variation in the rate at which the sun's [[right ascension]] changes, the days of longest and shortest daylight do not coincide with the solstices for locations very close to the equator. At the equator, the longest day is around 23 December and the shortest around 16 September (see graph). Inside the Arctic or Antarctic Circles the sun is up all the time for days or even months.