UT date and time of
solstices on Earth[1][2]

A solstice is an event occurring when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. The seasons of the year are determined by reference to both the solstices and the equinoxes.

The term solstice can also be used in a broader sense, as the day when this occurs. The day of a solstice in either hemisphere has either the most sunlight of the year (summer solstice) or the least sunlight of the year (winter solstice) for any place other than the Equator. Alternative terms, with no ambiguity as to which hemisphere is the context, are "June solstice" and "December solstice", referring to the months in which they take place every year. [3]

The word solstice is derived from the Latin sol ("sun") and sistere ("to stand still"), because at the solstices, the Sun's declination appears to "stand still"; that is, the seasonal movement of the Sun's daily path (as seen from Earth) stops at a northern or southern limit before reversing direction.

Definitions and frames of reference

For an observer on 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 comes to a stop 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 uses it a number of times in his Natural History with a similar meaning that it has today. It contains two Latin-language morphemes, sol, "sun", and -stitium, "stoppage".[4] 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.[5]

To an observer in an inertial frame of reference, planet Earth is seen to rotate about an axis and revolve around the Sun in an elliptical path with the Sun at one focus. Earth's axis is tilted with respect to the plane of Earth's orbit and this axis maintains a position that changes little with respect to the background of stars. An observer on Earth therefore sees a solar path that is the result of both rotation and revolution.

A 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.[6] 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 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 days are longer around the summer solstice and shorter around the winter solstice. 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.[7]

Other Languages
Afrikaans: Sonstilstand
Ænglisc: Sunnstede
العربية: انقلاب شمسي
asturianu: Solsticiu
azərbaycanca: Gündönümü
беларуская: Сонцастаянне
български: Слънцестоене
Boarisch: Sunwend
català: Solstici
čeština: Slunovrat
chiShona: Zuvaragodza
Cymraeg: Heuldro
dansk: Solhverv
Deutsch: Sonnenwende
Ελληνικά: Ηλιοστάσιο
emiliàn e rumagnòl: Sulstìsi
español: Solsticio
Esperanto: Solstico
euskara: Solstizio
فارسی: انقلابین
français: Solstice
Frysk: Sinnekear
furlan: Solstizi
Gaeilge: Grianstad
galego: Solsticio
한국어: 지점
हिन्दी: अयनांत
hrvatski: Suncostaj
Bahasa Indonesia: Titik balik matahari
íslenska: Sólstöður
italiano: Solstizio
Kreyòl ayisyen: Sòlstis
kurdî: Rojveger
Latina: Solstitium
latviešu: Saulgrieži
Lëtzebuergesch: Sonnewenn
lietuvių: Saulėgrįža
magyar: Napforduló
македонски: Сонцестој
Bahasa Melayu: Solstis
Nederlands: Zonnewende
Nedersaksies: Zunnewende
日本語: 至点
norsk: Solverv
norsk nynorsk: Solkverv
occitan: Solstici
ਪੰਜਾਬੀ: ਆਇਨੰਤ
polski: Przesilenie
português: Solstício
română: Solstițiu
саха тыла: Күн туруута
shqip: Solstiku
Simple English: Solstice
slovenščina: Sončev obrat
کوردی: خۆروەگەڕ
српски / srpski: Solsticij
srpskohrvatski / српскохрватски: Solsticij
svenska: Solstånd
Tagalog: Solstisyo
ไทย: อายัน
Türkçe: Gündönümü
українська: Сонцестояння
Tiếng Việt: Điểm chí
Winaray: Solstisyo
吴语: 至點
粵語: 至日
žemaitėška: Sauliegrōžā
中文: 至點