View source for Solstice

{{Short description|Twice-annual astronomical event when the Sun is farthest from above the Earth's equator}}
{{About|the astronomical event|other uses}}
{{Use mdy dates|date=March 2018}}
{{Solstice-equinox}}

A '''solstice''' is the time when the [[Sun]] reaches its most northerly or southerly [[sun path|excursion]] relative to the [[celestial equator]] on the [[celestial sphere]]. Two solstices occur annually, around 20–22 June and 20–22 December. In many countries, the [[season]]s of the year are defined by reference to the solstices and the [[equinox]]es.

The term ''solstice'' can also be used in a broader sense, as the day when this occurs. For locations not too close to the equator or the poles, the dates with the longest and shortest periods of [[daylight]] are the summer and winter solstices, respectively. 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.<ref>{{cite web |url=https://www.scholastic.com/teachers/articles/teaching-content/summer-and-winter-solstices |title=The Summer and Winter Solstices |publisher=Scholastic |date=2017 |access-date=October 1, 2017}}</ref>

== Etymology==
The word ''solstice'' is derived from the [[Latin]] {{lang|la|sol}} ({{gloss|sun}}) and {{lang|la|sistere}} ({{gloss|to stand still}}), because at the solstices, the Sun's [[declination]] appears to "stand still"; that is, the seasonal movement of the Sun's [[sun path|daily path]] (as seen from [[Earth]]) reaches a northern or southern limit before reversing direction.{{cn|date=May 2025}}

''Solstice'' first entered into English in the [[Middle English]] period.<ref>{{cite web |title=solstice |url=https://www.oed.com/dictionary/solstice_n?tab=factsheet#21683609 |website=OED |access-date=17 May 2025}}</ref> An older term in English is its [[calque]] ''sunstead'' ({{langx|ang|sunstede}}), which became rare after the 17th century. ''Sunstead'' is cognate with other terms with the same meaning in other [[Germanic language]] such as {{langx|non|sólstaðr}} and {{langx|gmh|sunnenstat}}.<ref>{{cite web |title=sunstead |url=https://www.oed.com/dictionary/sunstead_n?tab=meaning_and_use |website=OED |access-date=17 May 2025}}</ref> A similar English calque of the Latin term is ''sunstay'' which was first used in the 16th century and is now also rare.<ref>{{cite web |title=sunstay |url=https://www.oed.com/dictionary/sunstay_n?tab=factsheet#19993600 |website=OED |access-date=17 May 2025}}</ref>

== 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, at the moment of solstice the Sun's motion in declination (i.e. vertically) appears to stop for an instant, and then reverse. 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.

==Relationship to seasons==
{{main|Season}}
The seasons occur because the Earth's axis of rotation is not perpendicular to its orbital plane (the [[plane of the ecliptic]]) but currently makes an angle of about 23.44° (called the [[obliquity of the ecliptic]]), and because the axis keeps its orientation with respect to an [[inertial frame of reference]]. As a consequence, for half the year the Northern Hemisphere is inclined toward the Sun while for the other half year the Southern Hemisphere has this distinction. The two moments when the inclination of Earth's rotational axis has maximum effect are the solstices.

At the [[June solstice]] the [[subsolar point]] is further north than any other time: at latitude 23.44° north, known as the [[Tropic of Cancer]]. Similarly at the [[December solstice]] the subsolar point is further south than any other time: at latitude 23.44° south, known as the [[Tropic of Capricorn]]. The subsolar point will cross every latitude between these two extremes exactly twice per year.
{{subsolar_point_date_graph.svg|400px}}
Also during the June solstice, places on the [[Arctic Circle]] (latitude 66.56° north) will see the Sun just on the horizon during midnight, and all places north of it will see the Sun above horizon for 24 hours. That is the [[midnight sun]] or [[midsummer]]-night sun or polar day. On the other hand, places on the [[Antarctic Circle]] (latitude 66.56° south) will see the Sun just on the horizon during midday, and all places south of it will not see the Sun above horizon at any time of the day. That is the [[polar night]]. During the December Solstice, the effects on both hemispheres are just the opposite. This sees polar [[sea ice]] re-grow annually due to lack of sunlight on the air above and surrounding sea. The warmest and coldest periods of the year in temperate regions are offset by about one month from the solstices, delayed by the earth's thermal inertia.

<gallery class="center">
Image:Seasonearth.png|Orientation of the [[Terminator_(solar)|terminator]] (division between night and day) depends on the season. 
File:Earth-lighting-summer-solstice EN.png|Illumination of Earth by Sun on 21 June. The orientation of the terminator shown with respect to the Earth's orbital plane.
File:Earth-lighting-winter-solstice EN.png|Illumination of Earth by Sun on 21 December. The orientation of the terminator shown with respect to the Earth's orbital plane.
File:north season.jpg|Diagram of the Earth's seasons as seen from the north. Far right: [[southern solstice]]
File:south season.jpg|Diagram of the Earth's seasons as seen from the south. Far left: [[northern solstice]]
File:ReflectedSolarRadiation Solstices.jpg|The globe on an [[equirectangular projection]] to show the amount of reflected sunlight at southern and northern summer solstices, respectively (watts / m<sup>2</sup>).
</gallery>

==Cultural aspects==

===Ancient Greek names and concepts===
The concept of the solstices was embedded in ancient Greek [[celestial navigation]]. As soon as they discovered that the Earth was spherical<ref>{{cite book
|author=Strabo
|author-link=Strabo
|title=The Geography
|at=II.5.1
|quote=sphairikē ... tēs gēs epiphaneia, spherical is the surface of the Earth}}</ref> they devised the concept of the [[celestial sphere]],<ref>{{cite book
|author=Strabo
|author-link=Strabo
|title=The Geography
|pages=II.5.2
|quote=sphairoeidēs ... ouranos, spherical in appearance ... is heaven}}</ref> an imaginary spherical surface rotating with the heavenly bodies (''ouranioi'') fixed in it (the modern one does not rotate, but the stars in it do). As long as no assumptions are made concerning the distances of those bodies from Earth or from each other, the sphere can be accepted as real and is in fact still in use. The Ancient Greeks use the term'' "ηλιοστάσιο" (heliostāsio)'', meaning ''stand of the Sun''.

The [[star]]s move across the inner surface of the celestial [[sphere]] along the [[circumference]]s of [[circle]]s in [[Parallel (geometry)|parallel]] planes<ref>Strabo II.5.2., "aplaneis asteres kata parallēlōn pherontai kuklōn", "the fixed stars are borne in parallel circles"</ref> [[perpendicular]] to the Earth's axis extended indefinitely into the heavens and intersecting the celestial sphere in a celestial pole.<ref>Strabo II.5.2, "ho di'autēs (gē) aksōn kai tou ouranou mesou tetagmenos", "the axis through it (the Earth) extending through the middle of the sky"</ref> The Sun and the [[planet]]s do not move in these parallel paths but along another circle, the ecliptic, whose plane is at an angle, the [[obliquity of the ecliptic]], to the axis, bringing the Sun and planets across the paths of and in among the stars.*

[[Cleomedes]] states:<ref>{{cite book
|title=Cleomedes' Lectures on Astronomy: A Translation of The Heavens
|author=Cleomedes
|author-link=Cleomedes
|others=Translated by Alan C. Bowen and Robert B. Todd
|publisher=[[University of California Press]]
|location=Berkeley
|date=2004
|isbn=0-520-23325-5
|page=41}} This translation cites this passage at the end of Book I Chapter 2 but other arrangements have it at the start of Chapter 3. In the Greek version of {{cite book|title=Cleomedis De motu circulari corporum caelestium libri duo|url=https://archive.org/details/kleomedouskyklik00cleo|author=Cleomedes |editor-first=Hermann|editor-last=Ziegler |publisher=B. G. Teubneri|date=1891|pages=[https://archive.org/details/kleomedouskyklik00cleo/page/32 32]}} the passage starts Chapter 4.</ref>

<blockquote>The band of the [[Zodiac]] (''zōdiakos kuklos'', "zodiacal circle") is at an oblique angle (''loksos'') because it is positioned between the tropical circles and equinoctial circle touching each of the tropical circles at one point ... This Zodiac has a determinable width (set at 8° today) ... that is why it is described by three circles: the central one is called "heliacal" (''hēliakos'', "of the sun").</blockquote>

The term heliacal circle is used for the ecliptic, which is in the center of the zodiacal circle, conceived as a band including the noted constellations named on mythical themes. Other authors use Zodiac to mean ecliptic, which first appears in a gloss of unknown author in a passage of Cleomedes where he is explaining that the [[Moon]] is in the zodiacal circle as well and periodically crosses the path of the Sun. As some of these crossings represent [[eclipse]]s of the Moon, the path of the Sun is given a synonym, the ''ekleiptikos (kuklos)'' from ''ekleipsis'', "eclipse".

===English names===
[[File:McGraw-Hill sundial jeh.JPG|thumb|Sun Triangle in New York points at Sun upon equinox and solstices]]
The two solstices can be distinguished by different pairs of names, depending on which feature one wants to stress.
* '''[[Summer solstice]]''' and '''[[winter solstice]]''' are the most common names, referring to the seasons they are associated with. However, these can be ambiguous since the [[Northern Hemisphere]]'s summer is the [[Southern Hemisphere]]'s winter, and vice versa. The [[Latin]]ate names '''estival solstice''' (summer) and '''hibernal solstice''' (winter) are sometimes used to the same effect,<ref>{{cite book
|last=Kent
|first=April Elliott
|title=The Essential Guide to Practical Astrology
|url=http://google.com/books?id=DOfnC5vAff4C&pg=PA284
|access-date=December 9, 2015
|date=June 7, 2011
|publisher=Penguin
|isbn=978-1-61564-093-5
|page=284}}</ref> as are '''midsummer''' and '''midwinter'''.
* '''[[June solstice]]''' and '''[[December solstice]]''' refer to the months of year in which they take place,<ref>{{cite book
|last=Bass
|first=Charles O.
|title=Astronomy Essentials
|url=http://google.com/books?id=wwsqvLL10KEC&pg=PA35
|access-date=December 9, 2015
|date=1994
|publisher=Research & Education Assoc
|isbn=978-0-87891-965-9
|page=35}}</ref> with no ambiguity as to which hemisphere is the context. They are still not universal, however, as not all cultures use a solar-based calendar where the solstices occur every year in the same month (as they do not in the [[Islamic calendar]] and [[Hebrew calendar]], for example).
* '''Northern solstice''' and '''southern solstice''' indicate the hemisphere of the Sun's location.<ref name="Kiddle1877">{{cite book
|last=Kiddle
|first=Henry
|title=A new manual of the elements of astronomy, descriptive and mathematical: comprising the latest discoveries and theoretic views : with directions for the use of the globes, and for studying the constellations
|url=http://google.com/books?id=NkwAAAAAYAAJ&pg=PA82
|access-date=June 2, 2011
|date=1877
|publisher=Ivison, Blakeman, Taylor, & Company
|page=82}}</ref> The northern solstice is in June, when the Sun is directly over the [[Tropic of Cancer]] in the [[Northern Hemisphere]], and the southern solstice is in December, when the Sun is directly over the [[Tropic of Capricorn]] in the [[Southern Hemisphere]].<ref>{{cite book
|last=Jain
|first=R.K.
|title=ICSE Geography
|url=http://google.com/books?id=s4SdkMyKHNUC&pg=PA25
|access-date=December 9, 2015
|publisher=Ratna Sagar
|isbn=978-81-8332-579-0
|page=25}}</ref> These terms can be used unambiguously for other planets.
* '''First point of [[Cancer (astrology)|Cancer]]''' and '''first point of [[Capricorn (astrology)|Capricorn]]''' refer to the [[zodiac|astrological signs]] that the sun "is entering" (a system rooted in Roman Classical period dates).<ref>{{cite book
|last=Stewart
|first=Alexander
|title=A Compendium of ModernGeography
|url=http://google.com/books?id=Y3QDAAAAQAAJ&pg=PA383
|access-date=December 9, 2015
|date=1869
|publisher=Oliver & Boyd
|location=Edinburgh
|page=383}}</ref> Due to the [[precession of the equinoxes]], the [[constellation]]s the sun appears in at solstices are currently [[Taurus (constellation)|Taurus]] in June and [[Sagittarius (constellation)|Sagittarius]] in December.

{| class="wikitable" 
|+Names of the equinoxes and solstices
![[Celestial longitude|Ls]]
!By date
([[Gregorian calendar]])
!By sun position
([[subsolar point]])
!By season
([[Northern Hemisphere]])
!By season
([[Southern Hemisphere]])
|-
| style="text-align: right;" | 0°
|[[March equinox]]
|Northward equinox
|Vernal (spring) equinox
|Autumnal (fall) equinox
|-
| style="text-align: right;" | 90°
|[[June solstice]]
|Northern solstice
|[[Summer solstice|Estival (summer) solstice]]
|[[Winter solstice|Hibernal (winter) solstice]]
|-
| style="text-align: right;" | 180°
|[[September equinox]]
|Southward equinox
|Autumnal (fall) equinox
|Vernal (spring) equinox
|-
| style="text-align: right;" | 270°
|[[December solstice]]
|Southern solstice
|[[Winter solstice|Hibernal (winter) solstice]]
|[[Summer solstice|Estival (summer) solstice]]
|}

===Solstice terms in East Asia===
{{Main|Xiazhi|Dongzhi (solar term)}}

The traditional East Asian calendars divide a year into 24 [[solar term]]s (節氣). '''Xiàzhì''' <small>([[Pinyin|pīnyīn]])</small> or '''Geshi''' <small>([[Romanization of Japanese|rōmaji]])</small> ({{CJKV|t=夏至|s=夏至|j=夏至|k=하지(Haji)|Tibetan=དབྱར་ཉི་ལྡོག|v=Hạ chí|l=''summer's extreme''}}) is the 10th solar term, and marks the '''summer solstice'''. It begins when the Sun reaches the [[celestial longitude]] of 90° (around 21 June) and ends when the Sun reaches the longitude of 105° (around 7 July). Xiàzhì more often refers in particular to the day when the Sun is exactly at the celestial longitude of 90°.

'''Dōngzhì''' <small>([[Pinyin|pīnyīn]])</small> or '''Tōji''' <small>([[Romanization of Japanese|rōmaji]])</small> ({{CJKV|t=冬至|s=冬至|j=冬至|k=동지(Dongji)|Tibetan= དགུན་ཉི་ལྡོག|v=Đông chí|l=''winter's extreme''}}) is the 22nd solar term, and marks the '''winter solstice'''. It begins when the Sun reaches the [[celestial longitude]] of 270° (around 23 December) and ends when the Sun reaches the longitude of 285° (around 5 January). Dōngzhì more often refers in particular to the day when the Sun is exactly at the celestial longitude of 270°.

The solstices (as well as the [[equinox]]es) mark the middle of the seasons in East Asian calendars. Here, the Chinese character [[wikt:至|至]] means "extreme", so the terms for the solstices directly signify the summits of summer and winter.

===Solstice celebrations===
{{See also|Fête St-Jean-Baptiste|Festival of San Juan|Saint Jonas Day|St John's Day (Estonia)|Ivan Kupala Day|Golowan|Wheel of the Year}}
{{unreferenced section|date=June 2012}}
[[File:Summer Solstice Sunrise over Stonehenge 2005.jpg|2005 Summer solstice sunrise over [[Stonehenge]]|thumb]]
The term ''solstice'' can also be used in a wider sense, as the date (day) that such a passage happens. The solstices, together with the equinoxes, are connected with the seasons. In some languages they are considered to start or separate the seasons; in others they are considered to be centre points (in [[England]], in the Northern Hemisphere, for example, the period around the northern solstice is known as midsummer). [[Midsummer's Day]], defined as St. Johns Day by the [[Christian Church]], is 24 June, about three days after the solstice itself). Similarly 25 December is the start of the [[Christmas]] celebration, and is the day the Sun begins to return to the Northern Hemisphere. The traditional British and Irish main rent and meeting days of the year, "the usual [[quarter days]]," were often those of the solstices and equinoxes.

Many cultures celebrate various combinations of the winter and summer solstices, the equinoxes, and the midpoints between them, leading to various holidays arising around these events. During the southern or [[winter solstice]], Christmas is the most widespread contemporary holiday, while [[Yaldā Night|Yalda]], [[Saturnalia]], [[Karachun]], [[Hanukkah]], [[Kwanzaa]], and [[Yule]] are also celebrated around this time. In East Asian cultures, the [[Dongzhi Festival]] is celebrated on the winter solstice. For the northern or [[summer solstice]], Christian cultures celebrate the feast of [[John the Baptist|St. John]] from June 23 to 24 (see [[St. John's Eve]], [[Ivan Kupala Day]]), while [[Modern Paganism|Modern Pagans]] observe Midsummer, known as [[Wheel of the Year|Litha]] among [[Wicca]]ns. For the vernal (spring) equinox, several springtime festivals are celebrated, such as the [[Persia]]n [[Nowruz]], the observance in [[Judaism]] of [[Passover]], the rites of [[Easter]] in most Christian churches, as well as the Wiccan [[Ostara (Wicca)|Ostara]]. The autumnal equinox is associated with the Jewish holiday of [[Sukkot]] and the Wiccan [[Wheel of the Year|Mabon]]. 

In the southern tip of [[South America]], the [[Mapuche people]] celebrate [[We Tripantu]] (the New Year) a few days after the northern solstice, on 24 June. Further north, the [[Atacama people]] formerly celebrated this date with a noise festival, to call the Sun back. Further east, the [[Aymara people]] celebrate their New Year on 21 June. A celebration occurs at sunrise, when the sun shines directly through the [[Gate of the Sun]] in [[Tiwanaku]]. Other Aymara New Year feasts occur throughout [[Bolivia]], including at the site of [[El Fuerte de Samaipata]].

<!--In many cultures, the solstices and equinoxes traditionally determine the midpoint of the seasons, which can be seen in the celebrations called midsummer and [[Winter solstice|midwinter]]. In this vein, the [[Japan]]ese celebrate the start of each season with an occurrence known as [[Setsubun]]. The cumulative cooling and warming that result from the tilt of the planet become most pronounced ''after'' the solstices, leading to the more recent custom of using them to mark the ''beginning'' of summer and winter in most countries of [[Central Europe|Central]] and [[Northern Europe]], as well as in [[Canada]], the United States and [[New Zealand]].-->
In the [[Hindu calendar]], two sidereal solstices are named [[Makar Sankranti|Makara Sankranti]] which marks the start of [[Uttarayana]] and Karka [[Sankranti]] which marks the start of [[Dakshinayana]]. The former occurs around 14 January each year, while the latter occurs around 14 July each year. These mark the movement of the Sun along a sidereally fixed [[zodiac]] ([[precession (astronomy)|precession]] is ignored) into Makara, the zodiacal sign which corresponds with [[Capricorn (astrology)|Capricorn]], and into Karka, the zodiacal sign which corresponds with [[Cancer (astrology)|Cancer]], respectively.

The [[Amundsen–Scott South Pole Station]] celebrates every year on 21 June a midwinter party, to celebrate that the Sun is at its lowest point and coming back.

The [[Fremont Solstice Parade]] takes place every summer solstice in [[Fremont, Seattle, Washington]] in the [[United States]].

The reconstructed [[Cahokia Woodhenge]], a large [[timber circle]] located at the [[Mississippian culture]] [[Cahokia]] [[archaeological site]] near [[Collinsville, Illinois]],<ref name=VIS>{{Cite web| title= Visitors Guide to the Woodhenge| url= http://greatriverroad.com/somadco/collins/woodhenge.htm| access-date= December 19, 2017| archive-url= https://web.archive.org/web/20171222051732/http://greatriverroad.com/somadco/collins/woodhenge.htm| archive-date= December 22, 2017| url-status= dead}}</ref> is the site of annual equinox and solstice sunrise observances. Out of respect for [[Native Americans in the United States|Native American]] beliefs these events do not feature ceremonies or rituals of any kind.<ref>{{cite web| title= Welcome the Fall Equinox at Cahokia Mounds | publisher= Illinois Department of Natural Resources | url= https://www.illinois.gov/news/press-release.14832.html | access-date= November 25, 2024 }}</ref><ref>{{Cite web| title= Winter Solstice Sunrise Observance at Cahokia Mounds| publisher = Collinsville Chamber of Commerce| url = http://www.discovercollinsville.com/events/details/winter-solstice-sunrise-observance-at-cahokia-mounds-3895 | access-date= December 20, 2017 }}</ref><ref name=ICT>{{Cite news | title= Cahokia Mounds Mark Spring Equinox : The keepers of Cahokia Mounds will host a spring gathering to celebrate the vernal equinox | url= https://indiancountrymedianetwork.com/travel/destinations/spring-equinox-cahokia-mounds/ | newspaper= Indian Country Today | publisher= [[Indian Country Media Network]] | access-date= December 20, 2017 | archive-url= https://web.archive.org/web/20171222052220/https://indiancountrymedianetwork.com/travel/destinations/spring-equinox-cahokia-mounds/ | archive-date= December 22, 2017 | url-status= dead }}</ref>

===Solstice determination===
Unlike the equinox, the solstice time is not easy to determine. The changes in [[Position of the Sun|solar declination]] become smaller as the Sun gets closer to its maximum/minimum declination. The days before and after the solstice, the declination speed is less than 30 [[arcseconds]] per day which is less than {{frac|60}} of the [[angular size]] of the Sun, or the equivalent to just 2 seconds of [[right ascension]].

This difference is hardly detectable with indirect viewing based devices like [[sextant]] equipped with a [[Vernier scale|vernier]], and impossible with more traditional tools like a [[gnomon]]<ref>{{cite web
|title=Solstice Determination based on Observations
|last=Mollerup
|first=Asger
|date=January 12, 2008
|url=http://sundial.thai-isan-lao.com/solstice-determination.html
|access-date=September 27, 2010
|archive-url=https://web.archive.org/web/20090211044959/http://sundial.thai-isan-lao.com/solstice-determination.html
|archive-date=February 11, 2009}}</ref> or an [[astrolabe]]. It is also hard to detect the changes in sunrise/sunset azimuth due to the [[atmospheric refraction]]<ref>{{cite journal
|last = Exton
|first = Harold
|title = A Fresh Analysis of Some Recent Data on Atmospheric Refraction Near the Horizon with Implications in Archaeoastronomy
|journal = Journal of History of Astronomy, Archaeoastronomy Supplement
|volume = 23
|page = S57
|date = 1992
|issue = 17
|bibcode = 1992JHAS...23...57E
|doi = 10.1177/002182869202301707|s2cid = 118484271
}}</ref> changes. Those accuracy issues render it impossible to determine the solstice day based on observations made within the 3 (or even 5) days surrounding the solstice without the use of more complex tools.

Accounts do not survive but Greek astronomers must have used an approximation method based on interpolation, which is still used by some amateurs. This method consists of recording the declination angle at noon during some days before and after the solstice, trying to find two separate days with the same declination. When those two days are found, the halfway time between both noons is estimated solstice time. An interval of 45 days has been postulated as the best one to achieve up to a quarter-day precision, in the solstice determination.<ref>{{cite journal
|last = Hugh
|first = Thurston
|title = Early Greek Solstices and Equinoxes
|journal = Journal for the History of Astronomy
|volume = 32, Part 2
|issue = 107
|pages = 154–156
|date = 2001
|doi = 10.1177/002182860103200208
|bibcode = 2001JHA....32..154T
|s2cid = 118464897
|issn = 0021-8286|doi-access = 
}}</ref>
In 2012, the journal DIO [http://www.dioi.org/cot.htm#ngmw found] that accuracy of one or two hours with balanced errors can be attained by observing the Sun's equal altitudes about S = twenty degrees (or d = about 20 days) before and after the summer solstice because the average of the two times will be early by q arc minutes where q is (πe cosA)/3 times the square of S in degrees (e = earth orbit eccentricity, A = earth's perihelion or Sun's apogee), and the noise in the result will be about 41 hours divided by d if the eye's sharpness is taken as one arc minute.

Astronomical almanacs define the solstices as the moments when the Sun passes through the [[colure|solstitial colure]], i.e. the times when the apparent geocentric [[celestial longitude]] of the Sun is equal to 90° (June solstice) or 270° (December solstice).<ref>{{cite book
|last=Meeus
|first=Jean
|author-link1=Jean Meeus
|date=1998
|title=Astronomical Algorithms
|edition=Second English
|url=https://archive.org/details/astronomicalalgo00meeu
|url-access=limited
|location=Richmond
|publisher=Willmann-Bell, Inc.
|pages=[https://archive.org/details/astronomicalalgo00meeu/page/n184 177]–182
|isbn=0-943396-61-1}}</ref> The dates of the solstice varies each year and may occur a day earlier or later depending on the [[time zone]]. Because the earth's orbit takes slightly longer than a calendar year of 365 days, the solstices occur slightly later each calendar year, until a leap day re-aligns the calendar with the orbit. Thus the solstices always occur between June 20 and 22 and between December 20 and 23 <ref>{{cite web|title=December solstice|url=https://www.timeanddate.com/calendar/december-solstice.html|access-date=March 21, 2018}}</ref><ref>{{cite web|title=June solstice|url=https://www.timeanddate.com/calendar/june-solstice.html|access-date=March 21, 2018}}</ref> in a four-year-long cycle with the 21st and 22nd being the most common dates, as can be seen in the schedule at the start of the article.

== In the constellations ==
Using the current official [[International Astronomical Union|IAU]] constellation boundaries—and taking into account the variable precession speed and the rotation of the ecliptic—the solstices shift through the constellations as follows<ref>{{cite book
|last=Meeus
|first=Jean
|author-link1=Jean Meeus
|date=1997
|title= Mathematical astronomy morsels
|edition=1st English
|location=Richmond
|publisher=Willmann-Bell, Inc.
|isbn=0-943396-51-4}}</ref> (expressed in [[astronomical year numbering]] in which the year 0 = 1 BC, −1 = 2 BC, etc.):
* The northern solstice passed from [[Leo (constellation)|Leo]] into [[Cancer (constellation)|Cancer]] in year −1458, passed into [[Gemini (constellation)|Gemini]] in year −10, passed into [[Taurus (constellation)|Taurus]] in December 1989, and is expected to pass into [[Aries (constellation)|Aries]] in year 4609.
* The southern solstice passed from [[Capricornus]] into [[Sagittarius (constellation)|Sagittarius]] in year −130, is expected to pass into [[Ophiuchus]] in year 2269, and is expected to pass into [[Scorpius]] in year 3597.

== See also ==
<!-- Please keep entries in alphabetical order & add a short description [[WP:SEEALSO]] -->
{{Portal|Astronomy|Stars|Outer space|Solar System|Holidays|History of science}}
* [[Analemma]]
* [[Geocentric view of the seasons]]
* [[Iranian calendars]]
* [[Perihelion and aphelion]]
* [[Wheel of the Year]]
* [[Zoroastrian calendar]]
<!-- Please keep entries in alphabetical order -->

== References ==
{{reflist}}

== External links ==
{{Wiktionary}}
{{Commons}}
* [http://neoprogrammics.com/equinoxes_and_solstices/Equinoxes_and_Solstices_Calculator.php Equinoxes and Solstices Calculator (1600 to 2400)]
* {{cite web
|url=https://aa.usno.navy.mil/data/Earth_Seasons
|title=Earth's Seasons: Equinoxes, Solstices, Perihelion, and Aphelion (2000–2025)
|publisher=[[United States Naval Observatory]], Astronomical Applications Department
|access-date=December 9, 2015}}
* {{cite web
|url=http://scienceworld.wolfram.com/astronomy/SummerSolstice.html
|first=Eric
|last=Weisstein
|title=Summer Solstice
|publisher=Eric Weisstein's World of Astronomy
|date=1996–2007
|quote=The above plots show how the date of the summer solstice shifts through the Gregorian calendar according to the insertion of leap years.
|access-date=October 24, 2008
}}

{{Time measurement and standards}}
{{Greek astronomy}}
{{Authority control}}

[[Category:Solstices| ]]
[[Category:Astronomical events of the Solar System]]
[[Category:Calendars]]
[[Category:December observances]]
[[Category:Dynamics of the Solar System]]
[[Category:June observances]]
[[Category:Technical factors of astrology]]
[[Category:Time in astronomy]]