Asolstice is the time when theSun reaches its most northerly or southerlyexcursion relative to thecelestial equator on thecelestial sphere. Two solstices occur annually, around 20–22 June and 20–22 December. In many countries, theseasons of the year are defined by reference to the solstices and theequinoxes.
The termsolstice 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 ofdaylight 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.[7]
The wordsolstice is derived from theLatinsol ('sun') andsistere ('to stand still'), because at the solstices, the Sun'sdeclination appears to "stand still"; that is, the seasonal movement of the Sun'sdaily path (as seen fromEarth) reaches a northern or southern limit before reversing direction.[citation needed]
Solstice first entered into English in theMiddle English period.[8] An older term in English is itscalquesunstead (Old English:sunstede), which became rare after the 17th century.Sunstead is cognate with other terms with the same meaning in otherGermanic language such asOld Norse:sólstaðr andMiddle High German:sunnenstat.[9] A similar English calque of the Latin term issunstay which was first used in the 16th century and is now also rare.[10]
For an observer at theNorth 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 theSouth Pole, the Sun reaches the highest position on the December solstice day. When it is thesummer solstice at one Pole, it is thewinter 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 aLatin scientific word in use in the lateRoman Republic of the 1st century BC:solstitium.Pliny uses it a number of times in hisNatural History with a similar meaning that it has today. It contains two Latin-language morphemes,sol, "sun", and-stitium, "stoppage".[11] The Romans used "standing" to refer to a component of therelative 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 aframe of reference. From a fixed position on the ground, the Sun appears to orbit around Earth.[12]
Asolargraph taken from theAtacama Pathfinder Experiment at theLlano 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 southernwinter solstice in June 2010.[13] 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 theelevation 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 thehorizon 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, orecliptic, 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 theequator, the length of the nights at latitudes +L° and −L° are of equal length. This is known as anequinox. There are two solstices and two equinoxes in a tropical year.[14]
Derivative of −Δt, the so-calledEquation of time. The axis on the right shows the length of thesolar day, also called the synodic day.
Because of the variation in the rate at which the sun'sright 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.
The seasons occur because the Earth's axis of rotation is not perpendicular to its orbital plane (theplane of the ecliptic) but currently makes an angle of about 23.44° (called theobliquity of the ecliptic), and because the axis keeps its orientation with respect to aninertial 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 theJune solstice thesubsolar point is further north than any other time: at latitude 23.44° north, known as theTropic of Cancer. Similarly at theDecember solstice the subsolar point is further south than any other time: at latitude 23.44° south, known as theTropic of Capricorn. The subsolar point will cross every latitude between these two extremes exactly twice per year.
Also during the June solstice, places on theArctic 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 themidnight sun ormidsummer-night sun or polar day. On the other hand, places on theAntarctic 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 thepolar night. During the December Solstice, the effects on both hemispheres are just the opposite. This sees polarsea 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.
Orientation of theterminator (division between night and day) depends on the season.
Illumination of Earth by Sun on 21 June. The orientation of the terminator shown with respect to the Earth's orbital plane.
Illumination of Earth by Sun on 21 December. The orientation of the terminator shown with respect to the Earth's orbital plane.
Diagram of the Earth's seasons as seen from the north. Far right:southern solstice
Diagram of the Earth's seasons as seen from the south. Far left:northern solstice
The globe on anequirectangular projection to show the amount of reflected sunlight at southern and northern summer solstices, respectively (watts / m2).
The concept of the solstices was embedded in ancient Greekcelestial navigation. As soon as they discovered that the Earth was spherical[15] they devised the concept of thecelestial sphere,[16] 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), meaningstand of the Sun.
Thestars move across the inner surface of the celestialsphere along thecircumferences ofcircles inparallel planes[17]perpendicular to the Earth's axis extended indefinitely into the heavens and intersecting the celestial sphere in a celestial pole.[18] The Sun and theplanets do not move in these parallel paths but along another circle, the ecliptic, whose plane is at an angle, theobliquity of the ecliptic, to the axis, bringing the Sun and planets across the paths of and in among the stars.*
The band of theZodiac (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").
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 theMoon is in the zodiacal circle as well and periodically crosses the path of the Sun. As some of these crossings representeclipses of the Moon, the path of the Sun is given a synonym, theekleiptikos (kuklos) fromekleipsis, "eclipse".
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 andwinter solstice are the most common names, referring to the seasons they are associated with. However, these can be ambiguous since theNorthern Hemisphere's summer is theSouthern Hemisphere's winter, and vice versa. TheLatinate namesestival solstice (summer) andhibernal solstice (winter) are sometimes used to the same effect,[20] as aremidsummer andmidwinter.
June solstice andDecember solstice refer to the months of year in which they take place,[21] 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 theIslamic calendar andHebrew calendar, for example).
Northern solstice andsouthern solstice indicate the hemisphere of the Sun's location.[22] The northern solstice is in June, when the Sun is directly over theTropic of Cancer in theNorthern Hemisphere, and the southern solstice is in December, when the Sun is directly over theTropic of Capricorn in theSouthern Hemisphere.[23] These terms can be used unambiguously for other planets.
The traditional East Asian calendars divide a year into 24solar terms (節氣).Xiàzhì(pīnyīn) orGeshi(rōmaji) (Chinese andJapanese: 夏至; Korean:하지(Haji); Vietnamese:Hạ chí; lit.summer's extreme) is the 10th solar term, and marks thesummer solstice. It begins when the Sun reaches thecelestial 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(pīnyīn) orTōji(rōmaji) (Chinese andJapanese: 冬至; Korean:동지(Dongji); Vietnamese:Đông chí; lit.winter's extreme) is the 22nd solar term, and marks thewinter solstice. It begins when the Sun reaches thecelestial 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 theequinoxes) mark the middle of the seasons in East Asian calendars. Here, the Chinese character至 means "extreme", so the terms for the solstices directly signify the summits of summer and winter.
The termsolstice 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 (inEngland, 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 theChristian Church, is 24 June, about three days after the solstice itself). Similarly 25 December is the start of theChristmas 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 usualquarter 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 orwinter solstice, Christmas is the most widespread contemporary holiday, whileYalda,Saturnalia,Karachun,Hanukkah,Kwanzaa, andYule are also celebrated around this time. In East Asian cultures, theDongzhi Festival is celebrated on the winter solstice. For the northern orsummer solstice, Christian cultures celebrate the feast ofSt. John from June 23 to 24 (seeSt. John's Eve,Ivan Kupala Day), whileModern Pagans observe Midsummer, known asLitha amongWiccans. For the vernal (spring) equinox, several springtime festivals are celebrated, such as thePersianNowruz, the observance inJudaism ofPassover, the rites ofEaster in most Christian churches, as well as the WiccanOstara. The autumnal equinox is associated with the Jewish holiday ofSukkot and the WiccanMabon.
In the southern tip ofSouth America, theMapuche people celebrateWe Tripantu (the New Year) a few days after the northern solstice, on 24 June. Further north, theAtacama people formerly celebrated this date with a noise festival, to call the Sun back. Further east, theAymara people celebrate their New Year on 21 June. A celebration occurs at sunrise, when the sun shines directly through theGate of the Sun inTiwanaku. Other Aymara New Year feasts occur throughoutBolivia, including at the site ofEl Fuerte de Samaipata.
In theHindu calendar, two sidereal solstices are namedMakara Sankranti which marks the start ofUttarayana and KarkaSankranti which marks the start ofDakshinayana. 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 fixedzodiac (precession is ignored) into Makara, the zodiacal sign which corresponds withCapricorn, and into Karka, the zodiacal sign which corresponds withCancer, respectively.
TheAmundsen–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.
Unlike the equinox, the solstice time is not easy to determine. The changes insolar 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 30arcseconds per day which is less than1⁄60 of theangular size of the Sun, or the equivalent to just 2 seconds ofright ascension.
This difference is hardly detectable with indirect viewing based devices likesextant equipped with avernier, and impossible with more traditional tools like agnomon[29] or anastrolabe. It is also hard to detect the changes in sunrise/sunset azimuth due to theatmospheric refraction[30] 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.[31]In 2012, the journal DIOfound 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 thesolstitial colure, i.e. the times when the apparent geocentriccelestial longitude of the Sun is equal to 90° (June solstice) or 270° (December solstice).[32] The dates of the solstice varies each year and may occur a day earlier or later depending on thetime 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[33][34] 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.
Using the current officialIAU constellation boundaries—and taking into account the variable precession speed and the rotation of the ecliptic—the solstices shift through the constellations as follows[35] (expressed inastronomical year numbering in which the year 0 = 1 BC, −1 = 2 BC, etc.):
The northern solstice passed fromLeo intoCancer in year −1458, passed intoGemini in year −10, passed intoTaurus in December 1989, and is expected to pass intoAries in year 4609.
The southern solstice passed fromCapricornus intoSagittarius in year −130, is expected to pass intoOphiuchus in year 2269, and is expected to pass intoScorpius in year 3597.
^ThePrinciple of relativity was first applied to inertial frames of reference byAlbert Einstein. Before then, the concepts ofabsolute space and time applied byIsaac Newton prevailed. The motion of the Sun across the sky is still called "apparent motion" incelestial 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.
^Strabo.The Geography. II.5.1.sphairikē ... tēs gēs epiphaneia, spherical is the surface of the Earth
^Strabo.The Geography. pp. II.5.2.sphairoeidēs ... ouranos, spherical in appearance ... is heaven
^Strabo II.5.2., "aplaneis asteres kata parallēlōn pherontai kuklōn", "the fixed stars are borne in parallel circles"
^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"
^Cleomedes (2004).Cleomedes' Lectures on Astronomy: A Translation of The Heavens. Translated by Alan C. Bowen and Robert B. Todd. Berkeley:University of California Press. p. 41.ISBN0-520-23325-5. 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 ofCleomedes (1891). Ziegler, Hermann (ed.).Cleomedis De motu circulari corporum caelestium libri duo. B. G. Teubneri. pp. 32. the passage starts Chapter 4.
^Exton, Harold (1992). "A Fresh Analysis of Some Recent Data on Atmospheric Refraction Near the Horizon with Implications in Archaeoastronomy".Journal of History of Astronomy, Archaeoastronomy Supplement.23 (17): S57.Bibcode:1992JHAS...23...57E.doi:10.1177/002182869202301707.S2CID118484271.
Weisstein, Eric (1996–2007)."Summer Solstice". Eric Weisstein's World of Astronomy. RetrievedOctober 24, 2008.The above plots show how the date of the summer solstice shifts through the Gregorian calendar according to the insertion of leap years.
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