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A solstice is the time when the Sun reaches its most northerly or southerly sun path 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 of the year are defined by reference to the solstices and the .
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.
Solstice first entered into English in the Middle English period. An older term in English is its calque sunstead (), which became rare after the 17th century. Sunstead is cognate with other terms with the same meaning in other Germanic language such as and . A similar English calque of the Latin term is sunstay which was first used in the 16th century and is now also rare.
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". 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.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.
To an observer in an inertial frame of reference, planet Earth is seen to rotate about an rotation and orbit around the Sun in an ellipse path with the Sun at one focus. Earth's axis is axial tilt with respect to the plane of Earth's orbit and this axis maintains a position that changes little with respect to the background of . An observer on Earth therefore sees a solar path that is the result of both rotation and revolution.
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 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.For an introduction to these topics of astronomy refer to
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.
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.
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.
The move across the inner surface of the celestial sphere along the of in parallel planesStrabo II.5.2., "aplaneis asteres kata parallēlōn pherontai kuklōn", "the fixed stars are borne in parallel circles" perpendicular to the Earth's axis extended indefinitely into the heavens and intersecting the celestial sphere in a celestial pole.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" The Sun and the 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: 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 the passage starts Chapter 4.
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").
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 of the Moon, the path of the Sun is given a synonym, the ekleiptikos (kuklos) from ekleipsis, "eclipse".
| +Names of the equinoxes and solstices !Ls !By date (Gregorian calendar) !By sun position (subsolar point) !By season (Northern Hemisphere) !By season (Southern Hemisphere) | ||||
| 0° | March equinox | Northward equinox | Vernal (spring) equinox | Autumnal (fall) equinox |
| 90° | June solstice | Northern solstice | Summer solstice | Winter solstice |
| 180° | September equinox | Southward equinox | Autumnal (fall) equinox | Vernal (spring) equinox |
| 270° | December solstice | Southern solstice | Winter solstice | Summer solstice |
Dōngzhì (Pinyin) or Tōji (rōmaji) () 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 ) mark the middle of the seasons in East Asian calendars. Here, the Chinese character means "extreme", so the terms for the solstices directly signify the summits of summer and winter.
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 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 St. John from June 23 to 24 (see St. John's Eve, Ivan Kupala Day), while Modern Paganism observe Midsummer, known as Litha among . For the vernal (spring) equinox, several springtime festivals are celebrated, such as the Nowruz, the observance in Judaism of Passover, the rites of Easter in most Christian churches, as well as the Wiccan Ostara. The autumnal equinox is associated with the Jewish holiday of Sukkot and the Wiccan 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 the Hindu calendar, two sidereal solstices are named Makar 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 is ignored) into Makara, the zodiacal sign which corresponds with Capricorn, and into Karka, the zodiacal sign which corresponds with 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, is the site of annual equinox and solstice sunrise observances. Out of respect for Native American beliefs these events do not feature ceremonies or rituals of any kind.
This difference is hardly detectable with indirect viewing based devices like sextant equipped with a Vernier scale, and impossible with more traditional tools like a gnomon or an astrolabe. It is also hard to detect the changes in sunrise/sunset azimuth due to the atmospheric refraction 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. In 2012, the journal DIO 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, i.e. the times when the apparent geocentric celestial longitude of the Sun is equal to 90° (June solstice) or 270° (December solstice). 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 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.
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