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A solstice is an event occurring when the Sun
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 directly connected 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 the 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 of year in which they take place. [2] At latitudes outside the tropics, the summer solstice marks the day when the Sun
Sun
appears to reach its highest point in the sky. Within the tropics, the Sun
Sun
appears directly overhead at solar noon days to 3 months before and after the summer solstice. This means the subsolar point occurs twice each year at any tropical latitude. The word solstice is derived from the Latin
Latin
sol ("sun") and sistere ("to stand still"), because at the solstices, the Sun's declination "stands 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.

Contents

1 Definitions and frames of reference 2 Relationship to seasons 3 Cultural aspects

3.1 Ancient Greek names and concepts 3.2 English names 3.3 Solstice
Solstice
terms in East Asia 3.4 Solstice
Solstice
celebrations 3.5 Solstice
Solstice
determination

4 In the constellations 5 Solstices on other planets 6 See also 7 References 8 External links

Definitions and frames of reference[edit] For an observer on the North Pole, the Sun
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
Sun
reaches the highest position on the December solstice
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
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
Latin
scientific word in use in the late Roman Republic
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".[3] The Romans used "standing" to refer to a component of the relative velocity of the Sun
Sun
as it is observed in the sky. Relative velocity
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
Sun
appears to orbit around Earth.[4] To an observer in an inertial frame of reference, planet Earth
Earth
is seen to rotate about an axis and revolve around the Sun
Sun
in an elliptical path with the Sun
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
Earth
therefore sees a solar path that is the result of both rotation and revolution.

A solargraph taken from the Atacama Pathfinder Experiment
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.[5] 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
Sun
– is an observed daily increment (and lateral offset) of the elevation of the Sun
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
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.[6] Relationship to seasons[edit] Main article: 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
Northern Hemisphere
is inclined toward the Sun
Sun
while for the other half year the Southern Hemisphere
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
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
Arctic Circle
(latitude 66.56° north) will see the Sun
Sun
just on the horizon during midnight, and all places north of it will see the Sun
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
Antarctic Circle
(latitude 66.56° south) will see the Sun
Sun
just on the horizon during midday, and all places south of it will not see the Sun
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 also allows the polar sea ice to increase its annual growth and temporary extent at a greater level due to lack of direct sunlight.

Illumination of Earth
Earth
by Sun
Sun
at the northern solstice.

Illumination of Earth
Earth
by Sun
Sun
at the southern solstice.

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

Two images showing the amount of reflected sunlight at southern and northern summer solstices, respectively (watts / m²).

Cultural aspects[edit] Ancient Greek names and concepts[edit] The concept of the solstices was embedded in ancient Greek celestial navigation. As soon as they discovered that the Earth
Earth
is spherical[7] they devised the concept of the celestial sphere,[8] 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
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 stars move across the inner surface of the celestial sphere along the circumferences of circles in parallel planes[9] perpendicular to the Earth's axis extended indefinitely into the heavens and intersecting the celestial sphere in a celestial pole.[10] The Sun
Sun
and the planets 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
Sun
and planets across the paths of and in among the stars.* Cleomedes states:[11]

The band of the Zodiac
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
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
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
Moon
is in the zodiacal circle as well and periodically crosses the path of the Sun. As some of these crossings represent eclipses of the Moon, the path of the Sun
Sun
is given a synonym, the ekleiptikos (kuklos) from ekleipsis, "eclipse". English names[edit] The two solstices can be distinguished by different pairs of names, depending on which feature one wants to stress.

Summer solstice
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 Latinate names estival solstice (summer) and hibernal solstice (winter) are sometimes used to the same effect,[12] as are midsummer and midwinter. June solstice and December solstice
December solstice
refer to the months of year in which they take place,[13] 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
Islamic calendar
and Hebrew calendar, for example). Northern solstice and southern solstice indicate the hemisphere of the Sun's location.[14] The northern solstice is in June, when the Sun
Sun
is directly over the Tropic of Cancer
Tropic of Cancer
in the Northern Hemisphere, and the southern solstice is in December, when the Sun
Sun
is directly over the Tropic of Capricorn
Tropic of Capricorn
in the Southern Hemisphere.[15] These terms can be used unambiguously for other planets. First point of Cancer and first point of Capricorn refer to the astrological signs that the sun is entering.[16] Due to the precession of the equinoxes, however, the constellations where the solstices are currently located are Taurus and Sagittarius, respectively.

Solstice
Solstice
terms in East Asia[edit] Main articles: Xiazhi and Dongzhi (solar term) The traditional East Asian calendars divide a year into 24 solar terms (節氣). Xiàzhì (pīnyīn) or Geshi (rōmaji) (Chinese and Japanese: 夏至; Korean: 하지(Haji); Vietnamese: Hạ chí; "summer's extreme") is the 10th solar term, and marks the summer solstice. It begins when the Sun
Sun
reaches the celestial longitude of 90° (around June 21) and ends when the Sun
Sun
reaches the longitude of 105° (around July 7). Xiàzhì more often refers in particular to the day when the Sun
Sun
is exactly at the celestial longitude of 90°. Dōngzhì (pīnyīn) or Tōji (rōmaji) (Chinese and Japanese: 冬至; Korean: 동지(Dongji); Vietnamese: Đông chí; "winter's extreme") is the 22nd solar term, and marks the winter solstice. It begins when the Sun
Sun
reaches the celestial longitude of 270° (around December 22 ) and ends when the Sun
Sun
reaches the longitude of 285° (around January 5). 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 equinoxes) 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. Solstice
Solstice
celebrations[edit] See also: Fête St-Jean-Baptiste, Festival of San Juan, Saint Jonas Day, St John's Day
Day
(Estonia), Ivan Kupala Day, and Golowan

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2005 Summer Solstice
Solstice
Sunrise over Stonehenge

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
Day
by the Christian Church, is June 24, about three days after the solstice itself). Similarly December 25 is the start of the Christmas celebration, and is the day the Sun
Sun
begins to return to the Northern Hemisphere. 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. For the southern solstice, Christmas is the most popular holiday to have arisen. In addition, Yalda, Saturnalia, Karachun, Hanukkah, Kwanzaa
Kwanzaa
and Yule
Yule
(see winter solstice for more) are also celebrated around this time. For the northern solstice, Christian cultures celebrate the feast of St. John from June 23 to 24 (see St. John's Eve, Ivan Kupala Day, Midsummer), while Neopagans observe Midsummer, also known as Litha. For the vernal (spring) equinox, several spring-time festivals are celebrated, such as the Persian Nowruz, the observance in Judaism
Judaism
of Passover
Passover
and in most Christian churches of Easter. The autumnal equinox has also given rise to various holidays, such as the Jewish holiday of Sukkot. At the midpoints between these four solar events, cross-quarter days are celebrated. In the southern tip of South America, the Mapuche people
Mapuche people
celebrate We Tripantu (the New Year) a few days after the northern solstice, on June 24. Further north, the Atacama people
Atacama people
formerly celebrated this date with a noise festival, to call the Sun
Sun
back. Further east, the Aymara people
Aymara people
celebrate their New Year
Year
on June 21. A celebration occurs at sunrise, when the sun shines directly through the Gate of the Sun
Sun
in Tiwanaku. Other Aymara New Year
Year
feasts occur throughout Bolivia, including at the site of El Fuerte de Samaipata. In the Hindu calendar, two sidereal solstices are named Makara Sankranti
Sankranti
which marks the start of Uttarayana
Uttarayana
and Karka Sankranti which marks the start of Dakshinayana. The former occurs around January 14 each year, while the latter occurs around July 14 each year. These mark the movement of the Sun
Sun
along a sidereally fixed zodiac (precession is ignored) into Makara, the zodiacal sign which corresponds with Capricorn, and into Karkat, the zodiacal sign which corresponds with Cancer, respectively. The Amundsen–Scott South Pole
South Pole
Station celebrates every year on June 21 a midwinter party, to celebrate that the Sun
Sun
is at its lowest point and coming back. The Fremont Solstice Parade
Fremont Solstice Parade
takes place every summer solstice in Fremont, Seattle, Washington
Fremont, Seattle, Washington
in the United States. The reconstructed Cahokia
Cahokia
Woodhenge, a large timber circle located at the Mississippian culture
Mississippian culture
Cahokia
Cahokia
archaeological site near Collinsville, Illinois,[17] 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.[18][19][20] Solstice
Solstice
determination[edit] Unlike the equinox, the solstice time is not easy to determine. The changes in 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 ​1⁄60 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, and impossible with more traditional tools like a gnomon[21] or an astrolabe. It is also hard to detect the changes on sunrise/sunset azimuth due to the atmospheric refraction[22] 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.[23] 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 solstitial colure, i.e. the times when the apparent geocentric longitude of the sun is equal to 90° (summer solstice) or 270° (winter solstice).[24] The dates of the solstice varies each year and may occur a day earlier or later depending on the time zone. The solstices always occur between June 20 and 22 and between December 20 and 23 with the 21st and 22nd being the most common dates.[25][26] In the constellations[edit] Using the current official 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[27] (expressed in astronomical year numbering in which the year 0 = 1 BC, −1 = 2 BC, etc.):

The northern solstice passed from Leo into Cancer in year −1458, passed into Gemini in year −10, passed into Taurus in December 1989, and is expected to pass into Aries in year 4609. The southern solstice passed from Capricornus
Capricornus
into Sagittarius in year −130, is expected to pass into Ophiuchus
Ophiuchus
in year 2269, and is expected to pass into Scorpius
Scorpius
in year 3597.

Solstices on other planets[edit] Due to the 687-day orbit of Mars
Mars
around the Sun
Sun
(almost twice that of the Earth), the planet experiences its summer and winter solstices at approximately 23 month intervals.[28] See also[edit]

Analemma Geocentric view of the seasons Iranian calendars Perihelion and aphelion Zoroastrian calendar

References[edit]

^ United States
United States
Naval Observatory (21 September 2015). "Earth's Seasons: Equinoxes, Solstices, Perihelion, and Aphelion, 2000–2025". Retrieved 9 December 2015.  ^ "The Summer and Winter Solstices". Scholastic. 2017. Retrieved October 1, 2017.  ^ "solstice". The American Heritage Dictionary of the English Language (Fifth ed.). Houghton Mifflin Harcourt. 2015. Retrieved December 8, 2015.  ^ The Principle of relativity
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
Isaac Newton
prevailed. The motion of the Sun
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. ^ "A Solargraph taken from APEX at Chajnantor". European Southern Observatory. Retrieved December 9, 2015.  ^ For an introduction to these topics of astronomy refer to Bowditch, Nathaniel (2002). The American Practical Navigator: an Epitome of Navigation (PDF). Bethesda, Maryland: National Geospatial-Intelligence Agency. Chapter 15 Navigational Astronomy'. Retrieved December 9, 2015.  ^ 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
Strabo
II.5.2., "aplaneis asteres kata parallēlōn pherontai kuklōn", "the fixed stars are borne in parallel circles" ^ Strabo
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. ISBN 0-520-23325-5.  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 of Cleomedes (1891). Ziegler, Hermann, ed. Cleomedis De motu circulari corporum caelestium libri duo. B. G. Teubneri. p. 32.  the passage starts Chapter 4. ^ Kent, April Elliott (June 7, 2011). The Essential Guide to Practical Astrology. Penguin. p. 284. ISBN 978-1-61564-093-5. Retrieved December 9, 2015.  ^ Bass, Charles O. (1994). Astronomy Essentials. Research & Education Assoc. p. 35. ISBN 978-0-87891-965-9. Retrieved December 9, 2015.  ^ Kiddle, Henry (1877). 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. Ivison, Blakeman, Taylor, & Company. p. 82. Retrieved June 2, 2011.  ^ Jain, R.K. ICSE Geography. Ratna Sagar. p. 25. ISBN 978-81-8332-579-0. Retrieved December 9, 2015.  ^ Stewart, Alexander (1869). A Compendium of ModernGeography. Edinburgh: Oliver & Boyd. p. 383. Retrieved December 9, 2015.  ^ "Visitors Guide to the Woodhenge". Retrieved December 19, 2017.  ^ Iseminger, William. "Welcome the Fall Equinox
Equinox
at Cahokia
Cahokia
Mounds". Illinois Department of Natural Resources. Retrieved December 20, 2017.  ^ "Winter Solstice
Solstice
Sunrise Observance at Cahokia
Cahokia
Mounds". Collinsville Chamber of Commerce. Retrieved December 20, 2017.  ^ " Cahokia
Cahokia
Mounds Mark Spring Equinox : The keepers of Cahokia Mounds will host a spring gathering to celebrate the vernal equinox". Indian Country Today. Indian Country Media Network. Retrieved December 20, 2017.  ^ Mollerup, Asger (January 12, 2008). " Solstice
Solstice
Determination based on Observations". Archived from the original on February 11, 2009. Retrieved September 27, 2010.  ^ Exton, Harold (1992). "A Fresh Analysis of Some Recent Data on Atmospheric Refraction Near the Horizon
Horizon
with Implications in Archaeoastronomy". Journal of History of Astronomy, Archaeoastronomy Supplement. 23: S57. Bibcode:1992JHAS...23...57E.  ^ Hugh, Thurston (2001). "Early Greek Solstices and Equinoxes". Journal for the History of Astronomy. 32, Part 2 (107): 154–156. Bibcode:2001JHA....32..154T. ISSN 0021-8286.  ^ Meeus, Jean (1998). Astronomical Algorithms ( Second
Second
English ed.). Richmond: Willmann-Bell, Inc. pp. 177–182. ISBN 0-943396-61-1.  ^ "December solstice". Retrieved March 21, 2018.  ^ "June solstice". Retrieved March 21, 2018.  ^ Meeus, Jean (1997). Mathematical astronomy morsels (1st English ed.). Richmond: Willmann-Bell, Inc. ISBN 0-943396-51-4.  ^ Planetary Society. " Mars
Mars
Calendar". Retrieved December 9, 2015. 

External links[edit]

Look up solstice in Wiktionary, the free dictionary.

Wikimedia Commons has media related to Solstice.

Equinoxes and Solstices Calculator (1600 to 2400) "Earth's Seasons: Equinoxes, Solstices, Perihelion, and Aphelion (2000–2025)". United States
United States
Naval Observatory, Astronomical Applications Department. Retrieved December 9, 2015.  Weisstein, Eric (1996–2007). "Summer Solstice". Eric Weisstein's World of Astronomy. Retrieved October 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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