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Solstice

Solstice

Overview
A solstice is an astronomical event that happens twice each year when the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

's apparent position in the sky, as viewed from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, reaches its northernmost or southernmost extremes. The name is derived from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 sol (sun) and sistere (to stand still), because at the solstices, the Sun stands still in declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...

; that is, the apparent movement of the Sun's path north or south comes to a stop before reversing direction.

The term solstice can also be used in a broader sense, as the date (day) when this occurs.
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Encyclopedia
A solstice is an astronomical event that happens twice each year when the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

's apparent position in the sky, as viewed from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, reaches its northernmost or southernmost extremes. The name is derived from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 sol (sun) and sistere (to stand still), because at the solstices, the Sun stands still in declination
Declination
In astronomy, declination is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Declination in astronomy is comparable to geographic latitude, but projected onto the celestial sphere. Declination is measured in degrees north and...

; that is, the apparent movement of the Sun's path north or south comes to a stop before reversing direction.

The term solstice can also be used in a broader sense, as the date (day) when this occurs. The solstices, together with the equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

es, are connected with the seasons. In some cultures they are considered to start or separate the seasons, while in others they fall nearer the middle.

Definitions and frames of reference


Of the many ways in which solstice can be defined, one of the most common (and perhaps most easily understood) is by the astronomical phenomenon for which it is named, which is readily observable by anyone on Earth: a "sun-standing." This modern scientific word descends from a Latin scientific word in use in the late Roman republic
Roman Republic
The Roman Republic was the period of the ancient Roman civilization where the government operated as a republic. It began with the overthrow of the Roman monarchy, traditionally dated around 508 BC, and its replacement by a government headed by two consuls, elected annually by the citizens and...

 of the 1st century BC: solstitium. Pliny
Pliny the Elder
Gaius Plinius Secundus , better known as Pliny the Elder, was a Roman author, naturalist, and natural philosopher, as well as naval and army commander of the early Roman Empire, and personal friend of the emperor Vespasian...

 uses it a number of times in his Natural History with the same 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
Relative velocity
In non-relativistic kinematics, relative velocity is the vector difference between the velocities of two objects, as evaluated in terms of a single coordinate system....

 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
Frame of reference
A frame of reference in physics, may refer to a coordinate system or set of axes within which to measure the position, orientation, and other properties of objects in it, or it may refer to an observational reference frame tied to the state of motion of an observer.It may also refer to both an...

. From a fixed position on the ground, the sun appears to orbit around the Earth.

To an observer in an inertial frame of reference
Inertial frame of reference
In physics, an inertial frame of reference is a frame of reference that describes time homogeneously and space homogeneously, isotropically, and in a time-independent manner.All inertial frames are in a state of constant, rectilinear motion with respect to one another; they are not...

, the planet Earth is seen to rotate about an axis
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

 and revolve
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

 around the Sun in an elliptical
Ellipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...

 path with the Sun at one focus
Focus (geometry)
In geometry, the foci are a pair of special points with reference to which any of a variety of curves is constructed. For example, foci can be used in defining conic sections, the four types of which are the circle, ellipse, parabola, and hyperbola...

. The Earth's axis is tilted
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...

 with respect to the plane of the Earth's orbit
Orbital plane (astronomy)
All of the planets, comets, and asteroids in the solar system are in orbit around the Sun. All of those orbits line up with each other making a semi-flat disk called the orbital plane. The orbital plane of an object orbiting another is the geometrical plane in which the orbit is embedded...

 and this axis maintains a position that changes little with respect to the background of star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s. 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
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...

 stops and reverses direction.

The maximum elevation occurs at the summer solstice and the minimum at the winter solstice. The path of the Sun, or ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...

, 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
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

. There are two solstices and two equinoxes in a tropical year.

Heliocentric view of the seasons


The cause of the seasons is that the Earth's axis of rotation is not perpendicular to its orbital plane (the flat plane made through the center of mass (barycenter
Center of mass
In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...

) of the solar system (near or within the Sun) and the successive locations of Earth during the year), but currently makes an angle of about 23.44° (called the "obliquity of the ecliptic"), and that the axis keeps its orientation with respect to an inertial frame of reference
Inertial frame of reference
In physics, an inertial frame of reference is a frame of reference that describes time homogeneously and space homogeneously, isotropically, and in a time-independent manner.All inertial frames are in a state of constant, rectilinear motion with respect to one another; they are not...

. As a consequence, for half the year (from around 20 March to 22 September) the Northern Hemisphere is inclined toward the Sun, with the maximum around 21 June, while for the other half year the Southern Hemisphere has this distinction, with the maximum around 21 December. The two moments when the inclination of Earth's rotational axis has maximum effect are the solstices.

The table at the top of the article gives the instances of equinoxes and solstices over several years. Refer to the equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

 article for some remarks.

At the Northern Solstice the subsolar point reaches to 23.44° north, known as the Tropic of Cancer
Tropic of Cancer
The Tropic of Cancer, also referred to as the Northern tropic, is the circle of latitude on the Earth that marks the most northerly position at which the Sun may appear directly overhead at its zenith...

. Likewise at the Southern Solstice the same thing happens for latitude 23.44° south, known as the Tropic of Capricorn
Tropic of Capricorn
The Tropic of Capricorn, or Southern tropic, marks the most southerly latitude on the Earth at which the Sun can be directly overhead. This event occurs at the December solstice, when the southern hemisphere is tilted towards the Sun to its maximum extent.Tropic of Capricorn is one of the five...

. The sub-solar point will cross every latitude between these two extremes exactly twice per year.

Also during the Northern Solstice, places on the Arctic Circle
Arctic Circle
The Arctic Circle is one of the five major circles of latitude that mark maps of the Earth. For Epoch 2011, it is the parallel of latitude that runs north of the Equator....

 (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
Midnight sun
The midnight sun is a natural phenomenon occurring in summer months at latitudes north and nearby to the south of the Arctic Circle, and south and nearby to the north of the Antarctic Circle where the sun remains visible at the local midnight. Given fair weather, the sun is visible for a continuous...

 or midsummer-night sun or polar day. On the other hand, places on the Antarctic Circle
Antarctic Circle
The Antarctic Circle is one of the five major circles of latitude that mark maps of the Earth. For 2011, it is the parallel of latitude that runs south of the Equator.-Description:...

 (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
Polar night
The polar night occurs when the night lasts for more than 24 hours. This occurs only inside the polar circles. The opposite phenomenon, the polar day, or midnight sun, occurs when the sun stays above the horizon for more than 24 hours.-Description:...

. During the Southern Solstice, the effects on both hemispheres are just the opposite.

At the temperate
Temperate
In geography, temperate or tepid latitudes of the globe lie between the tropics and the polar circles. The changes in these regions between summer and winter are generally relatively moderate, rather than extreme hot or cold...

 latitudes, during summer the Sun remains longer and higher above the horizon, while in winter it remains shorter and lower. This is the cause of summer heat and winter cold.
The seasons are not caused by the varying distance of Earth from the Sun due to the orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 of the Earth's orbit. This variation does make a contribution, but is small compared with the effects of exposure because of Earth's tilt. Currently the Earth reaches perihelion at the beginning of January – during the northern winter and the southern summer. Although the Earth is at its closest to the Sun and therefore receiving more heat, the whole planet is not in summer. Although it is true that the northern winter is somewhat warmer than the southern winter, the placement of the continents may also play an important factor. In the same way, during aphelion at the beginning of July, the Sun is farther away, but that still leaves the northern summer and southern winter as they are with only minor effects.

Over thousands of years, the Earth's axial tilt
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...

 and orbital eccentricity vary (see Milankovitch cycles
Milankovitch cycles
Milankovitch theory describes the collective effects of changes in the Earth's movements upon its climate, named after Serbian civil engineer and mathematician Milutin Milanković, who worked on it during First World War internment...

). Thus, ten thousand years from now Earth's northern winter will occur at aphelion and northern summer at perihelion. The severity of seasonal change—the average temperature difference between summer and winter in location—will also change over time because the Earth's axial tilt fluctuates between 22.1 and 24.5 degrees.



Geocentric view of the seasons


The explanation given in the previous section is useful for observers in outer space. They would see how the Earth revolves around the Sun and how the distribution of sunlight on the planet would change over the year.
To observers on Earth, it is also useful to see how the Sun seems to revolve around them. These pictures show such a perspective as follows. They show the day arcs of the Sun, the paths the Sun tracks along the celestial dome in its diurnal movement
Diurnal motion
Diurnal motion is an astronomical term referring to the apparent daily motion of stars around the Earth, or more precisely around the two celestial poles. It is caused by the Earth's rotation on its axis, so every star apparently moves on a circle, that is called the diurnal circle. The time for...

. The pictures show this for every hour on both solstice days. The longer arc is always the summer track and the shorter one the winter track. The two tracks are at a distance of 46.88° (2 × 23.44°) away from each other.

In addition, some 'ghost' suns are indicated below the horizon, as much as 18° down. The Sun in this area causes twilight. The pictures can be used for both the northern and southern hemispheres. The observer is supposed to sit near the tree on the island in the middle of the ocean. The green arrows give the cardinal directions.
  • On the northern hemisphere the north is to the left, the Sun rises in the east (far arrow), culminates
    Culmination
    In astronomy, the culmination of a planet, star, constellation, etc. is the altitude reached when the object transits over an observer's meridian....

     in the south (to the right) while moving to the right and sets in the west (near arrow). Both rise and set positions are displaced towards the north in summer, and towards the south for the winter track.
  • On the southern hemisphere the south is to the left, the Sun rises in the east (near arrow), culminates in the north (to the right) while moving to the left and sets in the west (far arrow). Both rise and set positions are displaced towards the south in summer, and towards the north for the winter track.


The following special cases are depicted.
  • On the equator
    Equator
    An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

     the Sun is not overhead every day, as some people think. In fact that happens only on two days of the year, the equinoxes. The solstices are the dates that the Sun stays farthest away from the zenith
    Zenith
    The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...

    , only reaching an altitude of 66.56° either to the north or the south. The only thing special about the equator is that all days of the year, solstices included, have roughly the same length of about 12 hours, so that it makes no sense to talk about summer and winter. Instead, tropical areas often have wet and dry seasons.
  • The day arcs at 20° latitude. The Sun culminates at 46.56° altitude in winter and 93.44° altitude in summer. In this case an angle larger than 90° means that the culmination takes place at an altitude of 86.56° in the opposite cardinal direction. For example in the southern hemisphere, the Sun remains in the north during winter, but can reach over the zenith to the south in midsummer. Summer days are longer than winter days, but the difference is no more than two or three hours. The daily path of the Sun is steep at the horizon the whole year round, resulting in a twilight of only about one hour.
  • The day arcs at 50° latitude. The winter Sun does not rise more than 16.56° above the horizon at midday, and 63.44° in summer above the same horizon direction. The difference in the length of the day between summer and winter is striking – slightly less than 8 hours at midwinter, to slightly more than 16 hours in midsummer. Likewise is the difference in direction of sunrise and sunset. Also note the steepness of the daily path of the Sun above the horizon. It is much shallower than at 20° latitude. Therefore not only is the Sun not reaching as high, it also seems not to be in a hurry to do so. But conversely this means that the Sun is not in a hurry to dip deeply below the horizon at night. At this latitude at midnight the summer sun is only 16.56° below the horizon, which means that astronomical twilight continues the whole night. This phenomenon is known as the grey nights, nights when it does not get dark enough for astronomers to do their observations. Above 60° latitude the Sun would be even closer to the horizon, only 6.56° away from it. Then civil twilight continues the whole night. This phenomenon is known as the white nights. And above 66.56° latitude, of course, one would get the midnight sun
    Midnight sun
    The midnight sun is a natural phenomenon occurring in summer months at latitudes north and nearby to the south of the Arctic Circle, and south and nearby to the north of the Antarctic Circle where the sun remains visible at the local midnight. Given fair weather, the sun is visible for a continuous...

    .
  • The day arcs at 70° latitude. At local noon the winter Sun culminates at −3.44°, and the summer Sun at 43.44°. Said another way, during the winter the Sun does not rise above the horizon, it is the polar night
    Polar night
    The polar night occurs when the night lasts for more than 24 hours. This occurs only inside the polar circles. The opposite phenomenon, the polar day, or midnight sun, occurs when the sun stays above the horizon for more than 24 hours.-Description:...

    . There will be still a strong twilight though. At local midnight the summer Sun culminates at 3.44°, said another way, it does not set, it is the polar day.
  • The day arcs at the pole. At the time of the summer or winter solstices, the Sun is 23.44° degrees above or below the horizon respectively, irrespective of time of day. Whilst the Sun is up (during summer months) it will circle around the whole sky, appearing to stay at the same angle from the horizon, therefore the concept of day or night is meaningless. The angle of elevation will gradually change on an annual cycle, with the Sun reaching its highest point at the summer Solstice, and rising or setting at the Equinox
    Equinox
    An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

    , with extended periods of twilight lasting several days after the autumn equinox and before the spring equinox.



Ancient Greek names and concepts


The concept of the solstices was embedded in ancient Greek celestial navigation
Celestial navigation
Celestial navigation, also known as astronavigation, is a position fixing technique that has evolved over several thousand years to help sailors cross oceans without having to rely on estimated calculations, or dead reckoning, to know their position...

. As soon as they discovered that the Earth is spherical they devised the concept of the celestial sphere
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...

, 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 star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s move across the inner surface of the celestial sphere
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

 along the circumference
Circumference
The circumference is the distance around a closed curve. Circumference is a special perimeter.-Circumference of a circle:The circumference of a circle is the length around it....

s of circle
Circle
A circle is a simple shape of Euclidean geometry consisting of those points in a plane that are a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius....

s in parallel
Parallel (geometry)
Parallelism is a term in geometry and in everyday life that refers to a property in Euclidean space of two or more lines or planes, or a combination of these. The assumed existence and properties of parallel lines are the basis of Euclid's parallel postulate. Two lines in a plane that do not...

 planes perpendicular
Perpendicular
In geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...

 to the Earth's axis extended indefinitely into the heavens and intersecting the celestial sphere in a celestial pole. The Sun and the planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

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
Cleomedes
Cleomedes was a Greek astronomer who is known chiefly for his book On the Circular Motions of the Celestial Bodies.-Placing his work chronologically:...

 states:
The band of the Zodiac
Zodiac
In astronomy, the zodiac is a circle of twelve 30° divisions of celestial longitude which are centred upon the ecliptic: the apparent path of the Sun across the celestial sphere over the course of the year...

 (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
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 is in the zodiacal circle as well and periodically crosses the path of the Sun. As some of these crossings represent eclipse
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...

s of the Moon, the path of the Sun is given a synonym, the ekleiptikos (kuklos) from ekleipsis, "eclipse".

English names


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 but in the United Kingdom, the terms longest day and shortest day are more common. However, these can be ambiguous since seasons of the northern hemisphere
    Northern Hemisphere
    The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

     and southern hemisphere
    Southern Hemisphere
    The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

     are opposites, and the summer solstice of one hemisphere is the winter solstice of the other.
  • Northern Solstice and Southern Solstice indicate the direction of the Sun's apparent movement. The northern solstice is in June on Earth, when the Sun is directly over the Tropic of Cancer
    Tropic of Cancer
    The Tropic of Cancer, also referred to as the Northern tropic, is the circle of latitude on the Earth that marks the most northerly position at which the Sun may appear directly overhead at its zenith...

     in the Northern Hemisphere
    Northern Hemisphere
    The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

    , and the southern solstice is in December, when the Sun is directly over the Tropic of Capricorn
    Tropic of Capricorn
    The Tropic of Capricorn, or Southern tropic, marks the most southerly latitude on the Earth at which the Sun can be directly overhead. This event occurs at the December solstice, when the southern hemisphere is tilted towards the Sun to its maximum extent.Tropic of Capricorn is one of the five...

     in the Southern Hemisphere
    Southern Hemisphere
    The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

    .
  • June Solstice and December Solstice are an alternative to the more common "summer" and "winter" terms, but without the ambiguity as to which hemisphere is the context. They are still not universal, however, as not all people 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
    The Hijri calendar , also known as the Muslim calendar or Islamic calendar , is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to date events in many Muslim countries , and used by Muslims everywhere to determine the proper day on which to celebrate Islamic...

     and Hebrew calendar
    Hebrew calendar
    The Hebrew calendar , or Jewish calendar, is a lunisolar calendar used today predominantly for Jewish religious observances. It determines the dates for Jewish holidays and the appropriate public reading of Torah portions, yahrzeits , and daily Psalm reading, among many ceremonial uses...

    , for example), and the names are not useful for other planets (Mars
    Mars
    Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

    , for example), even though these planets do have seasons.
  • First point of Cancer
    Cancer (astrology)
    Cancer is the fourth astrological sign in the Zodiac. It is considered a water sign and one of four cardinal signs. Cancer is ruled by the Moon. Individuals born when the Sun is in this sign are considered Cancerian individuals...

    and first point of Capricorn. One disadvantage of these names is that, due to the precession of the equinoxes
    Precession of the equinoxes
    In astronomy, axial precession is a gravity-induced, slow and continuous change in the orientation of an astronomical body's rotational axis. In particular, it refers to the gradual shift in the orientation of Earth's axis of rotation, which, like a wobbling top, traces out a pair of cones joined...

    , the astrological signs
    Zodiac
    In astronomy, the zodiac is a circle of twelve 30° divisions of celestial longitude which are centred upon the ecliptic: the apparent path of the Sun across the celestial sphere over the course of the year...

     where these solstices are located no longer correspond with the actual constellation
    Constellation
    In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

    s. The solstices are currently in the constellations of Taurus
    Taurus (constellation)
    Taurus is one of the constellations of the zodiac. Its name is a Latin word meaning 'bull', and its astrological symbol is a stylized bull's head:...

     and Sagittarius
    Sagittarius (constellation)
    Sagittarius is a constellation of the zodiac, the one containing the galactic center. Its name is Latin for the archer, and its symbol is , a stylized arrow. Sagittarius is commonly represented as a centaur drawing a bow...

    .
  • The Latin
    Latin
    Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

     names Hibernal solstice (winter) and Aestival solstice (summer) are sometimes used.

Solstice terms in East Asia


The traditional East Asian calendars divide a year into 24 solar term
Solar term
A solar term is any of 24 points in traditional East Asian lunisolar calendars that matches a particular astronomical event or signifies some natural phenomenon. The points are spaced 15° apart along the ecliptic and are used by lunisolar calendars to stay synchronized with the seasons. Solar terms...

s (節氣). Xiàzhì (pīnyīn
Pinyin
Pinyin is the official system to transcribe Chinese characters into the Roman alphabet in China, Malaysia, Singapore and Taiwan. It is also often used to teach Mandarin Chinese and spell Chinese names in foreign publications and used as an input method to enter Chinese characters into...

)
or Geshi (rōmaji
Romanization of Japanese
The romanization of Japanese is the application of the Latin alphabet to write the Japanese language. This method of writing is known as , less strictly romaji, literally "Roman letters", sometimes incorrectly transliterated as romanji or rōmanji. There are several different romanization systems...

)
is the 10th solar term, and marks the summer solstice. It begins when the Sun reaches the celestial longitude of 90° (around June 21) and ends when the Sun reaches the longitude of 105° (around July 7). Xiàzhì more often refers in particular to the day when the Sun is exactly at the celestial longitude of 90°.

Dōngzhì (pīnyīn
Pinyin
Pinyin is the official system to transcribe Chinese characters into the Roman alphabet in China, Malaysia, Singapore and Taiwan. It is also often used to teach Mandarin Chinese and spell Chinese names in foreign publications and used as an input method to enter Chinese characters into...

)
or Tōji (rōmaji
Romanization of Japanese
The romanization of Japanese is the application of the Latin alphabet to write the Japanese language. This method of writing is known as , less strictly romaji, literally "Roman letters", sometimes incorrectly transliterated as romanji or rōmanji. There are several different romanization systems...

)
is the 22nd solar term, and marks the winter solstice. It begins when the Sun reaches the celestial longitude of 270° (around December 22 ) and ends when the 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 equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

es) 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 celebrations



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 equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

es, are connected with the seasons. In some languages they are considered to start or separate the seasons; in others they are considered to be center points (in England, in the Northern hemisphere, for example, the period around the June solstice is known as midsummer
Midsummer
Midsummer may simply refer to the period of time centered upon the summer solstice, but more often refers to specific European celebrations that accompany the actual solstice, or that take place on a day between June 21 and June 24, and the preceding evening. The exact dates vary between different...

, and Midsummer's Day 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.

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 December solstice, Christmas is the most popular holiday to have arisen. In addition, Yalda
Yalda
Yalda , Yalda Night / Shab-e Yaldâ , or Shab-e Chelleh is the Persian Winter Solstice Celebration which has been popular since ancient times. Yalda is celebrated on the Northern Hemisphere's longest night of the year, that is, on the eve of the Winter Solstice...

, Saturnalia
Saturnalia
Saturnalia is an Ancient Roman festival/ celebration held in honour of Saturn , the youngest of the Titans, father of the major gods of the Greeks and Romans, and son of Uranus and Gaia...

, Karachun, Hanukkah
Hanukkah
Hanukkah , also known as the Festival of Lights, is an eight-day Jewish holiday commemorating the rededication of the Holy Temple in Jerusalem at the time of the Maccabean Revolt of the 2nd century BCE...

, Kwanzaa
Kwanzaa
Kwanzaa is a week long celebration held in the United States honoring universal African-American heritage and culture, observed from December 26 to January 1 each year. It features activities such as lighting a candle holder with seven candles and culminates in a feast and gift giving...

 and Yule
Yule
Yule or Yuletide is a winter festival that was initially celebrated by the historical Germanic people as a pagan religious festival, though it was later absorbed into, and equated with, the Christian festival of Christmas. The festival was originally celebrated from late December to early January...

 (see winter solstice
Winter solstice
Winter solstice may refer to:* Winter solstice, astronomical event* Winter Solstice , former band* Winter Solstice: North , seasonal songs* Winter Solstice , 2005 American film...

 for more) are also celebrated around this time. For the June solstice, Christian cultures celebrate the feast of St. John
John the Baptist
John the Baptist was an itinerant preacher and a major religious figure mentioned in the Canonical gospels. He is described in the Gospel of Luke as a relative of Jesus, who led a movement of baptism at the Jordan River...

 from June 23 to 24 (see St. John's Eve
St. John's Eve
The evening of June 23, St John's Eve, is the eve of celebration before the Feast Day of St John the Baptist. The Gospel of Luke states that John was born about six months before Jesus, therefore the feast of John the Baptist was fixed on June 21~24, six months before Christmas...

, Ivan Kupala Day
Ivan Kupala Day
Kupala Day is celebrated in Poland, Russia, Belarus and Ukraine currently on the night of 6/7 July in the Gregorian or New Style calendar, which is 23/24 June in the Julian or Old Style calendar still used by many Orthodox Churches. Calendar-wise, it is opposite to the winter solstice holiday...

, 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
Nowruz
Nowrūz is the name of the Iranian New Year in Iranian calendars and the corresponding traditional celebrations. Nowruz is also widely referred to as the Persian New Year....

, the observance in Judaism of Passover
Passover
Passover is a Jewish holiday and festival. It commemorates the story of the Exodus, in which the ancient Israelites were freed from slavery in Egypt...

 and in most Christian churches of Easter. The autumnal equinox has also given rise to various holidays, such as the Jewish holiday of Sukkot
Sukkot
Sukkot is a Biblical holiday celebrated on the 15th day of the month of Tishrei . It is one of the three biblically mandated festivals Shalosh regalim on which Hebrews were commanded to make a pilgrimage to the Temple in Jerusalem.The holiday lasts seven days...

. At the midpoints between these four solar events, cross-quarter days are celebrated.

In the southern tip of South America, the Mapuche people celebrate We Tripantu (the New Year) a few days after the winter solstice, on June 24. 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 June 21. A particularly beautiful and significant celebration occurs at sunrise when the sun shines right through the Gate of the Sun
Gate of the Sun
For the Inca trail related to Machu Picchu, see Intipuncu.The so-called Gate of the Sun is a megalithic solid stone arch or gateway constructed by the ancient Tiwanaku culture of Bolivia over 1500 years before the present....

 in Tiwanaku
Tiwanaku
Tiwanaku, is an important Pre-Columbian archaeological site in western Bolivia, South America. Tiwanaku is recognized by Andean scholars as one of the most important precursors to the Inca Empire, flourishing as the ritual and administrative capital of a major state power for approximately five...

. Other Aymara New Year feasts occur throughout Bolivia, including at the site of El Fuerte de Samaipata
El Fuerte de Samaipata
El Fuerte de Samaipata , also known simply as 'El Fuerte', is an archaeological site and UNESCO World Heritage Site located in the Santa Cruz Department, Florida Province, Bolivia. It is situated in the eastern foothills of the Bolivian Andes, and is a popular tourist destination for Bolivians and...

.

In many cultures the solstices and equinoxes traditionally determine the midpoint of the seasons, which can be seen in the celebrations called midsummer and midwinter. Along this vein, the Japanese celebrate the start of each season with an occurrence known as Setsubun
Setsubun
is the day before the beginning of spring in Japan. The name literally means "seasonal division", but usually the term refers to the spring Setsubun, properly called Risshun celebrated yearly on February 3 as part of the...

. 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 and northern Europe, as well as in Canada, the US, Australia, and New Zealand.

In the Hindu calendar
Hindu calendar
The hindu calendar used in ancient times has undergone many changes in the process of regionalization, and today there are several regional Indian calendars, as well as an Indian national calendar. Nepali calendar, Bengali calendar, Malayalam calendar, Tamil calendar, Telugu calendar, Kannada...

, two sidereal solstices are named Makara Sankranti which marks the start of Uttarayana
Uttarayana
Uttarāyaṇa , or Uttarayana, is the six-month period between Makar Sankranti and Karka Sankranti , when the sun travels towards the north on the celestial sphere. The name Uttarayana comes from joining two different Sanskrit words "Uttara" and "ayana"...

 and Karkat 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 along a sidereally fixed zodiac
Zodiac
In astronomy, the zodiac is a circle of twelve 30° divisions of celestial longitude which are centred upon the ecliptic: the apparent path of the Sun across the celestial sphere over the course of the year...

 (precession is ignored) into Makara, the zodiacal sign which corresponds with Capricorn, and into Karkat, the zodiacal sign which corresponds with Cancer
Cancer (astrology)
Cancer is the fourth astrological sign in the Zodiac. It is considered a water sign and one of four cardinal signs. Cancer is ruled by the Moon. Individuals born when the Sun is in this sign are considered Cancerian individuals...

, respectively.

Solstice determination


Unlike the equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

, 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 of the angular size of the sun, or the equivalent to just 2 seconds of right ascension
Right ascension
Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...

.

This difference is hardly detectable with indirect viewing based devices like sextant
Sextant
A sextant is an instrument used to measure the angle between any two visible objects. Its primary use is to determine the angle between a celestial object and the horizon which is known as the altitude. Making this measurement is known as sighting the object, shooting the object, or taking a sight...

 equipped with a vernier
Vernier scale
A vernier scale is an additional scale which allows a distance or angle measurement to be read more precisely than directly reading a uniformly-divided straight or circular measurement scale...

, and impossible with more traditional tools like a gnomon
Gnomon
The gnomon is the part of a sundial that casts the shadow. Gnomon is an ancient Greek word meaning "indicator", "one who discerns," or "that which reveals."It has come to be used for a variety of purposes in mathematics and other fields....

 or an astrolabe
Astrolabe
An astrolabe is an elaborate inclinometer, historically used by astronomers, navigators, and astrologers. Its many uses include locating and predicting the positions of the Sun, Moon, planets, and stars, determining local time given local latitude and longitude, surveying, triangulation, and to...

. It is also hard to detect the changes on sunrise/sunset azimuth due to the atmospheric refraction
Atmospheric refraction
Atmospheric refraction is the deviation of light or other things like humanelectromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude...

 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.

Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

 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.

See also


  • Midsummer
    Midsummer
    Midsummer may simply refer to the period of time centered upon the summer solstice, but more often refers to specific European celebrations that accompany the actual solstice, or that take place on a day between June 21 and June 24, and the preceding evening. The exact dates vary between different...

  • Summer solstice
    Summer solstice
    The summer solstice occurs exactly when the axial tilt of a planet's semi-axis in a given hemisphere is most inclined towards the star that it orbits. Earth's maximum axial tilt to our star, the Sun, during a solstice is 23° 26'. Though the summer solstice is an instant in time, the term is also...

  • Winter solstice
    Winter solstice
    Winter solstice may refer to:* Winter solstice, astronomical event* Winter Solstice , former band* Winter Solstice: North , seasonal songs* Winter Solstice , 2005 American film...

  • Analemma
    Analemma
    In astronomy, an analemma is a curve representing the angular offset of a celestial body from its mean position on the celestial sphere as viewed from another celestial body relative to the viewing body's celestial equator...


  • Iranian calendar
    Iranian calendar
    The Iranian calendars or sometimes called Persian calendars are a succession of calendars invented or used for over two millennia in Greater Iran...

  • Zoroastrian calendar
    Zoroastrian calendar
    This article treats of the reckoning of days, months and years in the calendar used by adherents of the Zoroastrian faith. Zoroastrian religious festivals are discussed elsewhere, but have a fixed relationship to Nawruz, the New Year festival, whose timing is discussed below...

  • Equinox
    Equinox
    An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...