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 seasons of the year are defined by reference to the solstices and the equinoxes.
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.
Etymology The word solstice is derived from the Latin () and (), because at the solstices, the Sun's declination appears to "stand still"; that is, the seasonal movement of the Sun's daily path (as seen from Earth) reaches a northern or southern limit before reversing direction.
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.
Definitions and frames of reference
For an observer at the North Pole, the Sun reaches the highest position in the sky once a year in June. The day this occurs is called the June solstice day. Similarly, for an observer on the South Pole, the Sun reaches the highest position on the December solstice day. When it is the summer solstice at one Pole, it is the winter solstice on the other. The Sun's westerly motion never ceases as Earth is continually in rotation. However, at the moment of solstice the Sun's motion in declination (i.e. vertically) appears to stop for an instant, and then reverse. In that sense, solstice means "sun-standing".
This modern scientific word descends from a Latin scientific word in use in the late Roman Republic of the 1st century BC: solstitium. Pliny 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 axis and orbit around the Sun in an elliptical path with the 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 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.
Relationship to seasons
The seasons occur because the Earth's axis of rotation is not perpendicular to its orbital plane (the plane of the ecliptic) but currently makes an angle of about 23.44° (called the obliquity of the ecliptic), and because the axis keeps its orientation with respect to an inertial frame of reference. As a consequence, for half the year the Northern Hemisphere is inclined toward the Sun while for the other half year the Southern Hemisphere has this distinction. The two moments when the inclination of Earth's rotational axis has maximum effect are the solstices.
At the June solstice the subsolar point is further north than any other time: at latitude 23.44° north, known as the Tropic of Cancer. Similarly at the December solstice the subsolar point is further south than any other time: at latitude 23.44° south, known as the Tropic of Capricorn. The subsolar point will cross every latitude between these two extremes exactly twice per year.
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.
Cultural aspects
Ancient Greek names and concepts The concept of the solstices was embedded in ancient Greek celestial navigation. As soon as they discovered that the Earth was spherical they devised the concept of the celestial sphere, an imaginary spherical surface rotating with the heavenly bodies (ouranioi) fixed in it (the modern one does not rotate, but the stars in it do). As long as no assumptions are made concerning the distances of those bodies from Earth or from each other, the sphere can be accepted as real and is in fact still in use. The Ancient Greeks use the term "ηλιοστάσιο" (heliostāsio), meaning stand of the Sun.
The stars move across the inner surface of the celestial sphere along the circumferences of circles 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 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 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 eclipses 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, 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, as are midsummer and midwinter. June solstice and December solstice refer to the months of year in which they take place, with no ambiguity as to which hemisphere is the context. They are still not universal, however, as not all cultures use a solar-based calendar where the solstices occur every year in the same month (as they do not in the Islamic calendar and Hebrew calendar, for example). Northern solstice and southern solstice indicate the hemisphere of the Sun's location. The northern solstice is in June, when the Sun is directly over the Tropic of Cancer in the Northern Hemisphere, and the southern solstice is in December, when the Sun is directly over the Tropic of Capricorn in the Southern Hemisphere. 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" (a system rooted in Roman Classical period dates). Due to the precession of the equinoxes, the constellations the sun appears in at solstices are currently Taurus in June and Sagittarius in December.
+Names of the equinoxes and solsticesLsBy date (Gregorian calendar)By sun position (subsolar point)By season (Northern Hemisphere)By season (Southern Hemisphere) 0°March equinoxNorthward equinoxVernal (spring) equinoxAutumnal (fall) equinox 90°June solsticeNorthern solsticeEstival (summer) solsticeHibernal (winter) solstice 180°September equinoxSouthward equinoxAutumnal (fall) equinoxVernal (spring) equinox 270°December solsticeSouthern solsticeHibernal (winter) solsticeEstival (summer) solstice
Solstice terms in East Asia
The traditional East Asian calendars divide a year into 24 solar terms (節氣). Xiàzhì (pīnyīn) or Geshi (rōmaji) () is the 10th solar term, and marks the summer solstice. It begins when the Sun reaches the celestial longitude of 90° (around 21 June) and ends when the Sun reaches the longitude of 105° (around 7 July). Xiàzhì more often refers in particular to the day when the Sun is exactly at the celestial longitude of 90°.