Solstice

A solstice is one of two opposite points on the celestial sphere where the celestial equator and ecliptic reach their largest separation. They derive their name (sol stitium is Latin for sun standstill) from the fact that when the Sun passes a solstice, it stands still in declination, i.e. it reaches a maximum or a minimum. The term solstice can also be used in a narrower sense of being the instance in time that such a passage happens. The solstices then, together with the equinoxes, are the defining moments of the start of the (astronomical) seasons, except in China, where they mark their centers.

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. These names can be used when one wants to explain the cause of the seasons. But as the seasons of the northern hemisphere and southern hemisphere are opposites, the summer solstice of one hemisphere is the winter solstice of the other, which makes their use ambiguous.
June solstice and December solstice. An alternative to the previous set, but without the ambiguity for which hemisphere they are intended. Still not universal, however, as not all people on Earth use a solar based calendar where the solstices occur every year in the same month (Jewish calendar for example), and also not useful for other planets (Mars for example), even though they do have seasons.
First point of Cancer and first point of Capricorn. Alternative names for the previous set. Disadvantage is that due to the precession of the equinoxes these astrological signs do not correspond any longer with the actual constellations where these solstices are located.
Taurus solstice and Sagittarius solstice. Names to indicate in which constellations the two equinoxes are currently located. These terms are not widely used, the more so since until December 1989 the first solstice was in Gemini according to official IAU boundaries.

Outside view of the seasons

The cause of the seasons is that the rotation axis of the Earth is not perpendicular to its orbital plane, but makes an angle of about 23.44°, the obliquity of the ecliptic, and that this axis keeps its orientation in inertial space. As a consequence, for half a year (from around 20 March to 22 September) the northern hemisphere tips to the Sun, with the maximum around 21 June, while for the other half year the southern hemisphere has this honour, with the maximum around 21 December. The two instances that the Earth's rotation axis is maximally tipped are the solstices.

The table above gives the instances of equinoxes and solstices over several years. Refer to the equinox article for some remarks.

During the June solstice the Sun appears to be directly overhead at noon for places situated at latitude 23.44° north, known as the tropic of Cancer. Likewise during the December solstice the same thing happens for latitude 23.44° south, known as the tropic of Capricorn. All places on Earth in between these two latitudes are known as the tropics and will see the Sun in the zenith at least two days in the year.

Also during the June solstice places situated at latitude 66.56° north, known as the arctic circle will see the Sun just on the horizon during midnight, and all places north of it will see the Sun above horizon at any time of the day. That is the midnight sun or midsummernight sun or polar day. On the other hand, places at latitude 66.56° south, known as the antarctic circle 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. Of course during the December solstice the effects on both hemispheres are just the opposite.

At the temperate latitudes, during summer the Sun is 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 (see: effect of sun angle on climate).

The seasons are not caused by the varying distance of Earth to the Sun due to the orbital eccentricity of the Earth's orbit. This variation is so small that it only makes a minor contribution. Currently the Earth reaches perihelion at the beginning of January, which is during the northern winter and the southern summer. The Sun being closer to Earth and therefore hotter does not cause the whole planet enter summer. Although it is true that the northern winter is somewhat warmer than the southern winter, the placement of the continents, ice-covered Antarctica in particular, may be the more 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.

Inside 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 us, observers on Earth, it is more useful to see how the Sun seems to revolve around us. The pictures to the right try to do that. They show the day arcs of the Sun, the paths the Sun tracks along the celestial dome in its diurnal movement. 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 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 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, 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 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. Likewise is the difference in direction of sunrise and sunset. Also note the different steepness of the daily path of the Sun above the horizon in summer and winter. It is much shallower in winter. Therefore not only is the Sun not reaching as high, it also seems not to be in a hurry to do so. But conversly this means that in summer 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° latitude, of course, one would get the midnight sun.
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. There will be still a strong twilight though. At local midnight the Sun culminates at 3.44°, said another way, it does not set, it is the polar day.
The day arcs at the pole. All the time the Sun is 23.44° above or below the horizon, depending on whether it is the summer or winter solstice. In the latter case, that is enough to not even have any twilight. There is also no south or north, neither east or west being discernible.

Due to atmospheric refraction, the Sun may already appear above the horizon when the real, geometric Sun is still below it.

Cultural aspects

There are traditional holidays celebrated in Northern Europe and elsewhere around the time of the June solstice, which is their Midsummer. In Denmark and Norway midsummers eve is known as Sankt Hans Aften. Bonfires are lit on beaches throughout the land as darkness comes and effigies and fireworks are thrown into the fires.
The Slavs celebrate the shortest night of the year, i.e. the summer solstice in June, as St. John's Night (see also Juhannus of Finns).

There are also feasts around the December solstice in northern Europe, their midwinter, the best known being the Germanic festival of Yule.

The summer solstice is celebrated as a Wiccan sabbat, called Litha. Every year in June, a celebration takes place at Stonehenge; (see Sun mythology). Likewise the winter solstice is celebrated nowadays by Ásatrúar and is recognized by some Neopagan groups as a sabbat too.
Other December solstice (or days close to it) holidays are the Yalda, Saturnalia, Christmas, Karachun, Hanukkah, Kwanzaa and Zamenhof Day. The first civilization to celebrate the winter solstice were the ancient Persians, deriving from their Zoroastrian religion.
See the list of winter festivals for yet more.
In Japan, all four major season days are celebrated, see Setsubun.
The passage and chamber of Newgrange, a tomb in Ireland, are illuminated by the winter solstice sunrise. A shaft of sunlight shines through the roof box over the entrance and penetrates the passage to light up the chamber. The dramatic event lasts for 17 minutes at dawn from the 19th to the 23rd of December.

In several cultures the solstices and equinoxes do not determine the start but the midpoint of the seasons, see cross-quarter day and seasons.

Solar terms in Chinese astronomy

In the Chinese calendar, the December solstice marks midwinter and is called dōng zhì (冬至, "winter's extreme"). It is traditionally regarded as one of the year's most important jiéqìs (solar terms), comparable to Chinese New Year. Rather confusingly, the character zhì may also mean "arrival" in other contexts (here it means "extreme"), but it is clear that the Chinese consider "winter's arrival" (立冬, lì dōng, literally "establishment of winter") to be a separate jiéqì which falls on or around November 7 instead.

Xiazhi is a solar term that begins when the Sun reaches a celestial longitude of 90° and ends when its celestial longitude is 105°. It sometimes refers in particular to the day when Sun is exactly at the celestial longitude of 90°. It usually begins around June 21, and ends around July 7.

External links

The seasons begin at the time of the solstice or equinox (from the Bad Astronomer)
Solstice does not signal season's start? (from The Straight Dope)
Solstice dates and times
Online Calculator for Dates and Times of Equinoxes and Solstices
Earth's Seasons, Equinoxes, Solstices, Perihelion, and Aphelion, 1992-2020 (from the United States Naval Observatory's Astronomical Applications Department)
Plot that shows how the date of the summer solstice shifts through the Gregorian calendar
Wolfram Research solstice explanation
Winter Solstice (in Celtic mythology)
Solstice Dates and Times
Solstice, Equinox & Cross-Quarter Moments for 2006 and other years, for several timezones
Calculation of Length of Day (Formulas and Graphs)