Solstice year,

UTC date and time of solstices and equinoxes
year	Equinox Mar		Solstice June		Equinox Sept		Solstice Dec
	day	time	day	time	day	time	day	time
2004	20	06:49	21	00:57	22	16:30	21	12:42
2005	20	12:33	21	06:46	22	22:23	21	18:35
2006	20	18:26	21	12:26	23	04:03	22	00:22
2007	21	00:07	21	18:06	23	09:51	22	06:08
2008	20	05:48	20	23:59	22	15:44	21	12:04
2009	20	11:44	21	05:45	22	21:18	21	17:47
2010	20	17:32	21	11:28	23	03:09	21	23:38
2011	20	23:21	21	17:16	23	09:04	22	05:30
2012	20	05:14	20	23:09	22	14:49	21	11:12
2013	20	11:02	21	05:04	22	20:44	21	17:11
2014	20	16:57	21	10:51	23	02:29	21	23:03
2015	20	22:45	21	16:38	23	08:20	22	04:48
2016	20	04:30	20	22:34	22	14:21	21	10:44
2017	20	10:28	21	04:24	22	20:02	21	16:28

A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky reaches its northernmost or southernmost extremes. The name is derived from the Latin sol (sun) and sistere (to stand still), because at the solstices, the Sun stands still in declination; 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 equinoxes, 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 of the 1st century BC: solstitium. Pliny uses it a number of times in his Natural History with the same meaning that it has today. It contains two Latin-language segments, 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 the Earth.

To an observer in inertial space, the planet Earth is seen to rotate about an axis and revolve around the Sun in an elliptical path with the Sun at one focus. The Earth's axis is tilted with respect to the plane of the 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.

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.

Heliocentric view of the seasons

Illumination of Earth by Sun at the northern solstice.

Illumination of Earth by 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

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) 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 inertial space. 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 article for some remarks.

At the northern solstice the subsolar point reaches to 23.44° north, known as the Tropic of Cancer. Likewise at the southern solstice the same thing happens for latitude 23.44° south, known as the Tropic of Capricorn. The sub-solar point will cross every latitude between these two extremes exactly twice per year.

Also during the northern 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 for 24 hours. That is the midnight sun or midsummer-night 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. During the southern solstice the effects on both hemispheres are just the opposite.

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

At the temperate 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.

Further information: effect of sun angle on climate

The seasons are not caused by the varying distance of Earth from the Sun due to the orbital eccentricity 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 - the beginning of 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.

Due to Milankovitch cycles, the Earth's axial tilt and orbital eccentricity will change over thousands of years. Thus in 10,000 years one would find that Earth's northern winter occurs at aphelion and its 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

Day arcs at 0° latitude, equator

Day arcs at 20° latitude

Day arcs at 50° latitude

Day arcs at 70° latitude

Day arcs at 90° latitude, pole

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. 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 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.

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

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 is spherical they devised the concept of the celestial sphere,[6] 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 stars move across the inner surface of the celestial sphere along the circumferences of circles in parallel planes perpendicular to the Earth's axis extended indefinitely into the heavens and intersecting the celestial sphere in a celestial pole. 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:

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."

(source:wikipedia)