The
Solstice occurs twice each year (around June 21 and December 21) as the
Sun reaches its most northerly or southerly excursion relative to the
celestial equator on the
celestial sphere. 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 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.
At latitudes outside the
tropics, the summer solstice marks the day when the sun appears highest in the sky. Within the tropics, the sun appears directly overhead (called the
subsolar point) from days to months before the solstice and again after the solstice, which means the subsolar point occurs twice each year.
The word
solstice 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 seasonal movement of the Sun's path (as seen from
Earth) comes to a stop before reversing direction.
Definitions and frames of reference[edit]
For an observer on 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 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 the 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 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".
[2] 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.
[3]
A
solargraph taken from the
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.
[4] 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 – 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 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.
[5]
Relationship to seasons[edit]
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.
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 also allows the polar
sea ice to increase its annual growth and temporary extent at a greater level due to lack of direct sunlight.
-
-
-
-
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 is spherical
[6] they devised the concept of the
celestial sphere,
[7] 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
[8] perpendicular to the Earth's axis extended indefinitely into the heavens and intersecting the celestial sphere in a celestial pole.
[9] 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.*
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[edit]
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,[11] as are midsummer and midwinter.
- June solstice and December solstice refer to the months of year in which they take place,[12] 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.[13] 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.[14] 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.[15] Due to the precession of the equinoxes, however, the constellations where the solstices are currently located are Taurus and Sagittarius, respectively.
Solstice terms in East Asia[edit]
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; literally: "
summer's extreme") 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) or
Tōji (rōmaji) (
Chinese and
Japanese: 冬至;
Korean:
동지(Dongji);
Vietnamese:
Đông chí; literally: "
winter's extreme") 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
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 celebrations[edit]
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, 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 southern solstice, Christmas is the most popular holiday to have arisen. In addition,
Yalda,
Saturnalia,
Karachun,
Hanukkah,
Kwanzaa and
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 of
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 many cultures, the solstices and equinoxes traditionally determine the midpoint of the seasons, which can be seen in the celebrations called midsummer and
midwinter. In this vein, the
Japanese celebrate the start of each season with an occurrence known as
Setsubun. 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 United States and
New Zealand.
In the
Hindu calendar, two sidereal solstices are named
Makara Sankranti which marks the start of
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 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.
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[17] or an
astrolabe. It is also hard to detect the changes on sunrise/sunset azimuth due to the
atmospheric refraction[18] 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.
[19] 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).
[20]
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
[21] (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 into Sagittarius in year −130, is expected to pass into Ophiuchus in year 2269, and is expected to pass into Scorpius in year 3597.
