Solstice
event | equinox | solstice | equinox | solstice | ||||
---|---|---|---|---|---|---|---|---|
month | March[3] | June[4] | September[5] | December[6] | ||||
year | day | time | day | time | day | time | day | time |
2019 | 20 | 21:58 | 21 | 15:54 | 23 | 07:50 | 22 | 04:19 |
2020 | 20 | 03:50 | 20 | 21:43 | 22 | 13:31 | 21 | 10:03 |
2021 | 20 | 09:37 | 21 | 03:32 | 22 | 19:21 | 21 | 15:59 |
2022 | 20 | 15:33 | 21 | 09:14 | 23 | 01:04 | 21 | 21:48 |
2023 | 20 | 21:25 | 21 | 14:58 | 23 | 06:50 | 22 | 03:28 |
2024 | 20 | 03:07 | 20 | 20:51 | 22 | 12:44 | 21 | 09:20 |
2025 | 20 | 09:02 | 21 | 02:42 | 22 | 18:20 | 21 | 15:03 |
2026 | 20 | 14:46 | 21 | 08:25 | 23 | 00:06 | 21 | 20:50 |
2027 | 20 | 20:25 | 21 | 14:11 | 23 | 06:02 | 22 | 02:43 |
2028 | 20 | 02:17 | 20 | 20:02 | 22 | 11:45 | 21 | 08:20 |
2029 | 20 | 08:01 | 21 | 01:48 | 22 | 17:37 | 21 | 14:14 |
A solstice is an event that occurs when the Sun reaches its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. In many countries, the seasons of the year are determined by 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 a solstice in either hemisphere 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 in which they take place every year.[7]
The word solstice is derived from the Latin sol ("sun") and sistere ("to stand still"), 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) pauses at a northern or southern limit before reversing direction.
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, the Sun's motion in declination (i.e. vertically) comes to a stop, before reversing, 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".[8] 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.[9]
To an observer in an
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.[11]
Relationship to seasons
The seasons occur because the Earth's axis of rotation is not perpendicular to its orbital plane (the
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.
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Orientation of the terminator (division between night and day) depends on the season.
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Illumination of Earth by Sun on 21 June. The orientation of the terminator shown with respect to the Earth's orbital plane.
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Illumination of Earth by Sun on 21 December. The orientation of the terminator shown with respect to the Earth's orbital plane.
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Diagram of the Earth's seasons as seen from the north. Far right:southern solstice
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Diagram of the Earth's seasons as seen from the south. Far left:northern solstice
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The globe on an equirectangular projection to show the amount of reflected sunlight at southern and northern summer solstices, respectively (watts / m2).
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[12] they devised the concept of the celestial sphere,[13] 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
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,[17] as are midsummer and midwinter.
- June solstice and December solstice refer to the months of year in which they take place,[18] 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.[19] 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.[20] These terms can be used unambiguously for other planets.
- First point of precession of the equinoxes, the constellations the sun appears in at solstices are currently Taurus in June and Sagittariusin December.
Ls
|
By date | By sun position | By season | By season |
---|---|---|---|---|
0° | March equinox | Northward equinox | Vernal (spring) equinox | Autumnal (fall) equinox |
90° | June solstice | Northern solstice | Estival (summer) solstice | Hibernal (winter) solstice |
180° | September equinox | Southward equinox | Autumnal (fall) equinox | Vernal (spring) equinox |
270° | December solstice | Southern solstice | Hibernal (winter) solstice | Estival (summer) solstice |
Solstice terms in East Asia
The traditional East Asian calendars divide a year into 24
Dōngzhì (
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
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
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. During the southern or
.In the southern tip of
In the
The Amundsen–Scott South Pole Station celebrates every year on June 21 a midwinter party, to celebrate that the Sun is at its lowest point and coming back.
The
The reconstructed Cahokia Woodhenge, a large timber circle located at the Mississippian culture Cahokia archaeological site near Collinsville, Illinois,[22] is the site of annual equinox and solstice sunrise observances. Out of respect for Native American beliefs these events do not feature ceremonies or rituals of any kind.[23][24][25]
Solstice determination
Unlike the equinox, the solstice time is not easy to determine. The changes in
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[26] or an astrolabe. It is also hard to detect the changes in sunrise/sunset azimuth due to the atmospheric refraction[27] 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.[28] 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
In the constellations
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[32] (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.
On other planets
The 687-day orbit of Mars around the Sun (almost twice that of the Earth) causes its summer and winter solstices to occur at approximately 23-month intervals.[33]
See also
- Analemma
- Geocentric view of the seasons
- Iranian calendars
- Perihelion and aphelion
- Wheel of the Year
- Zoroastrian calendar
References
- . Retrieved August 1, 2022.
- ^ "Solstices and Equinoxes: 2001 to 2100". AstroPixels.com. February 20, 2018. Retrieved December 21, 2018.
- ^ Équinoxe de printemps entre 1583 et 2999
- ^ Solstice d’été de 1583 à 2999
- ^ Équinoxe d’automne de 1583 à 2999
- ^ Solstice d’hiver
- ^ "The Summer and Winter Solstices". Scholastic. 2017. Retrieved October 1, 2017.
- ^ "solstice". The American Heritage Dictionary of the English Language (Fifth ed.). Houghton Mifflin Harcourt. 2015. Retrieved December 8, 2015.
- ^ 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.
- ^ "A Solargraph taken from APEX at Chajnantor". European Southern Observatory. Retrieved December 9, 2015.
- ^ For an introduction to these topics of astronomy refer to Bowditch, Nathaniel (2002). The American Practical Navigator: an Epitome of Navigation (PDF). Bethesda, Maryland: National Geospatial-Intelligence Agency. Chapter 15 Navigational Astronomy'. Archived from the original (PDF) on February 8, 2016. Retrieved December 9, 2015.
- ^ Strabo. The Geography. II.5.1.
sphairikē ... tēs gēs epiphaneia, spherical is the surface of the Earth
- ^ Strabo. The Geography. pp. II.5.2.
sphairoeidēs ... ouranos, spherical in appearance ... is heaven
- ^ Strabo II.5.2., "aplaneis asteres kata parallēlōn pherontai kuklōn", "the fixed stars are borne in parallel circles"
- ^ 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"
- ISBN 0-520-23325-5. 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 Cleomedes (1891). Ziegler, Hermann (ed.). Cleomedis De motu circulari corporum caelestium libri duo. B. G. Teubneri. pp. 32. the passage starts Chapter 4.
- ISBN 978-1-61564-093-5. Retrieved December 9, 2015.
- ISBN 978-0-87891-965-9. Retrieved December 9, 2015.
- ^ Kiddle, Henry (1877). A new manual of the elements of astronomy, descriptive and mathematical: comprising the latest discoveries and theoretic views : with directions for the use of the globes, and for studying the constellations. Ivison, Blakeman, Taylor, & Company. p. 82. Retrieved June 2, 2011.
- ISBN 978-81-8332-579-0. Retrieved December 9, 2015.
- ^ Stewart, Alexander (1869). A Compendium of ModernGeography. Edinburgh: Oliver & Boyd. p. 383. Retrieved December 9, 2015.
- ^ "Visitors Guide to the Woodhenge". Archived from the original on December 22, 2017. Retrieved December 19, 2017.
- ^ Iseminger, William. "Welcome the Fall Equinox at Cahokia Mounds". Illinois Department of Natural Resources. Retrieved December 20, 2017.
- ^ "Winter Solstice Sunrise Observance at Cahokia Mounds". Collinsville Chamber of Commerce. Retrieved December 20, 2017.
- Indian Country Media Network. Archived from the originalon December 22, 2017. Retrieved December 20, 2017.
- ^ Mollerup, Asger (January 12, 2008). "Solstice Determination based on Observations". Archived from the original on February 11, 2009. Retrieved September 27, 2010.
- S2CID 118484271.
- S2CID 118464897.
- ISBN 0-943396-61-1.
- ^ "December solstice". Retrieved March 21, 2018.
- ^ "June solstice". Retrieved March 21, 2018.
- ISBN 0-943396-51-4.
- ^ Planetary Society. "Mars Calendar". Retrieved December 9, 2015.
External links
- Equinoxes and Solstices Calculator (1600 to 2400)
- "Earth's Seasons: Equinoxes, Solstices, Perihelion, and Aphelion (2000–2025)". United States Naval Observatory, Astronomical Applications Department. Retrieved December 9, 2015.
- Weisstein, Eric (1996–2007). "Summer Solstice". Eric Weisstein's World of Astronomy. Retrieved October 24, 2008.
The above plots show how the date of the summer solstice shifts through the Gregorian calendar according to the insertion of leap years.