Solstice

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20152022:452116:382308:212204:48
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A solstice is an event occurring when the Sun appears to reach 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. The seasons of the year are determined by reference to both the solstices and the equinoxes.

Sun Star at the center of the Solar System

The Sun, or Sol, is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers, or 109 times that of Earth, and its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System. Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.

Sun path

Sun path, sometimes also called day arc, refers to the daily and seasonal arc-like path that the Sun appears to follow across the sky as the Earth rotates and orbits the Sun. The Sun's path affects the length of daytime experienced and amount of daylight received along a certain latitude during a given season.

Celestial equator projection of the Earths equator out into space

The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract projection of the terrestrial equator into outer space. Due to Earth's axial tilt, the celestial equator is currently inclined by about 23.44° with respect to the ecliptic. The inclination has varied from about 22.0° to 24.5° over the past 5 million years.

Contents

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. [3]

Sunlight portion of the electromagnetic radiation given off by the Sun

Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is filtered through Earth's atmosphere, and is obvious as daylight when the Sun is above the horizon. When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When it is blocked by clouds or reflects off other objects, it is experienced as diffused light. The World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts per square meter. Other sources indicate an "Average over the entire earth" of "164 Watts per square meter over a 24 hour day".

Summer solstice Astronomical phenomenon when Earths axial tilt toward the Sun is a maximum (currently 23.44°)

The summer solstice, also known as midsummer, occurs when one of the Earth's poles has its maximum tilt toward the Sun. It happens twice yearly, once in each hemisphere. For that hemisphere, the summer solstice is when the Sun reaches its highest position in the sky and is the day with the longest period of daylight. Within the Arctic circle or Antarctic circle, there is continuous daylight around the summer solstice. On the summer solstice, Earth's maximum axial tilt toward the Sun is 23.44°. Likewise, the Sun's declination from the celestial equator is 23.44°.

Winter solstice astronomical phenomenon marking the day with the shortest period of daylight and the longest night of the year

The winter solstice, hiemal solstice or hibernal solstice, also known as midwinter, occurs when one of the Earth's poles has its maximum tilt away from the Sun. It happens twice yearly, once in each hemisphere. For that hemisphere, the winter solstice is the day with the shortest period of daylight and longest night of the year, when the Sun is at its lowest daily maximum elevation in the sky. At the pole, there is continuous darkness or twilight around the winter solstice. Its opposite is the summer solstice.

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) stops at a northern or southern limit before reversing direction.

Latin Indo-European language of the Italic family

Latin is a classical language belonging to the Italic branch of the Indo-European languages. The Latin alphabet is derived from the Etruscan and Greek alphabets and ultimately from the Phoenician alphabet.

Declination Astronomical coordinate analogous to latitude

In astronomy, declination is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the celestial equator, along the hour circle passing through the point in question.

Earth Third planet from the Sun in the Solar System

Earth is the third planet from the Sun and the only astronomical object known to harbor life. According to radiometric dating and other sources of evidence, Earth formed over 4.5 billion years ago. Earth's gravity interacts with other objects in space, especially the Sun and the Moon, which is Earth's only natural satellite. Earth orbits around the Sun in 365.26 days, a period known as an Earth year. During this time, Earth rotates about its axis about 366.26 times.

Definitions and frames of reference

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 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 comes to a stop at the moment of solstice. In that sense, solstice means "sun-standing".

North Pole Northern point where the Earths axis of rotation intersects its surface

The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is defined as the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface.

South Pole Southern point where the Earths axis of rotation intersects its surface

The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of Earth and lies on the opposite side of Earth from the North Pole.

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". [4] 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. [5]

Roman Republic Period of ancient Roman civilization (509–27 BC)

The Roman Republic was the era of classical Roman civilization beginning with the overthrow of the Roman Kingdom, traditionally dated to 509 BC, and ending in 27 BC with the establishment of the Roman Empire. It was during this period that Rome's control expanded from the city's immediate surroundings to hegemony over the entire Mediterranean world.

Pliny the Elder Roman military commander and writer

Pliny the Elder was a Roman author, a naturalist and natural philosopher, a naval and army commander of the early Roman Empire, and a friend of emperor Vespasian.

<i>Natural History</i> (Pliny) Encyclopedia published c. AD 77–79 by Pliny the Elder

The Natural History is a book about the whole of the natural world in Latin by Pliny the Elder, a Roman author and naval commander who died in 79 AD.

To an observer in an inertial frame of reference, planet Earth is seen to rotate about an axis and revolve 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.

An inertial frame of reference in classical physics and special relativity possesses the property that in this frame of reference a body with zero net force acting upon it does not accelerate; that is, such a body is at rest or moving at a constant velocity. An inertial frame of reference can be defined in analytical terms as a frame of reference that describes time and space homogeneously, isotropically, and in a time-independent manner. Conceptually, the physics of a system in an inertial frame have no causes external to the system. An inertial frame of reference may also be called an inertial reference frame, inertial frame, Galilean reference frame, or inertial space.

Earths rotation rotation of the solid Earth around its own axis

Earth's rotation is the rotation of Planet Earth around its own axis. Earth rotates eastward, in prograde motion. As viewed from the north pole star Polaris, Earth turns counterclockwise.

Rotation Movement of an object around an axis

A rotation is a circular movement of an object around a center of rotation. A three-dimensional object can always be rotated around an infinite number of imaginary lines called rotation axes. If the axis passes through the body's center of mass, the body is said to rotate upon itself, or spin. A rotation about an external point, e.g. the Earth about the Sun, is called a revolution or orbital revolution, typically when it is produced by gravity. The axis is called a pole.

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. 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. Solargraph APEX.tif
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. 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. [7]

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.

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 [8] they devised the concept of the celestial sphere, [9] 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 planes [10] perpendicular to the Earth's axis extended indefinitely into the heavens and intersecting the celestial sphere in a celestial pole. [11] 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: [12]

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.

Solstice terms in East Asia

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;"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;"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 means "extreme", so the terms for the solstices directly signify the summits of summer and winter.

Solstice celebrations

2005 Summer Solstice Sunrise over Stonehenge Summer Solstice Sunrise over Stonehenge 2005.jpg
2005 Summer Solstice Sunrise over Stonehenge

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). Midsummer's Day, defined as St. Johns Day by the Christian Church, is June 24, about three days after the solstice itself). Similarly December 25 is the start of the Christmas celebration, and is the day the Sun begins to return to the Northern Hemisphere. The traditional British and Irish (often) main rent and meeting days of the year: "the usual quarter days" was at first those of the solstices and equinoxes.

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 winter solstice, Christmas is the most widespread contemporary holiday, while Yalda, Saturnalia, Karachun, Hanukkah, Kwanzaa, and Yule are also celebrated around this time. For the northern or summer solstice, Christian cultures celebrate the feast of St. John from June 23 to 24 (see St. John's Eve, Ivan Kupala Day), while Modern pagans observe Midsummer, known as Litha among Wiccans. For the vernal (spring) equinox, several springtime festivals are celebrated, such as the Persian Nowruz, the observance in Judaism of Passover, the rites of Easter in most Christian churches, as well as the Wiccan Ostara. The autumnal equinox is associated with the Jewish holiday of Sukkot and the Wiccan Mabon.

In the southern tip of South America, the Mapuche people celebrate We Tripantu (the New Year) a few days after the northern solstice, on June 24. Further north, the Atacama people formerly celebrated this date with a noise festival, to call the Sun back. Further east, the Aymara people celebrate their New Year on June 21. A celebration occurs at sunrise, when the sun shines directly through the Gate of the Sun in Tiwanaku. Other Aymara New Year feasts occur throughout Bolivia, including at the site of El Fuerte de Samaipata.

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 Karka, the zodiacal sign which corresponds with Cancer, respectively.

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 Fremont Solstice Parade takes place every summer solstice in Fremont, Seattle, Washington in the United States.

The reconstructed Cahokia Woodhenge, a large timber circle located at the Mississippian culture Cahokia archaeological site near Collinsville, Illinois, [18] 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. [19] [20] [21]

Solstice determination

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 160 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 [22] or an astrolabe. It is also hard to detect the changes on sunrise/sunset azimuth due to the atmospheric refraction [23] 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. [24] 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). [25] The dates of the solstice varies each year and may occur a day earlier or later depending on the time zone. The solstices always occur between June 20 and 22 and between December 20 and 23 with the 21st and 22nd being the most common dates. [26] [27]

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 [28] (expressed in astronomical year numbering in which the year 0 = 1 BC, −1 = 2 BC, etc.):

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. [29]

See also

Related Research Articles

Equinox Astronomical event where the Sun is directly above the Earths equator

An equinox is commonly regarded as the instant of time when the plane of Earth's equator passes through the center of the Sun. This occurs twice each year: around 20 March and 23 September. In other words, it is the moment at which the center of the visible Sun is directly above the equator.

Northern Hemisphere Half of Earth that is north of the equator

The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's North Pole.

Zodiac celestial circle of twelve divisions centered upon the ecliptic

The zodiac is an area of the sky that extends approximately 8° north or south of the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year. The paths of the Moon and visible planets are also within the belt of the zodiac.

Celestial sphere imaginary sphere of arbitrarily large radius, concentric with the observer

In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, which may be centered on Earth or the observer. If centered on the observer, half of the sphere would resemble a hemispherical screen over the observing location.

Equatorial coordinate system A celestial coordinate system used to specify the positions of celestial objects

The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fundamental plane consisting of the projection of Earth's equator onto the celestial sphere, a primary direction towards the vernal equinox, and a right-handed convention.

Ecliptic coordinate system celestial coordinate system used for representing the positions of Solar System objects

The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions and orbits of Solar System objects. Because most planets and many small Solar System bodies have orbits with only slight inclinations to the ecliptic, using it as the fundamental plane is convenient. The system's origin can be the center of either the Sun or Earth, its primary direction is towards the vernal (March) equinox, and it has a right-hand convention. It may be implemented in spherical or rectangular coordinates.

Axial precession gravity-induced, slow, and continuous change in the orientation of an astronomical bodys rotational axis

In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In particular, it can refer to the gradual shift in the orientation of Earth's axis of rotation in a cycle of approximately 25,772 years. This is similar to the precession of a spinning-top, with the axis tracing out a pair of cones joined at their apices. The term "precession" typically refers only to this largest part of the motion; other changes in the alignment of Earth's axis—nutation and polar motion—are much smaller in magnitude.

Spring (season) One of the Earths four temperate seasons, occurring between winter and summer

Spring is one of the four temperate seasons, following winter and preceding summer. There are various technical definitions of spring, but local usage of the term varies according to local climate, cultures and customs. When it is spring in the Northern Hemisphere, it is autumn in the Southern Hemisphere and vice versa. At the spring equinox, days and nights are approximately twelve hours long, with day length increasing and night length decreasing as the season progresses.

Earths orbit The gravitationally curved trajectory Earth travels around the Sun

Earth orbits the Sun at an average distance of 149.60 million km, and one complete orbit takes 365.256 days, during which time Earth has traveled 940 million km. Earth's orbit has an eccentricity of 0.0167. Since the Sun constitutes 99.8% of the mass of the solar system, the center of the orbit is extremely close to the center of the Sun.

September equinox the equinox on the earth when the Sun appears to leave the nothern hemisphere and cross the celestial equator

The September equinox is the moment when the Sun appears to cross the celestial equator, heading southward. Due to differences between the calendar year and the tropical year, the September equinox can occur at any time from September 21 to 24.

March equinox the equinox on the earth when the Sun appears to leave the southern hemisphere and cross the celestial equator

The March equinox or Northward equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth. The March equinox is known as the vernal equinox in the Northern Hemisphere and as the autumnal equinox in the Southern.

Astronomy on Mars What an observer on Mars can see in the sky

In many cases astronomical phenomena viewed from the planet Mars are the same or similar to those seen from Earth but sometimes they can be quite different. For example, because the atmosphere of Mars does not contain an ozone layer, it is also possible to make UV observations from the surface of Mars.

The term Uttarāyaṇa or Uttarayan is derived from two different Sanskrit words "uttara" (North) and "ayana" (movement) thus indicating a semantic of the northward movement of the Earth on the celestial sphere. This movement begins to occur a day after the winter solstice in December which occurs around 22 December and continues for a six-month period through to the summer solstice around June 21. This difference is because the solstices are continually precessing at a rate of 50 arcseconds / year due to the precession of the equinoxes, i.e. this difference is the difference between the sidereal and tropical zodiacs. The Surya Siddhanta bridges this difference by juxtaposing the four solstitial and equinotial points with four of the twelve boundaries of the rashis.

A lunar standstill is the gradually varying range between the northern and the southern limits of the Moon's declination, or the lunistices, over the course of one-half a sidereal month, or 13.66 days. One major, or one minor, lunar standstill occurs every 18.6 years due to the precessional cycle of the lunar nodes at that rate.

Daytime period on any given point of the planets surface during which it experiences natural illumination from sunlight

On Earth, daytime is roughly the period of the day during which any given point in the world experiences natural illumination from especially direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's hemisphere facing the Sun. In direct sunlight the movement of the sun can be recorded and observed using a sundial that casts a shadow that slowly moves during the day. During daytime, an observer sees indirect sunlight while in the shade, which includes cloud cover. 'Day' is sometimes used instead of 'daytime', in this case 'day' will mean 'the period of light between dawn and nightfall; the interval from sunrise to sunset', which is synonymous with daytime. However, in this context, in order to be clear "daytime" should be used distinguish it from "day" which typically refers to a 24-hour period.

A season is a division of the year marked by changes in weather, ecology, and the amount of daylight. On Earth, seasons is the result from Earth's orbit around the Sun and Earth's axial tilt relative to the ecliptic plane. In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to undergo hibernation or to migrate, and plants to be dormant. Various cultures define the number and nature of seasons based on regional variations.

Position of the Sun

The position of the Sun in the sky is a function of both the time and the geographic location of observation on Earth's surface. As Earth orbits the Sun over the course of a year, the Sun appears to move with respect to the fixed stars on the celestial sphere, along a circular path called the ecliptic.

References

  1. United States Naval Observatory (January 4, 2018). "Earth's Seasons and Apsides: Equinoxes, Solstices, Perihelion, and Aphelion" . Retrieved September 18, 2018.
  2. Astro Pixels (February 20, 2018). "Solstices and Equinoxes: 2001 to 2100" . Retrieved December 21, 2018.
  3. "The Summer and Winter Solstices". Scholastic. 2017. Retrieved October 1, 2017.
  4. "solstice". The American Heritage Dictionary of the English Language (Fifth ed.). Houghton Mifflin Harcourt. 2015. Retrieved December 8, 2015.
  5. 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.
  6. "A Solargraph taken from APEX at Chajnantor". European Southern Observatory . Retrieved December 9, 2015.
  7. 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'. Retrieved December 9, 2015.
  8. Strabo. The Geography. II.5.1. sphairikē ... tēs gēs epiphaneia, spherical is the surface of the Earth
  9. Strabo. The Geography. pp. II.5.2. sphairoeidēs ... ouranos, spherical in appearance ... is heaven
  10. Strabo II.5.2., "aplaneis asteres kata parallēlōn pherontai kuklōn", "the fixed stars are borne in parallel circles"
  11. 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"
  12. Cleomedes (2004). Cleomedes' Lectures on Astronomy: A Translation of The Heavens. Translated by Alan C. Bowen and Robert B. Todd. Berkeley: University of California Press. p. 41. 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. p. 32. the passage starts Chapter 4.
  13. Kent, April Elliott (June 7, 2011). The Essential Guide to Practical Astrology. Penguin. p. 284. ISBN   978-1-61564-093-5 . Retrieved December 9, 2015.
  14. Bass, Charles O. (1994). Astronomy Essentials. Research & Education Assoc. p. 35. ISBN   978-0-87891-965-9 . Retrieved December 9, 2015.
  15. 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.
  16. Jain, R.K. ICSE Geography. Ratna Sagar. p. 25. ISBN   978-81-8332-579-0 . Retrieved December 9, 2015.
  17. Stewart, Alexander (1869). A Compendium of ModernGeography. Edinburgh: Oliver & Boyd. p. 383. Retrieved December 9, 2015.
  18. "Visitors Guide to the Woodhenge" . Retrieved December 19, 2017.
  19. Iseminger, William. "Welcome the Fall Equinox at Cahokia Mounds". Illinois Department of Natural Resources. Retrieved December 20, 2017.
  20. "Winter Solstice Sunrise Observance at Cahokia Mounds". Collinsville Chamber of Commerce. Retrieved December 20, 2017.
  21. "Cahokia Mounds Mark Spring Equinox : The keepers of Cahokia Mounds will host a spring gathering to celebrate the vernal equinox". Indian Country Today. Indian Country Media Network . Retrieved December 20, 2017.
  22. Mollerup, Asger (January 12, 2008). "Solstice Determination based on Observations". Archived from the original on February 11, 2009. Retrieved September 27, 2010.
  23. Exton, Harold (1992). "A Fresh Analysis of Some Recent Data on Atmospheric Refraction Near the Horizon with Implications in Archaeoastronomy". Journal of History of Astronomy, Archaeoastronomy Supplement. 23: S57. Bibcode:1992JHAS...23...57E.
  24. Hugh, Thurston (2001). "Early Greek Solstices and Equinoxes". Journal for the History of Astronomy. 32, Part 2 (107): 154–156. Bibcode:2001JHA....32..154T. ISSN   0021-8286.
  25. Meeus, Jean (1998). Astronomical Algorithms (Second English ed.). Richmond: Willmann-Bell, Inc. pp. 177–182. ISBN   0-943396-61-1.
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  27. "June solstice" . Retrieved March 21, 2018.
  28. Meeus, Jean (1997). Mathematical astronomy morsels (1st English ed.). Richmond: Willmann-Bell, Inc. ISBN   0-943396-51-4.
  29. Planetary Society. "Mars Calendar" . Retrieved December 9, 2015.