Solstices occur twice a year, when the tilt of the Earth's axis is oriented directly towards or away from the Sun, causing the Sun to appear to reach its northernmost and southernmost extremes. An equinox is the event of the Sun passing over the Earth's equator in its annual cycle The Sun (Sol is the Star at the center of the Solar System. 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, its apparent movement north or south comes to a standstill. Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. In Astronomy, declination (abbrev dec or δ) is one of the two coordinates of the Equatorial coordinate system, the other being either

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. An equinox is the event of the Sun passing over the Earth's equator in its annual cycle In some languages they are considered to start or separate the seasons; in others they are considered to be centre points (in English, in the Northern hemisphere, for example, the period around the June solstice is known as midsummer, and Midsummer's Day is 24 June, about three days after the solstice itself). Midsummer may simply refer to the period of time centered upon the summer solstice, but more often refers to specific European celebrations that accompany the actual solstice Events 972 - Battle of Cedynia, the first documented victory of Polish forces takes place Similarly 25 December is the start of the Christmas celebration, which was a Pagan festival in pre-Christian times, and is the day the sun begins to return back to the northern hemisphere. Events 274 - Roman Emperor Aurelian Paganism (from Latin paganus, meaning "country dweller rustic" is a word used to refer to various religions and religious beliefs from across the world

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. In Astronomy, axial tilt is the Inclination angle of a planet's rotational axis in relation to its orbital plane. In physics the expression inertial space refers to the background reference that is provided by the phenomenon of Inertia. As a consequence, for half the 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 distinction, with the maximum around 21 December. Events 1600 - The Linköping Bloodbath takes place on Maundy Thursday in Linköping, Sweden. Events 66 - Emperor Nero creates the Legion I Italica. 1236 - The Lithuanians Events 524 - Godomar, King of the Burgundians defeats the Franks at the Battle of Vézeronce. Events 69 - The end of the Year of the four emperors: Following Galba, Otho and Vitellius, Vespasian 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. An equinox is the event of the Sun passing over the Earth's equator in its annual cycle

During the northern solstice the Sun appears to be directly overhead at noon for places situated at latitude 23. 44° north, known as the tropic of Cancer. For the novel by Henry Miller, see Tropic of Cancer (novel. The Tropic of Cancer, or Northern tropic, is one of five Likewise during the southern solstice the same thing happens for latitude 23. 44° south, known as the tropic of Capricorn. For the novel by Henry Miller, see Tropic of Capricorn (novel. 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. The Tropics are centered on the Equator and limited in Latitude by the Tropic of Cancer in the northern hemisphere at approximately 23°26' (23 In broad terms the zenith is the direction pointing directly above a particular location ( Perpendicular, Orthogonal)

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. The Arctic Circle is one of the five major circles of latitude that mark maps of the Earth. That is the midnight sun or midsummer-night sun or polar day. The midnight sun is a phenomenon occurring in Latitudes north and nearby to the south of the Arctic Circle and south and nearby to the north of the 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. The Antarctic Circle is one of the five major circles (or parallels of latitude that mark maps of the Earth. That is the polar night. The polar night is the night lasting more than 24 hours usually inside the Polar circles The opposite phenomenon when the sun stays above the horizon for a long time is called 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). The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. M^2 redirects here For other uses see M². CM2 redirects here

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 to the Sun due to the orbital eccentricity of the Earth's orbit. The amount of heat energy received at any location on the globe is a direct effect of sun angle of climate, as the angle at which Sunlight strikes the earth In Astrodynamics, under standard assumptions, any Orbit must be of Conic section shape This variation does make such a contribution, but is small compared to the effects of exposure because of Earth's tilt. Currently the Earth reaches perihelion at the beginning of January, which is during the northern winter and the southern summer. In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from The Sun, being closer to Earth and therefore hotter, does not cause the whole planet to 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 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. In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from

Due to Milankovitch cycles, the Earth's axial tilt and orbital eccentricity will change over thousands of years. Milankovitch cycles are the collective effect of changes in the Earth 's movements upon its climate named after Serbian civil engineer and Mathematician Thus in 10,000 years one would find that Earth's northern winter occurs at aphelion and its northern summer at perihelion. In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from 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. Diurnal motion is an astronomical term referring to the apparent daily motion of Stars around the Earth, or more precisely around the two 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. Twilight is the time before Sunrise, called Dawn, and the time after Sunset, called Dusk. 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). In astronomy the culmination, at a given point of a Planet, Star, Constellation, etc 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. The equator (sometimes referred to colloquially as "the Line") is the intersection of the Earth 's surface with the plane perpendicular to the 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. In broad terms the zenith is the direction pointing directly above a particular location ( Perpendicular, Orthogonal) 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. 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 conversely 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. Historically Astronomy was more concerned with the classification and description of phenomena in the sky while Astrophysics attempted to explain these phenomena 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 midnight sun is a phenomenon occurring in Latitudes north and nearby to the south of the Arctic Circle and south and nearby to the north of the
• 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. 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. All directions are north at the South Pole and south at the North pole. There is also no south at the South Pole, no north at the North Pole, and neither east nor west is discernible at either pole.

Due to atmospheric refraction, the Sun may already appear above the horizon when the real, geometric Sun is still below it. Atmospheric Refraction is the deviation of Light or other Electromagnetic wave from a straight line as it passes through the atmosphere due to the

Cultural aspects

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 December 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. Shabe Yaldā ( or Shabe Chelle ( is an Iranian festival originally celebrated on the Northern Hemisphere's longest night of the year that is on the Saturnalia is the feast with which the Romans commemorated the dedication of the temple of the god Saturn Hanukkah (חנוכה alt Chanukah) also known as the Festival of Lights, is an eight-day Jewish holiday commemorating the rededication of the Kwanzaa is a week-long holiday celebrated primarily in the United States, honoring African-American heritage Yule is a winter festival historically celebrated primarily in northern Europe but now celebrated in many other countries in various forms The winter solstice occurs at the instant when the Sun 's position in the sky is at its greatest angular distance on the other side of the equatorial plane from the For the June solstice, Catholic and Nordic Protestant cultures celebrate the feast of St. John from June 23 to June 24 (see St. John's Eve, Ivan Kupala Day, Midsummer), while the Wiccan culture observes Litha or Summer Solstice. Saint John the Baptist ( heb. Jochanan ben Sacharja, arab. يحيى Yaḥyā or يوحنا Yūḥanna, aram. Events 1180 - First Battle of Uji, starting the Genpei War in Japan 1305 - The Flemish Events 972 - Battle of Cedynia, the first documented victory of Polish forces takes place The evening of June 23, St John's Eve, is the eve of celebration before the Feast Day of St Ivan Kupala Day ( John the Baptist Day; Russian: Иван-Купала Belarusian: Купалле Ukrainian: Івана Купала Midsummer may simply refer to the period of time centered upon the summer solstice, but more often refers to specific European celebrations that accompany the actual solstice Midsummer may simply refer to the period of time centered upon the summer solstice, but more often refers to specific European celebrations that accompany the actual solstice For the vernal (spring) equinox, several spring-time festivals are celebrated, such as the observance in Judaism of Passover. Judaism (from the Greek Ioudaïsmos, derived from the Hebrew יהודה Yehudah, " Judah " in Hebrew יַהֲדוּת Yahedut Passover ( Hebrew, Yiddish: פֶּסַח Pesach, Tiberian: pɛsaħ Israeli: Pesah, Pesakh, Yiddish The autumnal equinox has also given rise to various holidays, such as the Jewish holiday of Sukkot. Sukkot ( also known as Succoth, Sukkos, Feast of Booths or Feast of Tabernacles) is a Biblical Pilgrimage At the midpoints between these four solar events, cross-quarter days are celebrated. A cross-quarter day is a day falling approximately halfway between a Solstice and an Equinox.

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. A season is one of the major divisions of the Year, generally based on yearly periodic changes in Weather. Midsummer may simply refer to the period of time centered upon the summer solstice, but more often refers to specific European celebrations that accompany the actual solstice The winter solstice occurs at the instant when the Sun 's position in the sky is at its greatest angular distance on the other side of the equatorial plane from the Along this vein, the Japanese celebrate the start of each season with an occurrence known as Setsubun. The are the dominant Ethnic group of Japan. Worldwide approximately 130 million people are of Japanese descent of these approximately 127 million are residents of Japan In Japan, Setsubun (ja 節分 is the day before the beginning of each Season. In modern western culture, the solstices and equinoxes are instead designated the beginning of each season, as the cumulative cooling and warming that result from the tilt of the planet become most pronounced after the solstices.

In the Hindu calendar, two sidereal solstices are named Uttarayana and Dakshinayana. The Hindu calendar used in ancient times has undergone many changes in the process of regionalization and today there are several regional Indian Calendars, as For the kite-flying festival see Makar Sankranti Uttarāyana (उत्तरायन is the six month period between Makar Sankranti The former occurs around January 14 each year, while the latter occurs around July 14 each year. Events 1129 - Formal approval of the Order of the Templar at the Council of Troyes. Events 1223 - Louis VIII becomes King of France upon the death of his father Philip II of France. These mark the movement of the Sun along a sidereally fixed zodiac (precession is ignored) into Mesha, a zodiacal sign which corresponded with Aries about 285, and into Tula, the opposite zodiacal sign which corresponded with Libra about 285. Zodiac denotes an annual cycle of twelve stations along the Ecliptic, the apparent path of the sun across the heavens through the Constellations that divide the ecliptic In Astronomy, Precession refers to the movement of the rotational axis of a body such as a planet with respect to Inertial space. Aries, the ram, is the first Astrological sign in the Zodiac. Events By Place Roman Empire July — Rival Emperors Carinus and Diocletian face each other in the Battle of the Margus Libra is the seventh Astrological sign in the Zodiac, originating from the constellation of Libra. Events By Place Roman Empire July — Rival Emperors Carinus and Diocletian face each other in the Battle of the Margus

See also

• Midsummer
• Winter solstice

References and external links

Calculations, plots and tables

• Earth's Seasons, Equinoxes, Solstices, Perihelion, and Aphelion, 2000–2020 (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
• 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)
• Calculate the Time of Solstices in Excel, CAD or your other programs. The Sun API is free and extremely accurate. The United States Naval Observatory ( USNO) is one of the oldest Scientific agencies in the United States. For Windows Computers.

Debate about season start

• 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)

Pictures and videos

• NASA picture of day, winter solstice
• Video of Winter Solstices Celebration at Stonehenge