Everything about Solstice totally explained
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. 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.
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're considered to start or separate the seasons; in others they're 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). 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.
Heliocentric view of the seasons
Image:Earth-lighting-summer-solstice EN.png|Illumination of Earth by Sun at the northern solstice.
Image:Earth-lighting-winter-solstice EN.png|Illumination of Earth by Sun at the southern solstice.
Image:north season.jpg|Diagram of the Earth's seasons as seen from the north. Far right: southern solstice
Image:south season.jpg|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 isn't perpendicular to its orbital plane (the flat plane made through the center of mass (
barycenter) of the solar system (near or within the Sun) and the successive locations of Earth during the year), but currently makes an angle of about 23.44° (called the "
obliquity of the ecliptic"), and that the axis keeps its orientation with respect to
inertial space. As a consequence, for half the year (from around
20 March to
22 September) the northern hemisphere 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. 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.
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. Likewise during the southern solstice the same thing happens for latitude 23.44° south, known as the
tropic of Capricorn. 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.
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'll see the Sun above horizon for 24 hours. That is the
midnight sun or midsummer-night sun or polar day. On the other hand, places at latitude 66.56° south, known as the
Antarctic Circle will see the Sun just on the horizon during midday, and all places south of it won't see the Sun above horizon at any time of the day. That is the
polar night. During the southern solstice the effects on both hemispheres are just the opposite.
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.
The seasons are not caused by the varying distance of Earth to the Sun due to the
orbital eccentricity of the Earth's orbit. 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. The Sun, being closer to Earth and therefore hotter, doesn't cause the whole planet to enter summer. Although it's 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're with only minor effects.
Due to
Milankovitch cycles, the Earth's axial tilt and orbital eccentricity will change over thousands of years. Thus in 10,000 years one would find that Earth's northern winter occurs at aphelion and its northern summer at
perihelion. The severity of seasonal change — the average temperature difference between summer and winter in location — will also change over time because the Earth's axial tilt fluctuates between 22.1 and 24.5 degrees.
Geocentric view of the seasons
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's also useful to see how the Sun seems to revolve around them. These pictures show such a perspective as follows. They show the day arcs of the Sun, the paths the Sun tracks along the celestial dome in its
diurnal movement. The pictures show this for every hour on both solstice days. The longer arc is always the summer track and the shorter one the winter track. The two tracks are at a distance of 46.88° (2 × 23.44°) away from each other.
In addition, some 'ghost' suns are indicated below the horizon, as much as 18° down. The Sun in this area causes
twilight. The pictures can be used for both the northern and southern hemispheres. The observer is supposed to sit near the tree on the island in the middle of the ocean. The green arrows give the cardinal directions.
- On the northern hemisphere the north is to the left, the Sun rises in the east (far arrow), culminates in the south (to the right) while moving to the right and sets in the west (near arrow). Both rise and set positions are displaced towards the north in summer, and towards the south for the winter track.
- On the southern hemisphere the south is to the left, the Sun rises in the east (near arrow), culminates in the north (to the right) while moving to the left and sets in the west (far arrow). Both rise and set positions are displaced towards the south in summer, and towards the north for the winter track.
The following special cases are depicted.
On the equator the Sun isn't overhead every day, as some people think. In fact that happens only on two days of the year, the equinoxes. The solstices are the dates that the Sun stays farthest away from the zenith, only reaching an altitude of 66.56° either to the north or the south. The only thing special about the equator is that all days of the year, solstices included, have roughly the same length of about 12 hours, so that it makes no sense to talk about summer and winter. Instead, tropical areas often have wet and dry seasons.
The day arcs at 20° latitude. The Sun culminates at 46.56° altitude in winter and 93.44° altitude in summer. In this case an angle larger than 90° means that the culmination takes place at an altitude of 86.56° in the opposite cardinal direction. For example in the southern hemisphere, the Sun remains in the north during winter, but can reach over the zenith to the south in midsummer. Summer days are longer than winter days, but the difference is no more than two or three hours. The daily path of the Sun is steep at the horizon the whole year round, resulting in a twilight of only about one hour.
The day arcs at 50° latitude. The winter Sun doesn't 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 isn't 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 doesn't get dark enough for astronomers to do their observations. Above 60° latitude the Sun would be even closer to the horizon, only 6.56° away from it. Then civil twilight continues the whole night. This phenomenon is known as the white nights. And above 66° latitude, of course, one would get the midnight sun.
The day arcs at 70° latitude. At local noon the winter Sun culminates at −3.44°, and the summer Sun at 43.44°. Said another way, during the winter the Sun doesn't rise above the horizon, it's 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 doesn't set, it's 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's the summer or winter solstice. In the latter case, that's 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.
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. 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. For the vernal (spring) equinox, several spring-time festivals are celebrated, such as the observance in Judaism of Passover. 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. Along this vein, the Japanese celebrate the start of each season with an occurrence known as Setsubun. 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 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 Mesha, a zodiacal sign which corresponded with Aries about 285, and into Tula, the opposite zodiacal sign which corresponded with Libra about 285.
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