Solstice
For locations not too close to the equator or the poles, the dates with the longest and shortest periods of daylight are the summer and winter solstices, respectively.
For an observer at the North Pole, the Sun reaches the highest position in the sky once a year in June.
Similarly, for an observer on the South Pole, the Sun reaches the highest position on the December solstice day.
At maximum or minimum elevation, the relative yearly motion of the Sun perpendicular to the horizon stops and reverses direction.
[11] Because of the variation in the rate at which the sun's right ascension changes, the days of longest and shortest daylight do not coincide with the solstices for locations very close to the equator.
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.
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.
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.
Xiàzhì (pīnyīn) or Geshi (rōmaji) (Chinese and Japanese: 夏至; Korean: 하지(Haji); Vietnamese: Hạ chí; lit.
Dōngzhì (pīnyīn) or Tōji (rōmaji) (Chinese and Japanese: 冬至; Korean: 동지(Dongji); Vietnamese: Đông chí; lit.
Here, the Chinese character 至 means "extreme", so the terms for the solstices directly signify the summits of summer and winter.
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.
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 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.
In the southern tip of South America, the Mapuche people celebrate We Tripantu (the New Year) a few days after the northern solstice, on 24 June.
Further north, the Atacama people formerly celebrated this date with a noise festival, to call the Sun back.
Other Aymara New Year feasts occur throughout Bolivia, including at the site of El Fuerte de Samaipata.
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.
[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.
[29] The dates of the solstice varies each year and may occur a day earlier or later depending on the time zone.
Thus the solstices always occur between June 20 and 22 and between December 20 and 23 [30][31] in a four-year-long cycle with the 21st and 22nd being the most common dates, as can be seen in the schedule at the start of the article.
