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A solstice is either of the two events of the year when the sun is at its greatest distance from the equator plane. The name is derived from Latin sol (sun) and sistere (stand still), because at the solstice, the Sun stands still in declination, that is, it reaches a maximum or a minimum. The term solstice can also be used in a narrower sense of being the instance in time that such a passage happens. The solstices then, together with the equinoxes, are related to the seasons. In some languages they are considered to start or separate the seasons; in others they are considered to be center points (in English, for example, the period around the June solstice is known as midsummer, and Midsummer's Day is the 24 June — now two or three days after the solstice).
Names The two solstices can be distinguished by different pairs of names, depending on which feature one wants to stress. Heliocentric view of the seasons The cause of the seasons is that the rotation axis of the Earth is not perpendicular to its orbital plane, 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 a 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 honour, with the maximum around 21 December. The two moments when the inclination of Earth's rotation axis has maximum effect are the solstices. The table above gives the instances of equinoxes and solstices over several years. Refer to the equinox article for some remarks. During the June 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 December 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 June 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 at any time of the day. That is the midnight sun or midsummernight 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 will not see the Sun above horizon at any time of the day. That is the polar night. Of course during the December 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 it 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 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, albeit with some minor effects. Due to Milankovitch cycles, the Earth's axial tilt and orbital eccentricity will change over thousands of years. Thus in 12,000 CE one would find that Earth's northern winter occurs at aphelion and its northern summer at perihilion. 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 is more useful to see how the Sun seems to revolve around them. The pictures to the right 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. The following special cases are depicted. Cultural aspects Many cultures celebrate the winter and summer solstices, or both the solstices and equinoxes, or even the solstices, equinoxes and midpoints between them (e.g. in some pagan cultures). Perhaps the most obvious example of this is Christmas. Similarly, Easter began as a celebration of the vernal equinox. Most Catholic cultures, as well as Nordic Protestant cultures, also celebrate the summer solstice in the form of the feast of St. John (June 23–24; see St. John's Night, St. John's Eve, Juhannus, Sankt Hans Aften, etc.). Jews celebrate especially the equinoxes (Passover and Rosh Hashanah); in Japan, all four major season days are celebrated (see Setsubun). Many other summer solstice festivals exist (e.g. the Wiccan Litha); likewise for winter solstice festivals (Yalda, Saturnalia, Karachun, Hanukkah, Kwanzaa and Ásatrúar — see the list of winter festivals for yet more). In most cultures the solstices and equinoxes do not determine the start but the midpoint of the seasons, see midsummer, midwinter, cross-quarter day, and seasons. Solar terms in Chinese astronomy In the Chinese calendar, the December solstice marks midwinter and is called ''dōng zhì'' (冬至, "winter's extreme"). It is traditionally regarded as one of the year's most important jiéqìs (solar terms), comparable to Chinese New Year. Rather confusingly, the character zhì may also mean "arrival" in other contexts (here it means "extreme"), but it is clear that the Chinese consider "winter's arrival" (立冬, lì dōng, literally "establishment of winter") to be a separate ''jiéqì'' which falls on or around November 7 instead. Xiazhi (夏至, "summer's extreme") is the jieqi that marks the summer solstice and therefore midsummer. It begins when the Sun reaches a celestial longitude of 90°, and ends when its celestial longitude is 105°. It sometimes refers in particular to the day when Sun is exactly at the celestial longitude of 90°. It begins on or around June 21, and ends around July 7. See Also | ||||||||
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