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A solstice is an astronomical event that occurs twice each year (in June and December) as the Sun reaches its highest or lowest excursion relative to the celestial equator on the celestial sphere. Both the solstices and the equinoxes are directly connected with the seasons of the year. The term ''solstice'' can also be used in a broader sense, as the day when this occurs. The day of the solstice is either the longest day of the year (summer solstice) or the shortest day of the year (winter solstice) for any place outside of the tropics. At latitudes in the temperate zone, the summer solstice marks the day when the sun appears highest in the sky. However, in the tropics, the sun appears directly overhead (called the subsolar point) some days (or even months) before the solstice and again after the solstice, which means the subsolar point occurs twice each year. The word ''solstice'' is derived from the Latin ''sol'' (sun) and ''sistere'' (to stand still), because at the solstices, the Sun stands still in declination; that is, the seasonal movement of the Sun's path (as seen from Earth) comes to a stop before reversing direction. ==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 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 the 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". 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". 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 the Earth.〔The Principle of relativity was first applied to inertial frames of reference by Albert Einstein. Before then, the concepts of absolute time and space 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.〕 To an observer in an inertial frame of reference, the planet Earth is seen to rotate about an axis and revolve around the Sun in an elliptical path with the Sun at one focus. The Earth's axis is tilted with respect to the plane of the 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. 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.〔For an introduction to these topics of astronomy refer to 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「solstice」の詳細全文を読む スポンサード リンク
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