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equinox
An equinox is an astronomical event in which the imaginary plane of Earth's equator passes the center of the Sun,〔(【引用サイトリンク】work=USNO Astronomical Information Center FAQ )〕 making night and day of approximately equal length all over the planet. The equinoxes are the only times when the solar terminator (the "edge" between night and day) is perpendicular to the Equator. As a result, the northern and southern hemispheres are equally illuminated.
In other words, the equinoxes are the only times when the subsolar point is on the Equator, meaning that the Sun is exactly overhead at a point on the equatorial line. Equinoxes occur twice a year, around 21 March and 23 September. The subsolar point crosses the Equator moving northward at the March equinox and southward at the September equinox.
The equinoxes, along with solstices, are directly related to the seasons of the year. In the Northern Hemisphere, the vernal equinox (March) conventionally marks the beginning of Spring in most cultures, and the autumnal equinox (September) marks the beginning of Autumn. In the Southern Hemisphere, the vernal equinox occurs in September and the autumnal equinox in March.
==Terminology==
The oldest meaning of the word "equinox" is the day when daytime and night are of approximately equal duration.〔("equinox" ) at Oxford Dictionaries〕 The word ''equinox'' comes from this definition, derived from the Latin ''aequinoctium'', ''aequus'' (equal) and ''nox'' (genitive ''noctis'') (night). The equinox is not exactly the same as the day when period of daytime and night are of equal length for two reasons.〔 First, sunrise, which begins daytime, occurs when the top of the Sun's disk rises above the eastern horizon. At that instant, the disk's center is still below the horizon. Second, Earth's atmosphere refracts sunlight. As a result, an observer sees daylight before the Sun's disk rises above the horizon geometrically. To avoid this ambiguity, the word ''equilux'' is sometimes used to mean a day on which the periods of daylight and night are equal.〔This meaning of "equilux" is rather modern (c. 2006) and unusual; technical references since the beginning of the 20th century (c. 1910) use the terms "equilux" and "isophot" to mean "of equal illumination", in the context of curves showing how intensely lighting equipment will illuminate a surface. See for instance John William Tudor Walsh, (''Textbook of Illuminating Engineering (Intermediate Grade)'' ), I. Pitman, 1947.〕 Times of sunset and sunrise vary with an observer's location (longitude and latitude), so the dates when day and night are closest together in length depend on location.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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