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A day is a unit of time. In common usage, it is an interval equal to 24 hours.〔 It also can mean the consecutive period of time during which the Sun is above the horizon, also known as daytime. The period of time during which the Earth completes one rotation with respect to the Sun is called a ''solar day''.〔 〕〔 〕 Several definitions of this universal human concept are used according to context, need and convenience. In 1967, the second was redefined in terms of the wavelength of light, and was designated the SI base unit of time. The unit of measurement "day", redefined in 1967 as 86 400 SI seconds and symbolized ''d'', is not an SI unit, but is accepted for use with SI.〔 〕 A civil day is usually also 86 400 seconds, plus or minus a possible leap second in Coordinated Universal Time (UTC), and, in some locations, occasionally plus or minus an hour when changing from or to daylight saving time. The word ''day'' may also refer to a day of the week or to a calendar date, as in answer to the question "On which day?" The life patterns of humans and many other species are related to Earth's solar day and the day-night cycle (see circadian rhythms). In recent decades the average length of a solar day on Earth has been about 86 400.002 seconds〔The average over the last 50 years is about 86 400.002. The yearly average over that period has ranged between about 86 400 and 86 400.003, while the length of individual days has varied between about 86 399.999 and 86 400.004 seconds. See this graph: File:Deviation of day length from SI day.svg (data from ).〕 (24.000 000 6 hours) and there are about 365.242 2 solar days in one mean tropical year. Because celestial orbits are not perfectly circular, and thus objects travel at different speeds at various positions in their orbit, a solar day is not the same length of time throughout the orbital year. A ''day'', understood as the span of time it takes for the Earth to make one entire rotation〔 Some authors caution against identifying "day" with rotation period. For example: 〕 with respect to the celestial background or a distant star (assumed to be fixed), is called a ''stellar day''. This period of rotation is about 4 minutes less than 24 hours (23 hours 56 minutes and 4.1 seconds) and there are about 366.242 2 stellar days in one mean tropical year (one stellar day more than the number of solar days). Mainly due to tidal effects, the Earth's rotational period is not constant, resulting in further minor variations for both solar days and stellar "days". Other planets and moons also have stellar and solar days. == Introduction == Besides the day of 24 hours (86 400 seconds), the word ''day'' is used for several different spans of time based on the rotation of the Earth around its axis. An important one is the solar day, defined as the time it takes for the Sun to return to its culmination point (its highest point in the sky). Because the Earth orbits the Sun elliptically as the Earth spins on an inclined axis, this period can be up to 7.9 seconds more than (or less than) 24 hours. On average over the year this day is equivalent to 24 hours (86 400 seconds). A day, in the sense of daytime that is distinguished from night-time, is commonly defined as the period during which sunlight directly reaches the ground, assuming that there are no local obstacles. The length of daytime averages slightly more than half of the 24-hour day. Two effects make daytime on average longer than nights. The Sun is not a point, but has an apparent size of about 32 minutes of arc. Additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground even when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc. The difference in time depends on the angle at which the Sun rises and sets (itself a function of latitude), but can amount to around seven minutes. Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon (Italian reckoning, for example). The exact moment of, and the interval between, two sunrises or sunsets depends on the geographical position (longitude as well as latitude), and the time of year (as indicated by ancient hemispherical sundials). A more constant day can be defined by the Sun passing through the local meridian, which happens at local noon (upper culmination) or midnight (lower culmination). The exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year. The length of such a day is nearly constant (24 hours ± 30 seconds). This is the time as indicated by modern sundials. A further improvement defines a fictitious mean Sun that moves with constant speed along the celestial equator; the speed is the same as the average speed of the real Sun, but this removes the variation over a year as the Earth moves along its orbit around the Sun (due to both its velocity and its axial tilt). The Earth's day has increased in length over time. This phenomenon is due to tides raised by the Moon which slow Earth's rotation. Because of the way the second is defined, the mean length of a day is now about 86 400.002 seconds, and is increasing by about 1.7 milliseconds per century (an average over the last 2 700 years). (See tidal acceleration for details.) The length of a day circa 620 million years ago has been estimated from rhythmites (alternating layers in sandstone) as having been about 21.9 hours. The length of day for the Earth before the moon was created is still unknown. 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「Day」の詳細全文を読む スポンサード リンク
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