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A time standard is a specification for measuring time: either the rate at which time passes; or points in time; or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day. Standardized time measurements are made using a clock to count periods of some cyclic change, which may be either the changes of a natural phenomenon or of an artificial machine. Historically, time standards were often based on the Earth's rotational period. From the late 17th century to the 19th century it was assumed that the Earth's daily rotational rate was constant.〔Before the time of John Flamsteed it was widely believed that the Earth's rotation had seasonal variations comparable in size with what is now called the equation of time. See articles on Vincent Wing and Thomas Streete for examples of astronomers before Flamsteed who believed this. The equation of time, correctly based on the two major components of the Sun's irregularity of apparent motion, i.e. the effect of the obliquity of the ecliptic and the effect of the Earth's orbital eccentricity, was not generally adopted until after John Flamsteed's tables of 1672/3, published with the posthumous edition of the works of Jeremiah Horrocks. See S Vince, ("A Complete System of Astronomy", 2nd edition, volume 1, 1814, at p.49 ); see also Equation of time - history.〕 Astronomical observations of several kinds, including eclipse records, studied in the 19th century, raised suspicions that the rate at which Earth rotates is gradually slowing and also shows small-scale irregularities, and this was confirmed in the early twentieth century.〔See Ephemeris time - history, and sources shown there.〕 Time standards based on Earth rotation were replaced (or initially supplemented) for astronomical use from 1952 onwards by an ''ephemeris time'' standard based on the Earth's orbital period and in practice on the motion of the Moon. The invention in 1955 of the caesium atomic clock has led to the replacement of older and purely astronomical time standards, for most practical purposes, by newer time standards based wholly or partly on atomic time. Various types of second and day are used as the basic time interval for most time scales. Other intervals of time (minutes, hours, and years) are usually defined in terms of these two. ==Time standards based on Earth rotation== Apparent solar time ('apparent' is often used in English-language sources, but 'true' in French astronomical literature〔See for example a recent description of ("temps vrai" ) by the Bureau des Longitudes; and for an older example S Vince, ('A complete system of astronomy' (1814), esp. at page 46 ).〕) is based on the solar day, which is the period between one solar noon (passage of the real Sun across the meridian) and the next. A solar day is approximately 24 hours of mean time. Because the Earth's orbit around the sun is elliptical, and because of the obliquity of the Earth's axis relative to the plane of the orbit (the ecliptic), the apparent solar day varies a few dozen seconds above or below the mean value of 24 hours. As the variation accumulates over a few weeks, there are differences as large as 16 minutes between apparent solar time and mean solar time (see Equation of time). However, these variations cancel out over a year. There are also other perturbations such as Earth's wobble, but these are less than a second per year. Sidereal time is time by the stars. A sidereal rotation is the time it takes the Earth to make one revolution with respect to the stars, approximately 23 hours 56 minutes 4 seconds. For accurate astronomical work on land, it was usual to observe sidereal time rather than solar time to measure mean solar time, because the observations of 'fixed' stars could be measured and reduced more accurately than observations of the Sun (in spite of the need to make various small compensations, for refraction, aberration, precession, nutation and proper motion). It is well known that observations of the Sun pose substantial obstacles to the achievement of accuracy in measurement.〔See H A Harvey, ("The Simpler Aspects of Celestial Mechanics" ), in Popular Astronomy 44 (1936), 533-541.〕 In former times, before the distribution of accurate time signals, it was part of the routine work at any observatory to observe the sidereal times of meridian transit of selected 'clock stars' (of well-known position and movement), and to use these to correct observatory clocks running local mean sidereal time; but nowadays local sidereal time is usually generated by computer, based on time signals.〔A E Roy, D Clarke, ('Astronomy: Principles and Practice' (4th edition, 2003) at p.89 ).〕 Mean solar time was originally apparent solar time corrected by the equation of time. Mean solar time was sometimes derived, especially at sea for navigational purposes, by observing apparent solar time and then adding to it a calculated correction, the equation of time, which compensated for two known irregularities, caused by the ellipticity of the Earth's orbit and the obliquity of the Earth's equator and polar axis to the ecliptic (which is the plane of the Earth's orbit around the sun). Greenwich Mean Time (GMT) was originally mean time deduced from meridian observations made at the Royal Greenwich Observatory (RGO). The principal meridian of that observatory was chosen in 1884 by the International Meridian Conference to be the Prime Meridian. GMT either by that name or as 'mean time at Greenwich' used to be an international time standard, but is no longer so; it was initially renamed in 1928 as Universal Time (UT) (partly as a result of ambiguities arising from the changed practice of starting the astronomical day at midnight instead of at noon, adopted as from 1 January 1925). The more current refined version of UT, UT1, is still in reality mean time at Greenwich. Greenwich Mean Time is still the legal time in the UK (in winter, and as adjusted by one hour for summer time). But Coordinated Universal Time (UTC) (an atomic-based time scale which is always kept within 0.9 second of UT1) is in common actual use in the UK, and the name GMT is often inaccurately used to refer to it. (See articles Greenwich Mean Time, Universal Time, Coordinated Universal Time and the sources they cite.) Universal Time (UT) is mean solar time at 0° longitude; some implementations are * UT0 is the rotational time of a particular place of observation. It is observed as the diurnal motion of stars or extraterrestrial radio sources. * UT1 is computed by correcting UT0 for the effect of polar motion on the longitude of the observing site. It varies from uniformity because of the irregularities in Earth's rotation. 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「Time standard」の詳細全文を読む スポンサード リンク
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