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The second (symbol: s) (abbreviated s or sec) is the base unit of time in the International System of Units (SI).〔 〕 It is qualitatively defined as the ''second'' division of the hour by sixty, the first division by sixty being the minute. It is quantitatively defined in terms of a certain number of periods – about 9 billion – of a certain frequency of radiation from the caesium atom: a so-called atomic clock. Seconds may be measured using a mechanical, electric or atomic clock. In the year 1000 CE, the Persian Muslim scholar al-Biruni first used the term ''second'' in Arabic ( ') and defined it as (that is, ) of a mean solar day.〔 〕 In the 13th century, scientists who wrote in Latin, including Bacon, and later Kepler and Tycho, used the Latin term ''parte minutae secundae'' (or ''secunda'' for short) to mean a unit of time which represented the ''second small part'' of an hour as the division of one minute by 60 (with the minute being the ''pars minuta prima'' or ''first small part''). The use of the word ''second'' in English began in the late 16th century. The definition remained unchanged (and still applies in some astronomical and legal contexts)〔〔(International System of Units from NIST ) accessed March 25, 2012.〕 from 1000 until 1960, at which time it was defined as "the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.".〔 (【引用サイトリンク】 publisher=Time Service Department, United States Naval Observatory ) 〕 However, astronomical observations of the 19th and 20th centuries revealed that the mean solar day is slowly but measurably lengthening, and the length of a tropical year is not entirely predictable either. Thus the sun–earth motion was not considered a suitable basis for the definition. With the advent of atomic clocks, it became feasible to define the second based on a fundamental property of nature. Thus, a mere seven years later in 1967, the International Committee for Weights and Measures (abbreviated CIPM from the French Comité international des poids et mesures) changed the definition to "the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."〔 In 1997, the CIPM added that the preceding definition "refers to a caesium atom at rest at a temperature of 0 K."〔 SI prefixes are combined with the word ''second'' to denote subdivisions of the second, ''e.g.'', the millisecond (one thousandth of a second), the microsecond (one millionth of a second), and the nanosecond (one billionth of a second). Though SI prefixes may also be used to form multiples of the second such as kilosecond (one thousand seconds), such units are rarely used in practice. The more common larger non-SI units of time are not formed by powers of ten; instead, the second is multiplied by 60 to form a minute, which is multiplied by 60 to form an hour, which is multiplied by 24 to form a day. The second is also the base unit of time in other systems of measurement: the centimetre-gram-second, metre-kilogram-second, metre-tonne-second, and foot-pound-second systems of units. ==International second== Under the International System of Units (via the International Committee for Weights and Measures, or CIPM), since 1967 the second has been defined as the duration of periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.〔 In 1997 CIPM added that the periods would be defined for a caesium atom at rest, and approaching the theoretical temperature of absolute zero (0 K), and in 1999, it included corrections from ambient radiation.〔 Absolute zero implies no movement, and therefore zero external radiation effects (i.e., zero local electric and magnetic fields). The second thus defined is consistent with the ephemeris second, which was based on astronomical measurements. (See History below.) The realization of the standard second is described briefly in a special publication from the National Institute of Standards and Technology,〔 〕 and in detail by the National Research Council of Canada.〔 〕 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「second」の詳細全文を読む スポンサード リンク
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