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Time in physics is defined by its measurement: time is what a clock reads. In classical, non-relativistic physics it is a scalar quantity and, like length, mass, and charge, is usually described as a fundamental quantity. Time can be combined mathematically with other physical quantities to derive other concepts such as motion, kinetic energy and time-dependent fields. ''Timekeeping'' is a complex of technological and scientific issues, and part of the foundation of ''recordkeeping''. ==Markers of time== (詳細はGalileo measured the period of a simple harmonic oscillator with his pulse.〕 which were understandable to each epoch of civilization:〔 *the first appearance (see: heliacal rising) of Sirius to mark the flooding of the Nile each year〔Otto Neugebauer ''The Exact Sciences in Antiquity''. Princeton: Princeton University Press, 1952; 2nd edition, Brown University Press, 1957; reprint, New York: Dover publications, 1969. Page 82.〕 *the periodic succession of night and day, seemingly eternallly〔See, for example William Shakespeare ''Hamlet'': " ... to thine own self be true, And it must follow, as the night the day, Thou canst not then be false to any man."〕 *the position on the horizon of the first appearance of the sun at dawn *the position of the sun in the sky〔(Farmers have used the sun to mark time for thousands of years, as the most ancient method of telling time. )〕 *the marking of the moment of noontime during the day〔Eratosthenes used this criterion in his measurement of the circumference of Earth〕 *the length of the shadow cast by a gnomon〔Fred Hoyle (1962), ''Astronomy: A history of man's investigation of the universe'', Crescent Books, Inc., London LC 62-14108, p.31''〕 Eventually,〔The Mesopotamian (modern-day Iraq) astronomers recorded astronomical observations with the naked eye, more than 3500 years ago. P. W. Bridgman defined his operational definition in the twentieth c.〕〔Naked eye astronomy became obsolete in 1609 with Galileo's observations with a telescope. Galileo Galilei Linceo, (''Sidereus Nuncius'' ) (''Starry Messenger'') 1610.〕 it became possible to characterize the passage of time with instrumentation, using operational definitions. Simultaneously, our conception of time has evolved, as shown below.〔http://tycho.usno.navy.mil/gpstt.html http://www.phys.lsu.edu/mog/mog9/node9.html Today, automated astronomical observations from satellites and spacecraft require relativistic corrections of the reported positions.〕 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 in physics」の詳細全文を読む スポンサード リンク
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