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time, sequential arrangement of all events
Physical Time and Its Measurement
The accurate measurement of time by establishing accurate time standards poses difficult technological problems. In prehistory, humans recognized the alternation of day and night, the phases of the moon, and the succession of the seasons; from these cycles, they developed the day, month, and year as the corresponding units of time. With the development of primitive clocks and systematic astronomical observations, the day was divided into hours, minutes, and seconds.
Any measurement of time is ultimately based on counting the cycles of some regularly recurring phenomenon and accurately measuring fractions of that cycle. The earth rotates on its axis at a very nearly constant rate, and the angular positions of celestial bodies can be determined with great precision. Therefore, astronomical observations provide an almost ideal method of measuring time. The true period of rotation of the earth, that with respect to the fixed stars, defines the sidereal day, which is the basis of sidereal time. All sidereal days are equal. The period of rotation of the earth with respect to the sun (i.e., the interval between successive high noons) is the solar day, which is the basis for solar time. Because of the earth's motion in its orbit around the sun, the sun appears to move eastward against the fixed stars, and the earth must make slightly more than one complete rotation to bring the sun back to the observer's meridian. (The meridian is the great circle on the celestial sphere running through the north celestial pole and the observer's zenith; the passage of the sun across the meridian marks high noon.) But the earth's orbital motion is not uniform, and the plane of the orbit is inclined to the celestial equator by 231-2°. Hence the eastward motion of the sun against the stars is not uniform and the length of the true solar day varies seasonally, but on the average is four minutes longer than the sidereal day. True solar time, as measured by a sundial, does not move at a constant rate. Therefore the mean solar day, with a length equal to the annual average of the actual solar day, was introduced as the basis of mean solar time.
Mean solar time does move at a constant rate and is the basis for the civil time kept by clocks. Actually, the earth's rotation is being slightly braked by tidal and other effects so that even mean solar time is not strictly uniform. The law of gravitation allows prediction of the moon's position in its orbit at a given time; inversely, the exact position of the moon provides a kind of clock that is not running down. Time calculated from the moon's position is called ephemeris time and moves at a truly uniform rate. The accumulated difference between mean solar and ephemeris time since 1900 amounts to more than half a minute. However, the ultimate standard for time is provided by the natural frequencies of vibration of atoms and molecules. Atomic clocks, based on masers and lasers, lose only about three milliseconds over a thousand years. See standard time; universal time.
Psychology of Time
Philosophy and Science of Time
Developments of modern physics have forced a modification of the concept of simultaneity. As Albert Einstein demonstrated in his theory of relativity, when two observers are in relative motion, they will necessarily arrange events in a somewhat different time sequence. As a result, events that are simultaneous in one observer's time sequence will not be simultaneous in some other observer's sequence. In the theory of relativity, the intuitive notion of time as an independent entity is replaced by the concept that space and time are intertwined and inseparable aspects of a four-dimensional universe, which is given the name space-time.
One of the most curious aspects of the relativistic theory is that all events appear to take place at a slower rate in a moving system when judged by a viewer in a stationary system. For example, a moving clock will appear to run slower than a stationary clock of identical construction. This effect, known as time dilation, depends on the relative velocities of the two clocks and is significant only for speeds comparable to the speed of light. Time dilation has been confirmed by observing the decay of rapidly moving subatomic particles that spontaneously decay into other particles. Stated naively, particles in motion decay more slowly than stationary particles.
Time Reversal Invariance
See S. V. Toulmin and J. Goodfield, Discovery of Time (1965); S. Hawking, A Brief History of Time: From the Big Bang to Black Holes (1988).
time, in music
The dimension of the physical universe which orders the sequence of events at a given place; also, a designated instant in this sequence, such as the time of day, technically known as an epoch, or sometimes as an instant.
Time measurement consists of counting the repetitions of any recurring phenomenon and possibly subdividing the interval between repetitions. Two aspects to be considered in the measurement of time are frequency, or the rate at which the recurring phenomena occur, and epoch, or the designation to be applied to each instant.
Time units are the intervals between successive recurrences of phenomena, such as the period of rotation of the Earth or a specified number of periods of radiation derived from an atomic energy-level transition. Other units are arbitrary multiples and subdivisions of these intervals, such as the hour being 1/24 of a day, and the minute being 1/60 of an hour. See Time-interval measurement
Several phenomena are used as bases with which to determine time. The phenomenon traditionally used has been the rotation of the Earth, where the counting is by days. Days are measured by observing the meridian passages of stars and are subdivided with the aid of precision clocks. The day, however, is subject to variations in duration. Thus, when a more uniform time scale is required, other bases for time must be used.
The angle measured along the celestial equator between the observer's local meridian and the vernal equinox, known as the hour angle of the vernal equinox, is the measure of sidereal time. It is reckoned from 0 to 24 hours, each hour being subdivided into 60 sidereal minutes and the minutes into 60 sidereal seconds. Sidereal clocks are used for convenience in most astronomical observatories because a star or other object outside the solar system comes to the same place in the sky at virtually the same sidereal time.
The hour angle of the Sun is the apparent solar time. The only true indicator of local apparent solar time is a sundial. Mean solar time has been devised to eliminate the irregularities in apparent solar time that arise from the obliquity of the ecliptic and the varying speed of the Earth in its orbit around the Sun. It is the hour angle of a fictitious point moving uniformly along the celestial equator at the same rate as the average rate of the Sun along the ecliptic. Both sidereal and solar time depend on the rotation of the Earth for their time base.
The mean solar time determined for the meridian of 0° longitude from the rotation of the Earth by using astronomical observations is referred to as UT1. Observations are made at a number of observatories around the world. The International Earth Rotation Service (IERS) receives these data and maintains a UT1 time scale.
Because the Earth has a nonuniform rate of rotation and since a uniform time scale is required for many timing applications, a different definition of a second was adopted in 1967. The international agreement calls for the second to be defined as 9,192,631,770 periods of the radiation derived from an energy-level transition in the cesium atom. This second is referred to as the international or SI (International System) second and is independent of astronomical observations. International Atomic Time (TAI) is maintained by the International Bureau of Weights and Measures (BIPM) from data contributed by time-keeping laboratories around the world.
Coordinated Universal Time (UTC) uses the SI second as its time base. However, the designation of the epoch may be changed at certain times so that UTC does not differ from UT1 by more than 0.9 s. UTC forms the basis for civil time in most countries and may sometimes be referred to as Greenwich mean time. The adjustments to UTC to bring this time scale into closer accord with UT1 consist of the insertion or deletion of integral seconds. These “leap seconds” may be applied at 23 h 59 m 59 s of June 30 or December 31 of each year according to decisions made by the IERS. UTC differs from TAI by an integral number of atomic seconds.
Civil and standard times
Because rotational time scales are defined as hour angles, at any instant they vary from place to place on the Earth. Persons traveling westward around the Earth must advance their time 1 day, and those traveling eastward must retard their time 1 day in order to be in agreement with their neighbors when they return home. The International Date Line is the name given to a line where the change of date is made. It follows approximately the 180th meridian but avoids inhabited land. To avoid the inconvenience of the continuous change of mean solar time with longitude, zone time or civil time is generally used. The Earth is divided into 24 time zones, each approximately 15° wide and centered on standard longitudes of 0°, 15°, 30°, and so on. Within each of these zones the time kept is the mean solar time of the standard meridian.
Many countries, including the United States, advance their time 1 hour, particularly during the summer months, into “daylight saving time.”
timeThe continuous and nonreversible passage of existence, or some interval in this continuum. It is also a quantity measuring the duration of such an interval or of some process, etc. The unit of time in the SI system is the second. The astronomical unit of time is often the interval of one day of 86 400 seconds. The time systems used in astronomy are International Atomic Time, universal time, sidereal time, and dynamical time (which has replaced ephemeris time). See also spacetime.
timethe continuous passage of existence. Time may be measured with reference to any stable or periodic physical or social process. In the latter case, ‘time’ will often be stated with reference to clear physical periodicies that determine the units of social division, as for example, ‘days’ and ‘years’. In many other respects, however, the divisions of’time’ (though still stated partly in terms of physical periodicies) depend on a patterning of social events which is relatively independent of such natural periodicies, as is so for ‘weeks’ or ‘hours’. Various forms of generic ‘social time’ may also be identified, as for example, in the distinctions drawn by GIDDENS between:
- the repeated day-to-day durée - or ‘reversible time’ – of everyday social life;
- the longue durée involved in the persistence, as against the rise and fall, of social institutions and societies;
- the ‘life span’ of the individual -‘irreversible time’.
As well as this, in social life and in sociological and historical accounts an almost infinite number of more specific ‘periodizations’ can also be noticed, e.g. ‘Victorian times’, ‘the Age of Reason’. See also CLOCK-TIME, TIME – SPACE DISTANCIATION.
Since time always exists as a fourth coordinate of time-space in specifying any event, it must obviously be an important component in any sociological account. A number of sociologists recently, however, have suggested that time has been relatively neglected in sociology, in that sociology has often been concerned with static structural models and has tended to neglect the great variety of ways in which social life is both temporally structured and, as the result of social processes occurring in time, socially transformed – see MANN (1986) and GIDDENS (1984). A resurgence of interest in time has been a feature of recent sociology and is also evident in other disciplines (e.g. see TIME-GEOGRAPHY), from which sociology has also drawn.
a basic form (together with space) of the existence of matter; it consists of the regular coordination of phenomena that are occurring one after another. It exists objectively and is inseparably associated with moving matter.
Measurement. Various branches of science and technology deal with the problem of measuring time, independent of the means and system by which it is recorded. Chronometers— technical means for measuring time and reproducing its units and subdivisions (clocks and other instruments)—are. developed in chronometry. With the aid of special observations of celestial bodies, astronomy makes it possible to monitor the performance of time-recording devices and to determine corrections in time scales.
Even in earliest times, measurements of large and small time intervals were based on astronomical phenomena dependent on the motions of celestial bodies, especially the earth and moon. The year, which was defined by the period of the earth’s orbit around the sun, began to be used as the unit for measuring large time intervals. The cycle of changes in nature is associated with this unit. The cycle of changing phases of the moon (the synodic month) began to be used as a smaller unit of time and, with slight changes, became what is now our month. The day is based on the cycle of light and dark periods and is determined by the earth’s rotation. In order to record smaller intervals, the day was divided into hours; originally the daylight period was divided into 12 day-time hours, and the period of darkness into 12 nighttime hours, which differed in length and whose duration throughout the year was not constant. Later, division of the day into 24 equal hours was introduced. The development of human economic activity led to greater demands on time measurement. Instruments for measuring time—clocks—were perfected, which permitted the introduction of more and more accurate systems for recording time for practical and scientific purposes. In modern clocks, the system of recording time is based on various artificial periodic processes: the oscillation of a balance wheel (marine chronometers and household clocks), a pendulum (astronomical clocks), or a quartz plate (quartz clocks). In the most accurate quartz clocks, the stability of the oscillations is governed by quantum generators, whose operation is based on periodic processes occurring in atoms and molecules (atomic clocks).
The rotation of the earth about its axis relative to the stars determines sidereal time. Since the stars have motion of their own, which has been insufficiently studied, sidereal time is measured relative to the vernal equinox, whose motion among the stars is well known. The moment of its upper culmination is taken as the beginning of the sidereal day. The sidereal day is subdivided into sidereal hours, minutes, and seconds. Sidereal time is determined directly from astronomical observations and serves to coordinate the readings of clocks and chronometers with the astronomical system of recording time. Knowledge of sidereal time is essential in various astronomical observations, as well as in geodetic measurements, navigation, and other work involving observations of celestial bodies. It is impractical in everyday life, since it does not coincide with the change from day to night. For this reason, solar time is used in everyday life.
True solar time is determined by the apparent daily motion of the sun, whose upper and lower culminations are accordingly called true noon and true midnight. The interval of time between two consecutive like culminations of the center of the sun is called a true solar day. However, because of the uneven motion of the earth in its orbit and, consequently, the apparent annual motion of the sun along the ecliptic, as well as the fact that the earth’s axis is not perpendicular to the plane of its orbit, the true solar day is not constant in its duration—that is, the system for recording true solar time is irregular. The system of solar time that is uniform throughout the year is called mean solar time and is based on the daily motion of the so-called mean sun, an imaginary point that moves evenly along the equator with a speed such that in its annual motion it always crosses the vernal equinox simultaneously with the true sun. The moments of upper and lower culmination of the mean sun are correspondingly called mean noon and mean midnight. The time interval between two consecutive like culminations of the mean sun is called a mean solar day, and it begins from the mean sun’s lower culmination. The mean solar day is divided into mean solar hours, minutes, and seconds.
The discrepancy between mean and true solar time is called the time equation, and this varies during the year between -14 min, 22 sec, and 16 min, 24 sec. Mean solar time is checked against sidereal time by the following relationship, based on numerous observations:
(1) 365.2422 mean solar days = 366.2422 sidereal days, from which it follows that
(2) 24 hr of sidereal time = 23 hr, 56 min, 4.091 sec of mean solar time, and
(3) 24 hr of mean solar time = 24 hr, 3 min, 56.666 sec of sidereal time.
Clocks operating on mean solar time and on sidereal time are used to keep time determined by astronomical observations.
At different meridians of the earth, the moments of culmi-nation of both the vernal equinox and the true and mean sun do not occur at the same physical moment. Therefore, the time at different meridians is also different: a 15° eastward change in longitude corresponds to an increase of one hour in sidereal time, as well as in true and mean solar time. The time determined for a particular longitude is called local time (sometimes the zone time used at various points on the earth is erroneously called local time). Local mean solar time at the zero or Greenwich meridian reckoned from midnight is called universal or world time (Greenwich time). Universal time, which is the same worldwide, is extensively used in astronomy.
Local time, which is different at points with different geo-graphic longitude, causes inconvenience in its practical use in intercity and international communications. To eliminate these inconveniences, a system of zone time was adopted at the end of the 19th century in many countries of the world, whereby the entire surface of the earth was divided into 24 time zones, each 15° of longitude wide, extending along the meridians. Zone time was introduced in the USSR on July 1, 1919. To make practical use of daylight hours, clocks in some countries are advanced one hour in relation to zone time in summer. In the USSR clocks were moved ahead one hour in 1930 (so-called daylight saving time). Daylight saving time in the second time zone of the USSR is called Moscow time and is three hours ahead of universal time.
Exacting research has shown that the system for astronomical recording of time based on observations of the culminations of celestial bodies is not uniform (universal time in this system is designated UTO); this is due first to the migration of the earth’s poles, which alters the longitude of observation sites, and second to unevenness in the rotation of the earth, which was discovered by using highly stable quartz and atomic clocks. The introduction of corrections in UTO to take into account the shifting of the poles results in UT1 universal time, and further corrections to account for mean seasonal changes in the period of the earth’s rotation result in UT2 universal time. Even after the above corrections have been made, however, the uniform systems for recording time based on the period of the earth’s rotation are not adequate for certain branches of modern science and technology.
A uniform system for recording time—ephemeris time—is being introduced as an independent argument in the laws of celestial mechanics and is checked by observations of the rotation of the moon about the earth. Astronomical year-books are compiled on the basis of ephemeris time. This system is defined in terms of the difference between ephemeris time and mean solar time on the basis of the empirical relationship
Δt sec = + 24.349 + 72.318T + 29.950T2 + 1.821B
where T is calculated in Julian centuries of 36,525 mean solar days from the date Jan. 0, 1900, at 12 o’clock universal time, and B is the deviation of the longitude of the moon computed by Braun’s theory from the longitude observed at a given moment. Because of irregularities in the earth’s rotation, the magnitude of a mean solar day has increased over a period of 100 years by 1.640 msec; it fluctuates because of the existence of a factor dependent on B (over the past 120 years it has reached -4.8 msec in 1870 and 1.9 msec in 1911). Therefore the definition of a second in physical systems of units has now begun to be based not on the period of the earth’s rotation but on the period of its orbit about the sun, which is called the tropical year and is equal to the time interval between two consecutive passages of the sun through the vernal equinox. This interval is slowly changing over the course of time and equals 365.24219879 -0.00000614(7 - 1900) mean solar days. The General Conference on Weights and Measures (Paris, 1954) gave the following definition of a second of time in the centimeter-gram-second system: “A second is 1/31,556,925.9747 of a tropical year for the moment Jan. 0, 1900, at 12 o’clock ephemeris time.” Ephemeris time defined by this second for recording large time intervals is expressed in Julian centuries of 36,525 ephemeris days from the moment Jan. 0, 1900, at 12 o’clock ephemeris time.
The development of electronics in the 1960’s made it possible to obtain a system for recording time that is new in principle and independent of astronomical observations. It is based on the use of high-accuracy quartz clocks controlled by quantum generators (atomic clocks). This system of calculating time has been given the name atomic time and is designated TA1. An atomic second serves as a standard unit, and its magnitude is determined by the resonance frequency of one of the energy transitions in an atom of cesium 133.
Radio signals for exact time are broadcast by time services by means of atomic clocks in a special system for calculating TA atomic time that is coordinated with astronomical systems of timekeeping: the duration of a second of TA time is defined annually from astronomical observations. Thus, the TA time system provides a connection between universal time obtained by astronomical observations and TA1 atomic time.
All systems for calculating time are regularly compared with each other so that a shift can be made for any moment from one system to another. The results of the comparisons are published in the Bulletins of the International Time Bureau in Paris, and in the USSR also in the bulletin Etalonnoe vremia (Standard Time), published by the All-Union Scientific Research Institute of Physical Technology and Radio Measurement.
REFERENCEKulikov, K. A.Kurs sfericheskoi astronomii, 2nded. Moscow, 1969.
K. A. KULIKOV and V. V. PODOBED
a weekly US magazine. Time, founded in 1923, is published in New York. Owned by the publishing trust of Time, Inc., it publishes articles on US domestic and foreign policy issues and scientific and cultural topics. Time also publishes many international editions. Circulation, about 4 million (1975).
What does it mean when you dream about time?
An emphasis on time in a dream may indicate a great deal of stress in the dreamer’s life, perhaps the feeling that time is running out in either a business or a personal matter.