Time (redirected from give the time of day)

time, sequential arrangement of all events

time, sequential arrangement of all events, or the interval between two events in such a sequence. The concept of time may be discussed on several different levels: physical, psychological, philosophical and scientific, and biological.

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

The Evolution of Mechanical 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 23 1-2°. 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

As a practical matter, clocks and calendars regulate everyday life. Yet at the most primitive level, human awareness of time is simply the ability to distinguish which of any two events is earlier and which later, combined with a consciousness of an instantaneous present that is continually being transformed into a remembered past as it is replaced with an anticipated future. From these common human experiences evolved the view that time has an independent existence apart from physical reality.

Philosophy and Science of Time

The belief in time as an absolute has a long tradition in philosophy and science. It still underlies the common sense notion of time. Isaac Newton, in formulating the basic concepts of classical physics, compared absolute time to a stream flowing at a uniform rate of its own accord. In everyday life, we likewise regard each instant of time as somehow possessing a unique existence apart from any particular observer or system of timekeeping. Inherent in the concept of absolute time is the assumption that the simultaneity of two given events is also absolute. In other words, if two events are simultaneous for one observer, they are simultaneous for all observers.

Relativistic 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

In addition to relative time, another aspect of time relevant to physics is how one can distinguish the forward direction in time. This problem is apart from one's purely subjective awareness of time moving from past into future. According to classical physics, if all particles in a simple system are instantaneously reversed in their velocities, the system will proceed to retrace its entire past history. This property of the laws of classical physics is called time reversal invariance (see symmetry); it means that when all microscopic motions of individual particles are precisely defined, there is no fundamental distinction between forward and backward in time. If the motions of very large collections of particles are treated statistically as in thermodynamics, then the forward direction of time is distinguished by the increase of entropy, or disorder, in the system. However, recent discoveries in particle physics have shown that time reversal invariance is not valid even on the microscopic scale for certain phenomena governed by the weak force of nuclear physics.

Biological Time

In the life sciences, evidence has been found that many living organisms incorporate biological clocks that govern the rhythms of their behavior (see rhythm, biological). Animals and even plants often exhibit a circadian (approximately daily) cycle in, for instance, temperature and metabolic rate that may have a genetic basis. Efforts to localize time sense in specialized areas within the brain have been largely unsuccessful. In humans, the time sense may be connected to certain electrical rhythms in the brain, the most prominent of which is known as the alpha rhythm at about ten cycles per second.

Bibliography

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).

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time, in music

time, in music: see tempo; meter; rhythm; syncopation; metronome and musical notation.

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time

Measured or measurable period. More broadly, it is a continuum that lacks spatial dimensions. Philosophers have sought an understanding of time by focusing on the broad questions of the relation between time and the physical world and the relation between time and consciousness. Those who adopt an absolutist theory of time regard it as a kind of container within which the universe exists and change takes place, and believe that its existence and properties are independent of the physical universe. According to the rival relationist theory, time is nothing over and above change in the physical universe. Largely because of Albert Einstein, it is now held that time cannot be treated in isolation from space (see space-time). Some argue that Einstein's theories of relativity vindicate relationist theories, others that they vindicate the absolutist theory. The primary issue concerning the relation between time and consciousness is the extent, if any, to which time or aspects of time depend on the existence of conscious beings. Events in time are normally thought of in terms of notions of past, present, and future, which some philosophers treat as mind-dependent; others believe that time is independent of perception and hold that past, present, and future are objective features of the world. See also geologic time, Greenwich Mean Time, standard time, Universal Time.

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Time

Major U.S. weekly newsmagazine, published in New York City. It was founded in 1923 by Henry R. Luce (as business manager) and Briton Hadden (as editor). It became the most influential newsmagazine in the U.S., with a format of short articles arranged in subject “departments,” which became the standard for later general newsmagazines. After Hadden's death in 1929, Luce was long the magazine's guiding force, and it reflected his moderately conservative political viewpoint. By the 1970s it had assumed a more neutral, centrist stance in its reportage. In addition to the U.S. circulation, editions are published in Canada, Europe, Asia, and the Pacific.

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Time

Timelessness (See AGELESSNESS, IMMORTALITY.)

Antevorta

goddess of the future. [Rom. Myth.: Kravitz, 24]

Cronos

(Rom. Saturn) Titan; god of the world and time. [Gk. and Rom. Myth.: Kravitz, 69]

dance of Shiva

symbolizes the passage of time. [Hindu Tradition: Cirlot, 76]

Father Time

classic personification of time with scythe and hourglass. [Art: Hall, 119]

Marcel

the fast ebbing of time impels him to devote his life to recording it. [Fr. Lit.: Proust Remembrance of Things Past]

ring

represents the cyclical nature of time. [Pop. Culture: Cirlot, 273–274]

river

represents the irreversible passage of time. [Pop. Culture: Cirlot, 274]

Skulda

Norn of future time. [Norse Myth.: Wheeler, 260]

Urda

Norn of time past. [Norse Myth.: Wheeler, 260]

Verdandi

Norn of time present. [Norse Myth.: Wheeler, 260]

white poplar

traditional symbol of time. [Flower Symbolism: Flora Symbolica, 178]

Years, The

the seven decades of Eleanor Pargiter’s life. [Br. Lit.: Benét, 1109]