what is time

Time has long been a major subject of philosophy, art, poetry, and science. Though dictionaries present some (varied) definitions of time (some of which are presented below), it is difficult to provide an uncontroversial definition because there are widely divergent views about its meaning, and concerns about whether there are any simpler terms with which to define it. Scholars also disagree on whether time itself can be measured or is itself part of the measuring system. To avoid these definitional problems, many fields use an operational definition in which only the units of measurement are defined.

The measurement of time has also occupied scientists and technologists, and was a prime motivation in astronomy. Time is also a matter of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in our lives. Units of time have been agreed upon to quantify the duration of events and the intervals between them. Regularly recurring events and objects with apparent periodic motion have long served as standards for units of time. Examples are the apparent motion of the sun across the sky, the phases of the moon, and the swing of a pendulum.

Time has historically been closely related with space, most obviously with spacetime in Einstein's General Relativity.

Measurement

Time is currently one of the few fundamental quantities. These are quantities which cannot be defined via other quantities because there is nothing more fundamental than what is presently known. Thus, similar to definition of other fundamental quantities (like space and mass), time is defined via measurement.

Standards

The standard unit for time is the SI second, from which larger units such as the minute, hour and day are defined. The minute, hour, and day are "non-SI&;quot; units because they do not use the decimal system, and also because of the occasional need for a leap-second. They are, however, officially accepted for use with the International System. There are no fixed ratios between seconds and months or years as months and years have significant variations in length.

World Time


The measurement of time is so critical to the functioning of modern societies that it is coordinated at an international level. The basis for scientific time is a continuous count of seconds based on atomic clocks around the world, known as the International Atomic Time (TAI). This is the yardstick for other time scales, including Coordinated Universal Time (UTC), which is the basis for civil time.

Earth is split up into a number of time zones. Most time zones are exactly one hour apart, and by convention compute their local time as an offset from Greenwich Mean Time.

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