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In addition to physics, human psychology and language play roles in the conventional treatment of time. From the perspective of Western philosophy and physics, time is a relationship between events. From the perspective of language, we typically use the term "time" to distinguish two different ideas: |
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A defined event, or point in time, the value of t at a given point. |
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The amount of time that elapses between two defined events the value of the difference, t2 t1. |
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In each case, events are measured in terms of other events the ticking of a clock, the vibration of a crystal, the rotation of the Earth. We have difficulty in defining time, but we can measure it. |
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The events that we choose for measuring time have one characteristic they are regular. An event that occurs irregularly, like the eruption of a volcano, could serve as a time unit, but it would not be a useful one. |
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Whenever we discover that a unit we thought was safely regular contains variations, we try to choose another unit. We have discovered, for example, that the Earth's rotation on its axis is not uniform. The length of a day varies in a way we do not really understand. Physicists are looking now at certain astronomical objects that are extremely regular in their emissions and considering a change to Galactic Barycentric Time to make use of this phenomenon as a basis for timekeeping on Earth. |
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Human psychology enters into the discussion of time in the context of units. Western people, at least, generally distinguish two different kinds of time. The first kind of time is perceived as appropriate for events of relatively short duration. The second kind of time is perceived as appropriate for events of relatively long duration. A clock is a very different instrument from a calendar, although both measure time. |
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We use different units of time for different kinds of events, and we agree to relate the units by some arbitrary rules, so that the entire concept of continuity is not lost. We are comfortable using the unit of seconds for measuring events that take a few of them, but balk at the idea of using seconds to measure events that elapse over a period of days. |
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In the area of computer applications, this perceptual and psychological dichotomy is preserved in the language, so that we must talk about both date and time. We typically intend for the word "time" to refer to events that occur in the course of a fraction of one day and the word "date" to refer to events that occur in the course of more than one day. |
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2.1.2
Calendars As the Historical Basis of Timekeeping |
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Man seems to have formed his first ideas of time in terms of the larger units of days, months, and years. Statistical analyses of markings in the caves at Lascaux, for example, show an unusually high number of occurrences of the numbers 29 and 30, suggesting that early man was tracking the length of the lunar month in terms of days as early as 20 to 30 thousand years ago. |
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