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Definition: time from The Penguin Dictionary of Science

Symbol t; unit ➤second, s. One of the four coordinates of spacetime that define events. An event occurs at a particular position and a particular time. Time appears to differ from space, at least in the human perception of it, in that the world moves steadily forward in time. ➤➤charge-parity-time symmetry; relativity.


Summary Article: time from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Continuous passage of existence, recorded by division into hours, minutes, and seconds. Formerly the measurement of time was based on the Earth's rotation on its axis, but this was found to be irregular. Therefore the second, the standard SI unit of time, was redefined in 1956 in terms of the Earth's annual orbit of the Sun, and in 1967 in terms of a radiation pattern of the element caesium.

Universal time (UT), based on the Earth's actual rotation, was replaced by coordinated universal time (UTC) in 1972, the difference between the two involving the addition (or subtraction) of leap seconds on the last day of June or December. National observatories (in the UK until 1990 the Royal Greenwich Observatory) make standard time available, and the BBC broadcasts six ‘pips’ at certain hours (five short, from second 55 to second 59, and one long, the start of which indicates the precise minute). Its computerized clock has an accuracy greater than 1 second in 4,000 years. From 1986 the term Greenwich Mean Time was replaced by UTC. However, the Greenwich meridian, adopted in 1884, remains that from which all longitudes are measured, and the world's standard time zones are calculated from it.

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Time Quiz

Walk Through Time: The Evolution of Time Measurement

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