time
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) 1972, the difference between the two involving the addition (or subtraction) of leap seconds on the last day of June or December. National observatories make standard time available in various countries. From 1986 the term Greenwich Mean Time was replaced by UTC. However, the Greenwich meridian, adopted 1884, remains that from which all longitudes are measured, and the world's standard time zones are calculated from it. |
time
