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Universal Time (UT or UT1) is a time standard based on Earth's rotation. [1] While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle with respect to the International Celestial Reference Frame (ICRF), called the Earth Rotation Angle ...
Graph showing the difference between UT1 and UTC. Vertical segments correspond to leap seconds. In about AD 140, Ptolemy, the Alexandrian astronomer, sexagesimally subdivided both the mean solar day and the true solar day to at least six places after the sexagesimal point, and he used simple fractions of both the equinoctial hour and the seasonal hour, none of which resemble the modern second. [8]
This is an accepted version of this page This is the latest accepted revision, reviewed on 5 December 2024. Primary time standard "UTC" redirects here. For the time zone between UTC−1 and UTC+1, see UTC+00:00. For other uses, see UTC (disambiguation). It has been suggested that UTC offset be merged into this article. (Discuss) Proposed since December 2024. Current time zones Coordinated ...
[5] [6] Today, Universal Time usually refers to Coordinated Universal Time (UTC) or UT1; [7] English speakers often use GMT as a synonym for UTC. [8] For navigation, it is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude); but this meaning can differ from UTC by up to 0.9 s. The term "GMT" should thus not be ...
TT is slightly ahead of UT1 (a refined measure of mean solar time at Greenwich) by an amount known as ΔT = TT − UT1. ΔT was measured at +67.6439 seconds (TT ahead of UT1) at 0 h UTC on 1 January 2015; [16] and by retrospective calculation, ΔT was close to zero about the year 1900.
Plot showing the difference UT1−UTC in seconds. Vertical segments correspond to leap seconds. Red part of graph was prediction (future values) at the time the file was made. Graph showing the difference between UTC (based on an atomic clock with leap seconds) and UT1 (based on the movement of the Earth).
This is a list of the UTC time offsets, showing the difference in hours and minutes from Coordinated Universal Time (UTC), from the westernmost (−12:00) to the easternmost (+14:00). It includes countries and regions that observe them during standard time or year-round.
DUT1 is a time correction equal to the difference between Universal Time , which is defined by Earth's rotation, and Coordinated Universal Time (UTC), which is defined by a network of precision atomic clocks, with a precision of +/- 0.1s [1] [2]. DUT1 = UT1 − UTC (with a precision of +/- 0.1s)