UTC (Coordinated Universal Time)
utc
The primary time standard by which the world regulates clocks and time, serving as the basis for civil timekeeping globally.
UT1 (Universal Time 1)
ut1
Definition
UT1 (Universal Time 1) is a timescale derived from the observed rotation angle of the Earth. It is determined through Very Long Baseline Interferometry (VLBI) observations of distant quasars, which precisely measure the Earth's rotational orientation. Because UT1 directly reflects all irregular variations in the Earth's spin, including the effects of tidal friction, crustal movements, and atmospheric circulation, the length of a UT1 day is never exactly 86,400 SI seconds.
Relationship to UTC
UTC is a uniform timescale based on atomic clocks, whereas UT1 is a non-uniform timescale tied to the Earth's rotation. The difference between them (DUT1 = UT1 - UTC) is kept within 0.9 seconds by inserting leap seconds. The International Earth Rotation and Reference Systems Service (IERS) monitors DUT1 and announces upcoming leap seconds via Bulletin C. The 2022 resolution of the General Conference on Weights and Measures (CGPM) decided to widen this permitted difference by 2035, although the new limit and its implementation date were still undecided as of September 2026. Once the wider limit takes effect, leap second insertions will effectively stop and DUT1 will be allowed to gradually accumulate without correction.
Practical Use
UT1 is essential in astronomy (celestial position calculations), geodesy (determining Earth's orientation), and precision satellite orbit determination. For everyday life and IT systems, UTC is sufficient, but observatory software and precision satellite-tracking systems require the DUT1 correction. IERS publishes the current DUT1 value weekly in Bulletin A.
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Related Terms
TAI (International Atomic Time)
tai
TAI is a continuous timescale computed as a weighted average of over 450 atomic clocks worldwide, containing no leap seconds, and serving as the foundation for UTC.
Leap Second
leap-second
A one-second adjustment applied to UTC to keep it aligned with the Earth's irregular rotation.
IERS (International Earth Rotation and Reference Systems Service)
iers
IERS is the international body responsible for monitoring Earth's rotation, determining when leap seconds are needed, and maintaining the global terrestrial and celestial reference frames.
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