Universal Time

Universal Time is a family of time standards derived from Earth’s rotation. Its variants account for polar motion and irregularities in the planet’s rotation.

UT1: The Universal Time scale that tracks Earth’s rotation angle relative to the celestial reference frame. It is the rotation-based form most directly tied to Earth’s observed orientation.
Earth rotation angle: The angle locating Earth’s rotating body relative to a celestial reference frame. UT1 is defined through this angle rather than through a uniform clock.
Greenwich Mean Time: Mean solar time referenced to the meridian at Greenwich, historically used as an international time standard. Universal Time developed from the astronomical timekeeping tradition associated with Greenwich.
Very Long Baseline Interferometry: A radio-astronomy technique that combines observations from widely separated telescopes to measure positions and distances precisely. It supplies highly accurate measurements of Earth’s rotation used to determine UT1.
UT0: Universal Time derived from astronomical observations at a particular site, without correction for polar motion. It is the uncorrected local starting point in the family of UT scales.
International Atomic Time: A continuous time scale based on the combined readings of atomic clocks. Unlike Universal Time, it does not follow the variable rate of Earth’s rotation.
International Meridian Conference: The 1884 conference that selected Greenwich as the prime meridian and recommended a universal day. Its decisions helped establish the global framework from which Universal Time emerged.
Satellite navigation: Positioning systems that determine location from timed signals transmitted by satellites. Earth-orientation data, including UT1, supports precise transformations between reference frames.
UT2: A smoothed form of UT1 that removes modeled seasonal variations in Earth’s rotation rate. It illustrates an older attempt to regularize rotation-based time.
Coordinated Universal Time: The international civil time standard maintained near Earth-rotation time through leap seconds. It combines atomic-clock regularity with limits on its divergence from UT1.

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