Editing Universal Time (section)

==Measurement==
{{see also|Earth's rotation#Measurement}}

Historically, Universal Time was computed from observing the position of the [[Sun]] in the sky. But astronomers found that it was more accurate to measure the rotation of the Earth by observing stars as they crossed the meridian each day. Nowadays, UT in relation to [[International Atomic Time]] (TAI) is determined by [[Very Long Baseline Interferometry]] (VLBI) observations of the positions of distant celestial objects ([[star]]s and [[quasar]]s), a method which can determine UT1 to within 15 microseconds or better.{{sfn|McCarthy|Seidelmann|2009|pages=68–9}}{{sfn|Urban|Seidelmann|2013|page=175}}

[[File:Universal Dial Plate or Times of all Nations, 1854.jpg|thumb|An 1853 "Universal Dial Plate" showing the relative times of "all nations" before the adoption of universal time]]The rotation of the Earth and UT are monitored by the [[International Earth Rotation and Reference Systems Service]] (IERS). The [[International Astronomical Union]] also is involved in setting standards, but the final arbiter of broadcast standards is the [[International Telecommunication Union]] or ITU.{{sfn|McCarthy|Seidelmann|2009|loc=Ch. 18}}

The rotation of the Earth is somewhat irregular and also is very gradually slowing due to [[tidal acceleration]]. Furthermore, the length of the second was determined from observations of the [[Moon]] between 1750 and 1890. All of these factors cause the modern [[mean solar day]], on the average, to be slightly longer than the nominal 86,400 [[SI]] seconds, the traditional number of seconds per day.{{efn|24 hours of 60 minutes of 60 seconds}} As UT is thus slightly irregular in its rate, astronomers introduced [[Ephemeris Time]], which has since been replaced by [[Terrestrial Time]] (TT). Because Universal Time is determined by the Earth's rotation, which drifts away from more precise atomic-frequency standards, an adjustment (called a [[leap second]]) to this atomic time is needed since ({{as of|2019|lc=on}}) 'broadcast time' remains broadly synchronised with solar time.{{efn|Continuation of this principle is under active debate in standards bodies. See [[Leap second#Future of leap seconds]]}} Thus, the civil broadcast standard for time and frequency usually follows International Atomic Time closely, but occasionally step (or "leap") in order to prevent them from drifting too far from mean solar time.{{citation needed|date=February 2024}}

[[Barycentric Dynamical Time]] (TDB), a form of atomic time, is now used in the construction of the ephemerides of the [[planet]]s and other [[Solar System]] objects, for two main reasons.<ref>{{harvnb|Urban|Seidelmann|2013|page=7}}. Strictly speaking, a major producer of ephemerides, the [[Jet Propulsion Laboratory]], uses a time scale they derive, T<sub>eph</sub>, which is functionally equivalent to TDB.</ref> First, these ephemerides are tied to optical and [[radar]] observations of planetary motion, and the TDB time scale is fitted so that [[Newton's laws of motion]], with corrections for [[general relativity]], are followed. Next, the time scales based on Earth's rotation are not uniform and therefore, are not suitable for predicting the motion of bodies in the Solar System.{{citation needed|date=February 2024}}