# Precision timing

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This page summarizes some links and formulae related to the precise manner of determining the rotation of Earth and similar quantities (e.g. sidereal time and so).

## The Earth Orientation Parameters EOP

The IERS Earth Orientation Parameters (EOP) describe the irregularities of the Earth's rotation. These are the parameters which provide the rotation of the ITRS to the ICRS as a function of time:

• Universal Time UT1: UT1 is the time of the earth clock, which performs one revolution in one day (derived from the various astro-geodetic techniques (LLR, SLR, GPS, VLBI and DORIS)), the duration of the day is slowly changing. International Atomic Time (TAI) is a high-precision atomic coordinate time standard and the basis for UTC. UTC differs from TAI by an integer number of seconds (leap seconds, because the rotation of the earth is slowing). The value of difference of UT1-UTC (DUT1) is announced in IERS Bulletins. Note a leap second will be added at 2016 December 31, 23h 59m 60s. TT=TAI+32,184. Greenwich Mean Sidereal Time is derived from UT1.
• Celestial pole offsets ($d\psi,d\epsilon$). Large scale motion of celestial pole: precession and nutation. Based on VLBI observations.
• Coordinates of the pole x,y(polar motion): two small angles of the Celestial Ephemeris Pole relative to the geographic pole axis (ITRF pole axis). The variations of pole coordinates are mostly because of atmospheric and oceanic mass redistributions.

EOP series are provided by the IERS in bulletins and as permanently updated series (long term earth orientation data).