A measure of time that relates closely to the Sun’s daily apparent motion and serves as the basis for civil timekeeping. It is formally defined by a mathematical formula that links it to sidereal time, and is thus determined from observations of the stars. The timescale taken directly from the stars, designated UT0, depends slightly on the place of observation. UT0 corrected for the shift in longitude of the observing station caused by polar motion is known as UT1. The use of the abbreviation UT normally implies UT1. Coordinated Universal Time UTC is the time used for broadcast time signals. It differs from International Atomic Time TAI by an integral number of seconds, and is maintained to within ±0.90 second of UT1 by introducing when necessary one-second steps - leap seconds.