Universal Time (UT) is a worldwide standard that serves as the basis of civil and most astronomical timekeeping, also known as Greenwich Mean Time (GMT). UT is defined as the hour angle of the mean sun as seen by an observer on the prime meridian plus 12 hours (so that 0000 UT corresponds to midnight rather than noon). In practice, it is determined from a formula that links it to sidereal time so that it actually comes from observations of the daily motion of stars or extraterrestrial radio sources. The version of UT thus produced, known as UT0, depends slightly on the place of observation. When UT0 is corrected for the shift in longitude of the observing station caused by polar motion, the result is UT1. If a further correction is made for seasonal changes in Earth's rotation rate, the result is UT2. So-called Coordinated Universal Time (UTC) is used for broadcast time signals (available via shortwave radio, for example). It differs from International Atomic Time by an integral number of seconds and is kept within +/- 0.9 second of UT1 by introducing leap-seconds when necessary.