Greenwich Mean Time (GMT/UT1)

Greenwich Mean Time, GMT, is the mean solar time at the Greenwich meridian, the time kept by the average position of the sun over zero degrees longitude. The navigator treats it as UT1, the version of Universal Time tied to the actual rotation of the Earth and so to the position of celestial bodies. The Nautical Almanac tabulates Greenwich Hour Angle and declination against GMT/UT1, so a sight must be timed in it to reduce correctly. UT1 can differ from UTC by up to 0.9 seconds, the offset DUT1 broadcast in time signals, because UTC is held to atomic seconds with leap seconds while UT1 follows the irregular spin of the Earth. For ordinary navigation the two are equal.