Points To Remember GMT now has to be considered ‘a figment A Definitions. of the general public’s imagination’. It UT0: mean solar time on the Greenwich meridian hasn’t, any longer, any scientific basis at all! Briefer History obtained from direct astronomical observation. ‘GMT’ MUST NOT BE CONSIDERED AS The time 00 hours being at Greenwich mean ANY EXACT TIME-SCALE. mid-night. UT1: UT0 corrected for polar motion. of Time UT2: UT1 corrected for seasonal variations in the Earth’s rotation (a smoothed scale that doesn’t When the Royal Observatory, Greenwich, by reflect the real periodic variations in the Earth’s used the term GMT, it was measured from angular position). Greenwich mean MID-DAY, 12 hours Howard Barnes UTC: civil atomic time-scale on or about the BEHIND Universal Time (UT). Greenwich meridian kept within 0·9 seconds of UT1. UT: An abbreviation that should be used to show with apologies to Stephen Hawking approximation of all of the above. Atomic clocks cannot keep ‘mean time’. Mean time is a measurement of the Earth’s rotation, whereas atomic time is based on References. the frequency of the cæsium 133 atom which * Bureau International de l’Heure (BIH) oscillates at a frequency of 9 192 631 770 Hz. Annual Reports 1977-1987. * National Bureau of Standards Monograph 140. * International Earth Rotation & Reference Systems Service (IERS) All civil times are now ‘atomic’ and Annual Reports 1988-2000. therefore, are on, or are a certain number of * Astronomical Almanacs, Section B, (Royal hours (and minutes) ahead of, or behind, Greenwich Observatory) 1981-2001. Co-ordinated Universal Time (UTC). * Time Amendment Act 1987 (New Zealand). * Greenwich Time and the Discovery of Longitude by Derek Howse, OUP 1980. The abbreviation UTC is language- * Dr Bernard Guinot, [then] Director BIH, personal communications independent. It is neither English nor French. It stands for both Co-ordinated Universal Time and Temps Universel Coordonné. A Georgi Dobrovolski Solar Observatory Georgi Dobrovolski New Zealand New Zealand Standard Time (NZST) and Solar Observatory New Zealand Daylight Time (NZDT) are Publication Website: gdso.webs.com deemed, both scientifically and legally, as being 12 and 13 hours ahead of E-mail: [email protected] Co-ordinated Universal Time (UTC) respectively. © Howard Barnes 2010. When the Royal Observatory was proper) GMT was renamed ‘Greenwich THE ESTABLISHMENT OF ATOMIC established at Greenwich, in 1675, time Mean Astronomical Time’ (GMAT), and TIME-SCALES. was considered mainly as a measurement civil ‘GMT’ was, at first, ‘officially’ named With the development of atomic of the Earth’s rotation. ‘Mean Time’ is the as ‘Greenwich Civil Time’ (GCT). clocks, it definitely became clear that all UT measurement of the Earth’s orientation However, as the abbreviation GCT was a scales were irregular because of the rotation with respect to the fictitious ‘mean sun’, military term, ‘Greenwich Civil Time’ was of the Earth. Various atomic scales were which is at the centre of the Earth’s orbit, objected to by the British Admiralty. invented, but two were considered as the and which ‘appears’ to travel around the Because of this objection, the term GCT main time-scales. These are International Earth’s equator. These two ‘conditions’ never got established, and because the Atomic Time (TAI), a laboratory time-scale create what is known as the Equation of general public didn’t know otherwise, GMT with no perceivable step adjustments, Time, the wavy line which appears on continued in public use. which was started in 1958, in a manner that many, if not all, sundials. Because of the objection to GCT, the 1958 January 1, 00 hours 00 minutes TAI In 1675, this ‘mean time’ and the Royal Observatory left the naming of equalled that of UT2; and Co-ordinated Earth’s rotation were considered constant. ‘Greenwich civil time’ to the International Universal Time (UTC) which was started at As this time was ‘invented’ by Greenwich Astronomical Union (IAU), which the beginning of 1960. astronomers for, and only for, Greenwich eventually adopted the name ‘Universal From 1960 to 1971, inclusive, UTC astronomy, and called ‘Greenwich Mean Time’ (UT) in 1928. was ‘stretched’ to try to match the Earth’s Time’ (GMT) it was measured from rotation. The UTC second was at first, 15 Greenwich mean mid-day. The practice of nanoseconds longer than the TAI second. It calling mid-day 00 h 00 m GMT, was to THE SPLITTING OF was then changed to 13 ns longer in 1962. avoid changing dates while observing. UNIVERSAL TIME (UT). Two further changes occurred in 1964 and Throughout the middle of the 20th 1966, when the UTC second reverted to 15 However, as the United Kingdom century, it became increasingly noticeable and then set at 30 ns longer respectively. began to use the Greenwich meridian as that, amongst other things, UT was different However, as synchronisation was its standard meridian, the term GMT was if observed from different locations. needed all the more, UTC was altered in ‘hijacked’ to mean ‘Greenwich civil time’, Therefore, Universal Time had to be split 1972, to having a second equal to the TAI 12 hours ahead of original (and proper) into three main component scales. These second, and ‘leap seconds’ were introduced. GMT. As the UK officially adopted the scales are named UT0, UT1 and UT2, and These are inserted whenever necessary, Greenwich meridian in 1880, followed by deviate from each other by up to 50 usually every 12 to 36 months. There has the world adopting the meridian as the milliseconds, a noticeable difference given not been any need, yet, to delete any ‘leap prime meridian (0º) in 1884, the term today’s technology. These 3 scales, which seconds’. UTC is now over ½ minute GMT was increasingly misused and have been split again, to create UT0R, behind TAI. The difference is always an abused from that date. UT1R and UT2R, are not at all constant. Of integral number of seconds. UTC is the The practice of having two GMTs (12 all these six scales, UT1 is the most system of Greenwich ‘civil time’, currently hours apart) was increasingly annoying important. [2010] in use. for astronomers, so in 1925 old (and ... and that, is a brief history of time..