A Few Facts Concerning GMT [and] UT…

(excerpts)
(original document by Richard B. Langley
Geodetic Research Laboratory
Dept. of Geodesy and Geomatics Engineering
University of New Brunswick
Fredericton, N.B., Canada E3B 5A3
original version: 3 February 1990; this version: 20 January 1999)

Here are a few facts concerning Greenwich Mean Time [and] Universal Time. Various versions of this document have been posted to Usenet newsgroups and made available on Web sites over the years under the original title “A Few Facts Concerning RGO, GMT, and UT”. The current title has been in use since the 23 December 1995 version.

Greenwich Mean Time

• Greenwich Mean Time is a time scale based on the apparent motion of the “mean” sun with respect to the meridian through the Old Greenwich Observatory (zero° longitude). The “mean” sun is used because time based on the actual or true apparent motion of the sun doesn’t “tick” at a constant rate. The earth’s orbit is slightly eccentric and the plane of the earth’s orbit is inclined with respect to the equator (about 23½°) hence at different times of the year the sun appears to move faster or slower in the sky. That’s why an uncorrected sundial can be “wrong” (if it is supposed to be telling mean time) by up to 16 minutes. So if the mean (i.e. corrected) sun is directly over the meridian through Greenwich, it is exactly 12 noon GMT or 1200 GMT.

Universal Time

• In 1928, the International Astronomical Union recommended that the time used in the compilation of astronomical almanacs, essentially GMT, be referred to as Universal Time (UT). The term “Universal Time” [had been] introduced at the various conferences in the 1800s held to set up the standard time system [it makes for an interesting back story—essentially the proliferation of railroads was the driving force towards the adoption of standard time]
• There are actually a couple of variants of UT. UT as determined by actual astronomical observations at a particular observatory is known as UTØ (“UT-zero”). It is affected by the motion of the earth’s rotation pole with respect to the crust of the earth. If UTØ is corrected for this effect, we get UT1… In an effort to derive a more uniform time scale, scientists established UT2. UT2 is obtained from UT1 by applying an adopted formula that approximates the seasonal oscillations in the earth’s rotation. UT2 is also not a uniform time scale.
• So rather than base our civil time keeping on the rotation of the earth we now use Atomic Time, time based on the extremely constant frequency of a radio emission from cesium atoms when they change between two particular energy states. The time scale based on the atomic second but corrected every now and again to keep it in approximate sync with the earth’s rotation is known as UTC or Coordinated Universal Time [see below The Origin of UTC].
• In 1928, when the term Universal Time was introduced, variations in the earth’s spin were not yet known, so the term GMT was, in essence, replaced by UT1. GMT now usually means the civil (atomic-second-based) time kept in the United Kingdom which is the standard time of the time zone centred on the Ø° meridian. In this (the most common) usage, the terms GMT and UTC are identical. But because there are two possible meanings for GMT differing by up to 0.9 seconds, the term GMT should not be used for precise purposes—particularly not in reference to GPS observations!

The Origin of UTC

• The concept of a coordinated universal time was introduced in 1960 when the British and American national time services initiated a program to coordinate the offsets of the frequencies and epochs (phases) of transmitted time service radio signals from Atomic Time in approximating UT2. Subsequently, other national time services joined the program. The BIH [formerly the Bureau International de I’Heure, now the Bureau International des Poids et Mesures in Sèvres, France] was charged with the task of monitoring and maintaining the program and introduced the term Temps Universel Coordinné or Coordinated Universal Time for the coordinated time scale in 1964.
• Sometimes the term “World Time” is used to denote UTC. This strange and potentially confusing term (“UTC for dummies”?) should be avoided. Similarly, there is no clear need for the Swatch watch company’s recently introduced “Internet Time” (Central European Time measured in 1/1000 of a day (a “beat”)).

LRod’s Interpretation

Nowhere in the foregoing is there a direct correlation by order between the initials “UTC” and the French words which make it up, nor have I been able to find anything substantive on the interweb which supports this opinion. However, I believe that since the term “UT” predates the establishment of “UTC” by more than 30 years, and possibly before France became the center of time (and the highly parochial guardian of the French language) it may be that Universel Temps might have been the acceptable French version of “UT”. Adding “coordinné” when UTC was officially introduced in 1960 might have been the step that gave us UTC, despite that pure French doesn’t support that arrangement of the initials. If any Francophone has a better (and supportable) opinion, I’d love to hear about it.

Last updated: 02 February 2010