A Few Facts Concerning GMT, UT, and the RGO
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Richard B. Langley
Geodetic Research Laboratory
Dept. of Geodesy and Geomatics Engineering
University of New Brunswick
Fredericton, N.B., Canada E3B 5A3
E-mail: lang@unb.ca
(original version: 3 February 1990; this version: 29 December 2007)
In answer to the question "Does anyone know the exact difference between
GMT and UTC?" here are a few facts concerning Greenwich Mean Time,
Universal Time, and the Royal Greenwich Observatory. 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.
The Royal Greenwich Observatory
-------------------------------
o Prior to 1948, the observatory at Greenwich (located on a hill back
from the River Thames with a view of the London Docks) was known as
the Royal Observatory.
o In 1948, the observatory moved to Herstmonceux Castle in Sussex,
becoming the Royal Greenwich Observatory (yes, even though it wasn't
at Greenwich any more!).
o The site at Greenwich became known as the Old Greenwich Observatory
and the historic buildings and instruments were progressively
incorporated into the National Maritime Museum, the main buildings
of which are located at the foot of Observatory Hill, close to the
river. Highly recommended for a visit if you're in London!
o Following the closing of the RGO in the autumn of 1998, the Old
Greenwich Observatory was renamed the Royal Observatory Greenwich (see
"Where's the RGO Now?" below).
Greenwich Mean Time
-------------------
o 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 degrees 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 23-1/2 degrees) 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 12:00 GMT (Prior to 1925, astronomers reckoned mean solar time
from noon so that when the mean sun was on the meridian, it was
actually 00:00 GMT. This practice arose so that astronomers wouldn't
have a change in date during a night's observing. Some in the
astronomical community still use the pre-1925 definition of GMT in the
analysis of old data although it is recommended that the term
Greenwich Mean Astronomical Time now be used to refer to time reckoned
from noon.)
o There have been several different zero-degree meridians used at the
observatory over the years, each associated with a particular observing
instrument. The last of these is the one through the transit circle
established by Sir George Airy, the 7th Astronomer Royal, in 1851. It
is this meridian which is identified by a stainless steel strip in the
observatory courtyard. Interestingly, the reference meridian for the
Navstar Global Positioning System (GPS), consistent with the
International Reference Meridian, actually passes about 100 metres to
the east of the steel strip.
o Mean time on selected meridians 15 degrees apart is generally known as
standard time. For example, Eastern Standard Time (EST) is the mean
solar time of the meridian at 75 degrees W.
Universal Time
--------------
o In 1928, the International Astronomical Union recommended that the
time used in the compilation of astronomical almanacs, essentially
GMT, or what was also sometimes called Greenwich Civil Time, be
referred to as Universal Time (UT). The terms "Universal Time" and
"Universal Day" were introduced at the various conferences in the
1800's held to set up the standard time system.
o There are actually a couple of variants of UT. UT as determined by
actual astronomical observations at a particular observatory is known
as UT0 ("UT-zero"). It is affected by the motion of the earth's
rotation pole with respect to the crust of the earth. If UT0 is
corrected for this effect, we get UT1 which is a measure of the true
angular orientation of the earth in space. However, because the earth
does not spin at exactly a constant rate, UT1 is not a uniform time
scale. The variation in UT1 is dominated by seasonal oscillations due
primarily to the exchange of angular momentum between the atmosphere
and the solid earth and seasonal tides. 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. However, due to other
variations including those associated with the secular effects of
tidal friction (the earth's spin is continually but gradually slowing
down), high frequency tides and winds, and the exchange of angular
momentum between the earth's core and its shell, UT2 is also not a
uniform time scale.
o 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 unit of Atomic Time is the
atomic second. 86,400 atomic seconds define the length of a nominal
"reference" day -- the length of the day as given by the earth's
rotation around the year 1900. But because of the variations in the
earth's spin, the length of the actual day can be shorter or longer
than the nominal day of 86,400 seconds. 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. The corrections show up as the leap
seconds put into UTC from time to time - usually on New Year's Eve.
With these leap second adjustments, UTC is kept within 0.9 seconds of
UT1. Currently, the need for leap seconds is primarily due to the
effects of tidal friction. The earth's rotation in space is monitored
by the International Earth Rotation and Reference Systems Service
(IERS) now headquartered in Frankfurt am Main, Germany, using a global
network of satellite and lunar laser ranging, very long baseline
interferometry, and GPS stations. The IERS, in consultation with the
Bureau International des Poids et Mesures in Sèvres, France,
determine when a leap second is needed.
o 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. Despite the official adoption of the term UT, the
navigational publications of English-speaking countries retained the
term GMT as a synonym for UT1 for some time. So, even today, in
astronavigation, GMT can imply UT1. But in general usage (including
that of shortwave broadcasters such as the BBC, for example), 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 0
degree 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
-----------------
o 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 Bureau International de l'Heure (a forerunner of the
IERS) 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.
Initially, the time scale was derived by offsetting its rate from that
of Atomic Time to agree with the average rate of UT2 over the past
year and was held fixed at that rate for the following year. If the
rate of UT2 changed significantly during the year, then an offset
(from 1962, in multiples of 100 milliseconds) could be introduced on
the first day of a month. This system of frequency and epoch offsets
was continued until 1972 when the current practice was adopted of
keeping the rate of UTC equal to that of Atomic Time and introducing
leap seconds when needed to keep UTC to within 0.9 seconds (it was 0.7
seconds until January 1975) of UT1.
o There is ongoing discussion about terminating the practice of adding
leap seconds to UTC. Those in favour of terminating the practice argue
that adding leap seconds causes potential difficulties for many
applications requiring precise civil time stamps and that there is no
longer sufficient demand for keeping UTC close to UT1 so that it can be
used as a proxy in navigation and astronomy applications.
o 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")).
GMT and the BBC
---------------
o The BBC began transmitting time signals in 1924. The chimes of Big
Ben were first broadcast at midnight beginning 1 January and on 5
February, at the recommendation of the then Astronomer Royal, Frank
Dyson, the six pips time signal (officially known as the Greenwich
Time Signal) was inaugurated.
o Control of the BBC's six pips was taken over by the Royal Observatory
in 1949 from Abinger to where the time service had moved during the
war. The time service moved to Herstmonceux in 1957.
o The time service at Herstmonceux closed down during February 1990 when
the BBC took over the generation of the six pips. The old Greenwich
Time Service control panel is now on display at the Science Museum in
Kensington, London. Since 5 February 1990, the 66th anniversary of the
start of the Greenwich Time Service, the six pips have been
synchronised to UTC by using the GPS satellite signals which are picked
up by a pair of GPS receivers atop Broadcasting House in London.
o The actual time reference is the beginning of the sixth pip. The
accuracy of the received signal from analogue radio transmitters in the
U.K. is around a few thousandths of a second with the most accurate
reception being from the BBC Radio 4 Droitwich transmitter on 198 kHz
at a distance of about 160 km from the transmitter. As the audio from a
Digital Audio Broadcasting (DAB) receiver is delayed by about two
seconds or more, the six-pip signal received via the BBC's DAB stations
is not accurate. And due to buffering and other factors, the delay of
the signal on the BBC's Internet-delivered streamed radio channels can
be 10 seconds or more.
Where's the RGO Now?
--------------------
o In March 1990, RGO officially moved from Herstmonceux Castle to the
grounds of Cambridge University's Institute of Astronomy. On 31
October 1998, the RGO was closed by the U.K. Particle Physics and
Astronomy Research Council as a cost-saving measure. Some of its
research activities have been transferred to the Royal Observatory
Edinburgh. Her Majesty's Nautical Almanac Office (HMNAO) was
transferred to the Rutherford Appleton Laboratory at Chilton in
Oxfordshire. (In December 2007, part of the HMNAO infrastructure was
transferred to the U.K. Hydrographic Office in Taunton, Somerset.)
With the closure of the RGO, the Old Greenwich Observatory has been
renamed the Royal Observatory Greenwich. A laser ranging station and a
GPS tracking station still operate at Herstmonceux but the castle and
estate is now owned by Queen's University in Kingston, Ontario,
Canada, who use it as a satellite campus for their International Study
Centre. Queen's purchased the castle in early 1993 for about $8
million (CDN). This money, and an additional $4 million for
renovations were gifts from Dr. Alfred and Mrs. Isabel Bader of
Milwaukee, WI. Dr. Bader is a Queen's alumnus.
To Learn More
-------------
o If you'd like to learn more about time you might look for the book
"Greenwich Time and the Discovery of Longitude" by Derek Howse
originally published in 1980 by the Oxford University Press. A second
edition, titled "Greenwich Time and the Longitude: Official Millennium
Guide" was published by the National Maritime Museum and Philip Wilson
Publishers in 1997 (ISBN 0-85667-468-0). A special paperback edition
is available exclusively from the museum. An excellent reference on
all matters concerning time is the "Explanatory Supplement to the
Astronomical Almanac" edited by P. Kenneth Seidelmann of the U.S.
Naval Observatory (USNO) and published by University Science Books,
Mill Valley, CA (ISBN 0-935702-68-7). There is also a wealth of
information on time at USNO's Directorate of Time Web site
<http://tycho.usno.navy.mil/time.html>. For information on Queen's
University's International Study Centre at Herstmonceux Castle, visit
their Web site <http://www.queensu.ca/isc/>. A GPS World article
discussing the future of the leap second can be found here:
<http://gauss.gge.unb.ca/papers.pdf/gpsworld.november99.pdf>.