WWVB Receivers and UTC Timescale
Bob Foxworth
One reason we got the WWVB receiver is to obtain a data time stream
that we propose encoding on a UDP packet to transmit over our local
ethernet system to time-sync over 50 control system micros in our
two accelerator rings at the NSLS at BNL. The order of magnitude
accuracy (1 ms.) is not that important. Relative drift and dispersion
over time, between the different micros here is more important. This
is a proposed use now being developed. The GPS system may have been a
better alternative, I don't know exactly what data is available on
the serial output. Does it show local time-of-day with DST and
leapsecond corrections like the Spectracom does?
Posters ask why Coordinated Universal Time is NOT called "CUT". I
am paraphrasing part of RFC-1305 "Network Time Protocol" (this
appears on page 76 of the postscript version) by David Mills of
the Univ. of Delaware:
E.7. Determination of Time and Leap Seconds
The Internation Bureau of Weights and Measures (IBWM) uses
astronomical observations provided by the U.S. Naval Observatory
and other observatories to determine UTC. Starting from
apparent mean solar time as observed, the UT0 timescale is
determined using corrections for Earth orbit and inclination
(The Equation of Time as used by sundials)., The UT1 (navigator's)
timescale by adding corrections for polar migration and the UT2
timescale by adding corrections for known periodicity variations.
While standard frequencies are based on TAI, conventional
civil time is based on UT1, which is presently slowing relative
to TAI by a fraction of a second per year. When the magnitude of
correction approaches 0.7 second, a leap second is inserted or
deleted in the TAI timescale on the last day of June or
December.
I hope it is apparent why UTC (as an arrangement of letters)
are ordered the way that they happen. This text is part of an
excellent, short history of Calendars, the origin of the Julian
Date, why the Leap Second, etc. 73.
BNL is not a party to nor responsible for these remarks
Bradley Grigor
> E.7. Determination of Time and Leap Seconds
> The Internation Bureau of Weights and Measures (IBWM) uses
> astronomical observations provided by the U.S. Naval Observatory
> and other observatories to determine UTC. Starting from
> apparent mean solar time as observed, the UT0 timescale is
[...]
> to TAI by a fraction of a second per year. When the magnitude of
> correction approaches 0.7 second, a leap second is inserted or
> deleted in the TAI timescale on the last day of June or
> December.
>I hope it is apparent why UTC (as an arrangement of letters)
>are ordered the way that they happen. This text is part of an
>excellent, short history of Calendars, the origin of the Julian
>Date, why the Leap Second, etc. 73.
The implication from the quoted passage is the 'C' is a form of
subscript appended to "UT", just like the '0', '1' and '2' are
for the other base time scales. I concur and my other sources
(e.g. the Observer's Handbook, Royal Astron. Soc. of Canada)
support the same interpretation. However, the OH 1993 pg. 19
says that UTC is maintained within +/- 0.9 s of UT1:
"...UTC runs at the SI [International Standard] rate and is
offset by an integral number of seconds from TAI so that it
approximates UT1. When required ... leap seconds are added
so that the difference UT1 - UTC == deltaUT1 does not exceed
+/- 0.9 s."
While it matters very little on a gross scale, I would have
expected better consistency between our references on such a
topic of minutae. :-)
-bag Holland Landing, Ontario, Canada Email: bradley...@canrem.com
---
ş DeLuxeı 1.26b #4613 ş You have no tail... I have no 9«!
Peter Whisker
>Subject: UTC Time Scale
>From: bradley...@canrem.com (Bradley Grigor)
>Date: Sun, 10 Oct 93 19:37:00 -0400
>Organization: CRS Online (Toronto, Ontario)
Can anyone give a *DEFINITIVE* statement on UT0, UT1, UTC, GMT? So that we all
know once and for all?
Thanks hopefully
Peter Whisker
~~~~~~~~~~~~~~~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Peter Whisker | Internet: Whis...@lgwct.logica.com
Logica DCG Ltd, | X400 : WhiskerP/O=LG/OU=LGWCT/P=LOGICA/A=TMAILUK/C=GB
Cobham, Surrey | "Opinions are mine, not Logica's"
Great Britain | "B'shin tuairim phearsanta, nach leargas Logica"
~~~~~~~~~~~~~~~~~~~'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dave Tholen
> Can anyone give a *DEFINITIVE* statement on UT0, UT1, UTC, GMT? So that we
> all know once and for all?
The recent information here has been accurate.
Richard Langley
>
I posted the following article on UT to the Internet sometime ago. Seems
like it's worth a re-post.
A Few Facts Concerning RGO, GMT, and UT
---------------------------------------
Richard B. Langley
Geodetic Research Laboratory
Dept. of Surveying Engineering
University of New Brunswick
Fredericton, N.B., Canada E3B 5A3
E-mail: la...@unb.ca
(original version: 3 February 1990; revised: 6 July 1993)
In answer to the question "Does anyone know the exact difference between
GMT and UTC?" here are a few facts concerning the Royal Greenwich
Observatory, Greenwich Mean Time, and Universal Time.
o Prior to 1948, the observatory at Greenwich (located on a hill back
from the Thames River 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 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 0:00 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 cling to the pre-1925 definition of GMT
although it is recommended that the term Greenwich Mean Astronomical
Time be used to refer to time reckoned from noon.)
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.
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. 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. 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. 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 the nominal day.
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.
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. So in astronavigation, GMT can imply
UT1, but in general communications (as it is used by shortwave
broadcasters for example) GMT usually means UTC.
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 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 six pips are
synchronized to UTC by using signals from the Navstar Global
Positioning System (GPS) picked up by a receiver atop one of the BBC's
buildings in London.
o In March 1990, RGO officially moved from Herstmonceux Castle to the
grounds of Cambridge University's Institute of Astronomy. A laser
ranging station and a GPS tracking station still operate at
Herstmonceux but the castle itself has been sold -- to a Canadian
university, I think.
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 published
in 1980 by the Oxford University Press. Although the book is out of
print, you may be able to find it in your public library.
===========================================================================
===
Richard B. Langley Internet: LA...@UNB.CA or
S...@UNB.CA
Geodetic Research Laboratory BITnet: LANG@UNB or SE@UNB
Dept. of Surveying Engineering Phone: (506) 453-5142
University of New Brunswick FAX: (506) 453-4943
Fredericton, N.B., Canada E3B 5A3 Telex: 014-46202
===========================================================================
===
Scott Wills
Reading all this debate about the significance of the "leap second" makes
me wonder:
We add a leap day every four years, not because the earth has slowed to
the point where we have gained an extra 24 hours, but because the year is
actually 365 1/4 days long. Might we also add a second every once in a
while, not because the earth is slower, but because the second is a
somewhat arbitrarily/imprecisely defined unit, and not equal to
1/(60*60*24) of a day?
Of course, I am sure that the earth is in fact slowing down (due to the
tides or whatever) but I would tend to believe this has a more sublte
effect on timekeeping than good old human error.
Note that this is just speculation and I have no sources to back me up; I
would welcome a rebuttal from someone w/ more expertise or resources than I.
At least I'm not making a guess as to the letter-order of "UTC"!
-Scott
peace
--
The opinions expressed are not necessarily those of the University of
North Carolina at Chapel Hill, the Campus Office for Information
Technology, or the Experimental Bulletin Board Service.
internet: laUNChpad.unc.edu or 152.2.22.80
John R. Grout
>Hello,
>Reading all this debate about the significance of the "leap second" makes
>me wonder:
>We add a leap day every four years, not because the earth has slowed to
>the point where we have gained an extra 24 hours, but because the year is
>actually 365 1/4 days long.
Actually, we don't... that's the Julian calendar (in which the average
year is given 365.25 days). The Gregorian calendar (which most of the
world has been on for centuries) adds a leap day every four years
_except_ in century-ending years not divisble by 400... by the way,
decades end in years ending in 0, centuries in 00, millenia in 000...
so, 1700, 1800 and 1900 were not leap years in the Gregorian calendar,
but 2000 will be. The Gregorian calendar gives an average year
365.2425 days... which is closer to the 365.2422(?) days which is the
observed length of the mean solar year.
>Might we also add a second every once in a
>while, not because the earth is slower, but because the second is a
>somewhat arbitrarily/imprecisely defined unit, and not equal to
>1/(60*60*24) of a day?
I'm not quite sure what this means... either you are saying the same
thing in two different ways (when the length of a solar day isn't
86,400 seconds, add a second to get us back when things are far enough
out of whack) or you are confusing solar _years_ (how long it takes
the Earth to go around the sun) and solar _days_ (how long it takes
the Earth to turn on its axis)... there's no question that seconds are
reckoned from solar days... and there's no reason why a solar year
should be expected to be an even multiple of solar days, or to use
leap _seconds_ to deal with the length of the solar year... that's
what the Gregorian calendar is for.
--
John R. Grout j-g...@uiuc.edu
Center for Supercomputing Research and Development
Coordinated Science Laboratory University of Illinois at Urbana-Champaign
William L. Sebok
>We add a leap day every four years, not because the earth has slowed to
>the point where we have gained an extra 24 hours, but because the year is
>actually 365 1/4 days long. Might we also add a second every once in a
>while, not because the earth is slower, but because the second is a
>somewhat arbitrarily/imprecisely defined unit, and not equal to
>1/(60*60*24) of a day?
As I recall, the second was defined as being 1/86400th of the day of Jan 1,
1900 (if it was not that date, some date around then). The earth has slowed
since then and is still slowing further.
Now the second is defined in terms of some number of atomic oscillations
(cesium? or is it now krypton? I haven't quite kept up-to-date).
--
Bill Sebok Univ. of Maryland, Astronomy Internet: w...@astro.umd.edu
John Dundas
(P. Kenneth Seidelmann, ed., 1992) [p.40]:
"The fundamental unit of atomic time is the Systeme International (SI)
second. It is defined as the duration of 9,192,631,770 periods of the
radiation corresponding to the transition between two hyperfine levels
of the ground state of the cesium-133 atom."
This is an excellent source for clearing up some of the other questions that
have been discussed as well.
John C Sager
> Hello,
> Reading all this debate about the significance of the "leap second" makes
> me wonder:
> We add a leap day every four years, not because the earth has slowed to
> the point where we have gained an extra 24 hours, but because the year is
> actually 365 1/4 days long. Might we also add a second every once in a
> while, not because the earth is slower, but because the second is a
> somewhat arbitrarily/imprecisely defined unit, and not equal to
> 1/(60*60*24) of a day?
The leap year stuff is because the rotation period of the Earth & the
period of its orbit round the sun are not simply related, but we like to
use integers, hence the 365/366 stuff.
The leap second stuff is because the Earth's rotation period itself has
changed & is still changing. It's got nothing to do with the ratio between
two periods not being a simple rational fraction.
The (365 day) year is 365*24*3600 seconds, where the second is defined
in terms of a microwave spectral line of Caesium. The value used just
happens also to be 1/(365*24*3600) of the length of the year 1900, because
that was how the second was originally defined. Because the earth has slowed
down, the period between similar celestial measurements has got slightly
longer.
As an example, consider going out at midnight on December 31st to one of
the measuring telescopes at Greenwich and measuring the crossing time of
a particular star at the telescope crosshairs. If you then went back
one year later & did the same thing, the star would cross fractions of a
second later. If you had done this in 1900, then it would have crossed
at exactly the same time.
If we let this carry on, then these crossings would get progressively
later & later, so sunrise, etc would start getting later. The solution is
to add a leap second every so often, so the star crossing time stays
within 1 second of where it should be. Also, the rate of slowing down
of the Earth changes, so one cannot predict too far in advance when
to add leap seconds. It's certainly not a fixed rule like leap years,
in fact leap second was probably the wrong term to use because it seems
to cause plenty of confusion - hence your question.
John C Sager Mail: B67 G18, BT Labs
Email: j...@zoo.bt.co.uk Martlesham Heath
Tel: +44 473 642623 IPSWICH IP5 7RE
Fax: +44 473 637614 England
Disclaimer: This is me, not BT.
Dave Tholen
> Might we also add a second every once in a
> while, not because the earth is slower, but because the second is a
> somewhat arbitrarily/imprecisely defined unit, and not equal to
> 1/(60*60*24) of a day?
The second is a very precisely defined unit. I don't have the number
handy, but it's an exact number of cycles between two hyperfine levels
of some isotope of cesium. What is imprecise is 1/86,400 of an Earth
rotation. Because of tides, the rotation period is variable, so to
keep the atomic clocks in step with the Earth, leap seconds are inserted
from time to time (or they could be removed, if the Earth were to
temporarily speed up).
Before the days of atomic clocks, the second was defined as the
ephemeris second, which was determined from the motions of celestial
bodies, notably the Moon. The orbit of Mars, for example, has nothing
to do with the Earth's rotation, so gravitational theories require an
independent variable (time) that is not tied to the Earth's rotation,
which gives us the concept of the ephemeris second.