[HCRA] FW: Synchronizing atomic clocks with the Earth's varying rotational speed: the "

Tue Dec 27 09:48:00 EST 2005

----Original Message Follows----
From: Richard Rucker <rrucker at verizon.net>
To: Ch91 outreach
Subject: Synchronizing atomic clocks with the Earth's varying rotational 
speed: the "Leap Seconds" debate
Date: Mon, 26 Dec 2005 12:00:47 -0500

On a recent Sunday morning net on the 146.79 repeater, John Reiser,  WQ4L, 
suggested that a future topic for discussion be the "leap  second" now used 
to synchronize the time kept by atomic clocks with  clock time based on the 
time-varying rotational speed of planet Earth.

In today's Washington Post (Monday, 12/26/05, page A17), there is a  good 
explanation of why the difference between the two ways of  keeping clock 
time occurs and why there is now a vigorous debate over  whether it is good 
idea to continue to try to keep time kept by  atomic clocks in synch with 
the vagaries of the Earth's rotation.

According to the article: "Last month, a working group of the  [ITU]... 
decided to postpone discussions of [a] U.S. proposal which  would have 
abandoned leap seconds in 2007 and let Coordinated  Universal Time (UTC) and 
astronomical time diverge for several  hundred years before inserting a 
"leap hour" [to re-synch them].  The  working group "suggested that this 
year's leap second offered a  welcome opportunity to determine whether the 
change is necessary;"  that is, whether the problems alleged to result from 
the current  synchronization scheme are important enough to break the chain 
of  practices that either support it or are dependent upon it.

In googling around this morning, I came across an email written in  late 
1998 by Tom Clark, W3IWI, a well-known opinion-leader within  AMSAT. I found 
it quite interesting, and thought you might also.

Dick Rucker, KM4ML

=====================

1. "Temps Atomique International (TAI), or 'International Atomic  Time,' is 
a weighted average of the time kept by about 300 atomic  clocks, including a 
large number of cesium atomic clocks, in over 50  national laboratories 
worldwide. It has been available since 1955.   On January 1, 1972, TAI 
became the international standard on which  UTC is based."

2. "Coordinated Universal Time (UTC), also sometimes referred to as  'Zulu 
time (Z), is an atomic realization of 'Universal Time (UT),' or  'Greenwich 
Mean Time,' the astronomical basis for civil time."

3. On the other hand, according to W3ITI (see below), “UT1” is based  on the 
actual rotational speed of Earth.

4. "Time zones around the world are expressed as positive and  negative 
offsets from UT. UTC differs by an integral number of  seconds from 
International Atomic Time (TAI), as measured by atomic  clocks and a 
fractional number of seconds from UT."

5. “The International Earth Rotation and Reference Systems Service  (IERS) 
is the body responsible for maintaining global time and  reference frame 
standards, notably through its Earth Orientation  Parameter (EOP) and 
International Celestial Reference System (ICRS)  groups.  Among its other 
functions, the IERS is responsible for  announcing leap seconds.”

All but item 3 were copied from:
   http://en.wikipedia.org/wiki/International_Atomic_Time:

======================
Now, Tom Clark’s email:

Date: Thu, 01 Oct 1998
From: "Dr Thomas A Clark (W3IWI)" <clark at tomcat.gsfc.nasa.gov>

To: TACGPS Mailing List <tacgps at tapr.org>
Subject: Leap Second Pending

... The next leap second will be 1999.0; i.e.. Dec.31, 1998 with an  added 
second named 23:59:60.

To explain what leap seconds are all about, the following is taken from

                ftp://hpiers.obspm.fr/iers/bul/bulc/BULLETINC.GUIDE:

"UTC is defined by the CCIR Recommendation 460-4 (1986). It differs  from 
TAI* by an integral number of seconds, in such a way that the  difference, 
(UT1-UTC), stays smaller than 0.9s in absolute value.

"The decision to introduce a leap second in UTC to meet this  condition is 
the responsibility of the IERS. According to the CCIR  Recommendation, first 
preference is given to the opportunities at the  end of December and June, 
and second preference to those at the end  of March and September. Since the 
system was introduced in 1972, only  dates in June and December have been 
used.

Since 1972, Leap Seconds have been added on these dates, on average  18 
months apart:
               added
               leap
date      seconds
--------  -------
1972 JUL  1
1973 JAN  1
1974 JAN  1
1975 JAN  1
1976 JAN  1
1977 JAN  1
1978 JAN  1
1979 JAN  1
1980 JAN  1 -- GPS-UTC=  0 sec, the zero point for GPS time
1981 JUL  1 -- GPS-UTC=  1 sec
1982 JUL  1 -- GPS-UTC=  2 sec
1983 JUL  1 -- GPS-UTC=  3 sec
1984
1985 JUL  1 -- GPS-UTC=  4 sec
1986
1987
1988 JAN  1 -- GPS-UTC=  5 sec
1989
1990 JAN  1 -- GPS-UTC=  6 sec
1991 JAN  1 -- GPS-UTC=  7 sec
1992 JUL  1 -- GPS-UTC=  8 sec
1993 JUL  1 -- GPS-UTC=  9 sec
1994 JUL  1 -- GPS-UTC= 10 sec
1995
1996 JAN  1 -- GPS-UTC= 11 sec
1997 JUL  1 -- GPS-UTC= 12 sec
1998
1999 JAN  1 -- GPS-UTC= 13 sec

The reason for Leap Seconds is that the RATE of UTC is defined as the  same 
as Atomic Time (AT); AT is defined by atomic frequency  standards, 
specifically the ~9.2 GHz Cesium hyperfine resonance.

But the earth does not rotate at a "perfect" (i.e. atomic) speed --  the 
length of a day is influenced by tidal drag from the moon  orbiting  the 
earth (over time scales like millennia), changes in the  rotation properties 
of the molten core of the earth (over decadal  time scales), seasonal 
re-distribution of mass on the surface of the  earth (like snow/ice cover, 
soil moisture, etc), and the angular  momentum  associated with atmospheric 
winds and ocean tides and some  other smaller effects.

All these geophysical effects, except for tidal dissipation, come  about 
because the rotating earth system must conserve angular  momentum. As the 
winds speed up, the earth slows down, and the same  applies to the other 
effects I listed.

As a recent example, last winter's El Nino caused the length of day  to 
increase by ~600 usec/day for ~100  days. By July 1 '98, this  summer's La 
Nina speeded up the earth by ~500 usec/day from the  nominal value -- i.e. a 
change from last winter to this summer of  about 1 msec/day!

Both our human existence (making the sun be overhead at local noon)  and 
scientific studies (testing ocean and atmosphere models) require  us to keep 
time related to the real rotation of the earth -- UT1.

   [My group here at NASA specializes in this type of research --  see  
http://lupus.gsfc.nasa.gov and http://cddisa.gsfc.nasa.gov for  some more 
details. We are now able to measure the length of a day to  a few usec, and 
to do this with ~1 hour time resolution using radio  telescopes observing 
quasars at the far fringes of the universe.]

AT and GPS time keep an unbroken record of time with no discontinuities.

UT1 follows the minute vagaries of the rotation of the earth  irrespective 
of AT.

UTC provides the bridge between AT & UT1 by defining a clock that  runs at 
the atomic time rate, and which has discrete jumps to keep  within ~1 second 
of the wanderings of the earth.

73, Tom