Date: Tue, 5 Jul 2005 13:50:31 +0200
From: "services.iers {at} obspm.fr" <services.iers {at} obspm.fr>
To: adresd1 {at} callisto.obspm.fr
Subject: Note to IERS Users

July 2005
Letter to IERS Bulletins C and D users

Dear colleague,

As you are probably aware, international discussions are in progress on a redefinition of UTC, including a possible suppression of the leap seconds procedure.

To reply to the request of several international organizations on a possible evolution of UTC the International Telecommunication Union, ITU decided in October 2000 the creation of a Working Group called "Special Rapporteur Group", SRG chaired by Mr Ron Beard of the Naval Research Laboratory (NRL-USA), in charge of studying by a large consultation the different possible solutions. The SRG reports its conclusions to the ITU working party WP-7A.

The Special Rapporteur Group (SRG) organized a colloquium in Torino, Italy in May 2003. No general agreement emerged from this colloquium.

In parallel, the US delegation to the WP-7A submitted, at the last meeting in Geneva in October 2004, a proposal which contained the following items:

1 - Maintenance of a time scale called UTC.

2 - Suppression of the leap seconds adjustments which maintains UTC close to UT1, a time scale based on the Earth's rotation (currently UT1-UTC < .9 s)

3 - The difference of UT1 from UTC should not exceed 1 hour.

4 - The change should take effect at 21 December 2007, 00:00 UTC

If your activity is affected by the content of the US proposal which will be discussed in November 2005 at the WP-7A, you are urged to react. This could be the last opportunity before a recommendation is issued by the WP-7A.

If you wish you can express your opinion to your representative(s) at the WP-7A of ITU (for the list see the ITU website, http://www.itu.int/home/index.html ) with a copy to Daniel Gambis
daniel.gambis {at} obspm.fr , IERS EOP Center.

Daniel Gambis
Head
Earth Orientation Center of the IERS
Observatoire de Paris

_________________________________________________________________

…. just a second!

While the concept of a Sun-Sirius orbit is of such gravity that it goes beyond the ordinary world of modern astronomy, the measurements of Sirius’ transit periods are an observed scientific fact. From April 1994 to April 2000 team members of the Sirius Research Group have determined that the mean transit period of Sirius (i.e. Earth’s mean spin period relative to Sirius) is 86164.0924 seconds (UTC). It is argued, that the result of such data cannot be explained by conventional “lunisolar precession” dynamics.

Interestingly enough, in 2001 several renowned experts (R. A. Nelson, D.D. McCarthy et al) wrote together an Abstract in ‘Metrologia’, entitled “The leap second: its history and possible future”.
http://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf

On page 511 under section 3 “Time measured by the rotation of the Earth”, sub-section 3.2 “Sidereal Time”, we can read that

“The sidereal day is the time interval between two successive transits of the vernal equinox. It represents the Earth’s period of rotation relative to the stars and is approximately 86 164.0905 mean solar seconds. Owing to precession of the Earth’s axis with respect to the celestial reference system, the sidereal day is about 0.0084 s shorter than the actual period in inertial space. Thus the true rotational period of the Earth is approximately 86 164.0989 mean solar seconds. However, the mean solar day presently exceeds a day of exactly 8[6]400 SI seconds by about by about 2.5 ms. Therefore, the Earth’s period of rotation is currently about 86 164.1014 SI seconds.”

Does this imply that at the time the ‘mean sidereal day’ (equinoctial day) was about 86 164.093 SI seconds? In other words, do observations confirm the fact that the mean sidereal day as measured relative to Sirius is ONLY about 0.6 milliseconds shorter than the defined time interval of the equinoctial or mean sidereal day, the primary astrodynamical constant?!

In order to understand the major ramifications of this issue, we need to know what the most fundamental basis of the physical unit ‘Second’ is in reality. Only then can we be certain that our reckoning and measurement of time on Earth does NOT depend upon the presumed gravitational influence of the Moon on our planet’s equatorial bulge.

Historically, in science the unit 'second' has been derived from Earth's very stable solar orbit period and no longer from Earth's fluctuating daily rotation period. Nowadays atomic clocks took over our "reckoning of time", but contrary to popular scientific belief, modern leap-seconds are actually required to synchronize the defined and nearly constant tropical or equinoctial time span with the extremely stable but (to equinoctial) non-resonant atomic time scale. As a matter of fact, the frequency of the ‘atomic second’ is about 3×10E-8 seconds shorter than the SI second, which implies that a leap second would be required about once a year. Hence, the atomic time scale is not directly synchronized to Earth's mean rotation period relative to the sun, which in turn is based on the motion of a hypothetical sun moving 360 degrees at an even rate around a 'geocentric' Earth in one tropical year.

Yet it appears that astronomers and physicists at the IERS (International Earth Rotation Service) are going "back in time", using once again - in spite of Simon Newcomb's great scientific accomplishment - the apparently irregular spin period of the Earth to determine its orientation parameters relative to inertial space. Torn between rotation and revolution, astronomers always wanted to decouple atomic time from Earth’s orbital motion and the equinox. According to a new proposal, the IERS intends to abolish the leap second altogether in favor of a "leap-hour" to be introduced supposedly some several thousand years from now. Judging from the intense emotions that usually occur in a NASA/JPL control room during missions in crisis, it would take extraordinary faith in the “God of Science” to accomplish such a task.

Theoretically, if the unit 'second' and Earth's rotational velocity remains stable, then the length of the mean solar day must change if any significant variations were to occur to the length of Earth's orbital path or to the mean orbital velocity of the Earth. Also, any changes to the number of rotations per orbit or the duration of the solar year will directly affect the length and the measurement of the mean solar day. In other words, one cannot expect that a stable rotational velocity of the Earth in combination with a different mean orbital velocity or a different orbital period (year) would cause the duration of the mean solar day to remain the same.
Even if Earth’s rotation has been slowing down for millennia, the length of the tropical year has not changed significantly. Thus the Earth could stop spinning and it still takes about 31 556 925.97 seconds to make a complete orbit from one vernal equinox to the next.

It should be noted that in 1999 experts have discovered that after 28 years of leap second insertions the Earth is apparently no longer slowing down in its orbit. In 2004 the Associated Press reported that "At the National Institute for Science and Technology in Boulder, spokesman Fred McGehan said most scientists agree the Earth's orbit around the sun has been gradually slowing for millennia. But he said they don't have a good explanation for why it's suddenly on schedule."

In practice, the mean solar day is derived from the exact time period of Earth's 360-degree orbit around the Sun. That is because in reality it takes the Earth 86164.0905382 seconds (mean sidereal day) to make a complete (absolute or 360-degree) rotation on its axis, regardless of its orbital speed around the Sun. Again, any major changes in the length or period of that orbit would result in a notable change in the duration of the mean solar day, but not in the mean sidereal day!

The 'second', Earth's absolute rotation on its axis and the defined time interval of the mean solar day of 86400 s are rigorously tied to the length of the equinoctial or tropical year, as the "Equation of Time" clearly proves. That is the real scientific reason why the tropical year is Earth's true 360-degree elliptical orbit period and NOT a roughly twenty minutes longer sidereal year.

If the latter case were to be true, it would have required a different definition of the physical unit 'second' in order to maintain the standard mean solar day of 86400 seconds. However, in 1955 the International Astronomical Union (IAU) passed the resolution to substitute the time interval of the tropical year for 1900.0 for the sidereal year for 1900.0 to define the unit 'second', as it had already been proposed in 1952. (see also http://www.journaloftheoretics.com/Articles/3-3/uwe.pdf ) The reason being, that the length of the tropical year is more fundamental than the length of the sidereal year, the latter depending upon the adopted value of the precession. The IAU experts had realized that the new standard of time ought to be based on the period of revolution of the Earth around the Sun, instead of the period of rotation of the Earth on its axis.

But the authors of the above mentioned abstract on the leap second noted on page 512, section 4.1: “Initially, the period of revolution of the Earth was understood to be the sidereal year.”
So thanks to our astronomers, who at first understood Earth’s period of revolution around the Sun to be something else, the time interval of the unit “second” was never allowed to depend on the mysterious actions of our Moon, which will continue to cause us many sleepless nights.

It is a shame that after more than 50 years of technological and scientific advancements no astronomer has been able to say what the exact length of the sidereal year for 1900.0 is in SI seconds.

This is really “Bad Astronomy”.

Uwe Homann

July 28, 2005