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\begin{document}
\title{Discussion at the IAU JD16}
\author{P.~Kenneth Seidelmann}
\affil{University of Virginia, Department of Astronomy, 
   Box 3818, Univ.~Station, Charlotteville, VA 22903}
\author{Norbert Zacharias}
\affil{U.S. Naval Observatory, 3450 Massachusetts Ave. NW, Washington DC, 20392-5420}

\begin{abstract}
The last session of the JD16 meeting was devoted to a general discussion.
A list of discussion items was presented.
Discussions of the implementation and IAU organization responsibilities 
for reference systems resulted.
The current organization of the IAU WG ICRS was extended for 6 months.

\end{abstract}

\section{Introduction}

  Following the scheduled presentations for Joint Discussion 16, there was a
discussion of open items. This was prepared and led by Ken Seidelmann. To
initiate the discussion, a list of items was presented for consideration. 
They appear as number items in outline form.  The discussion appears as 
paragraphs with the speakers identified, where possible.


\section{Items for Discussion}
\begin{enumerate}
\item Precession-nutation Model
  \begin{itemize}
  \item New precession Theory?
  \item What angles- Newcomb or Williams?
  \item What nutation model?
  \item An abbreviated nutation theory with less accuracy?
  \item Include geodesic precession \& nutation?
  \item Is it a BCRS or GCRS model?
  \end{itemize}

\item Future of Equinox
  \begin{itemize}
  \item Introduce Earth Rotation Angle?
  \item Revise definition of ERA?
  \item Dual system by IERS?
  \item Dual system in almanacs?
  \item Transition period specified?
  \item For how long? 2004? Indefinitely?
  \end{itemize}

\item Definition of equinox
  \begin{itemize}
  \item inertial or rotating
  \end{itemize}

\item Introduce Conventional ecliptic
  \begin{itemize}
  \item How defined, through x axis of ICRF?
  \item \hspace*{10mm} by node angle and obliquity?
  \item For what purpose?
  \item With what accuracy?
  \end{itemize}

\begin{flushleft}
{\bf Fr.Mignard} said that the ecliptic is moving and will change with time.
{\bf D.McCarthy} said this is a subject for the WG on precession. 
 The ecliptic is needed for planetary precession.
\end{flushleft}

\item Terminology issues
  \begin{itemize}
  \item CIO and TIO or CEO and TEO?
  \item Stellar angle or Earth Rotation Angle?
  \item right ascensions from equinox only?
  \item right ascensions from CIO?
  \item other terms?
  \end{itemize}

\begin{flushleft}
{\bf C.Hohenkerk} said some concepts seem odd, the user is not concerned 
"which" RA,Dec is used, as long as there is some RA,Dec.

{\bf After consideration} of the old meaning of CIO, it was generally felt that
the new use of CIO would not cause confusion. For consistency, the use of
"intermediate" in CIP, CIO, and TIO was desirable.
\end{flushleft}

\item Unification of Lists of Constants
  \begin{itemize}
  \item IAU Best Estimates
  \item IAU 1976 Astronomical Constants
  \item IERS Best Estimates
  \item IUGG List of Constants
  \item JPL DE 405 Constants
  \item Astronomical Almanac Constants Used
  \item Are all these necessary?
  \item A joint IAU/IUGG committee?
  \end{itemize}

\item Redefinition of UTC
  \begin{itemize}
  \item UTC tied to UT1?
  \item Use of "mean solar time"?
  \item Leap seconds or not?
  \end{itemize}

\item Implementation Issues
  \begin{itemize}
  \item Who are the users?
  \item What do users need?
  \item What is really used by the IERS?
  \item What is necessary for almanacs offices?
  \item Standardized software?
  \item Documentation required?
  \item Dual availability for how long?
  \item What to do now?
  \end{itemize}

\item Roles of Organizations
  \begin{itemize}
  \item WG on Reference systems of IAU Div I
  \item IERS
  \item IUGG
  \item IAU Comm 5
  \item Others involved?
  \end{itemize}

\item Possible Procedure
  \begin{itemize}
  \item IAU /IUGG? WG on Reference Systems with subgroups established now
  \item Dec 2004  WG proposals circulated
  \item Mid 2005  Colloquium for discussion of proposals and draft resolutions
  \item IAU 2006  Clarifying resolutions
  \item Note: Almanac offices have a March 2004 decision deadline
  \end{itemize}

\item Education Plan
  \begin{itemize}
  \item Clear and convincing presentation of reference systems and justifications
  \item Dissemination of information with proposals in Dec 2004
  \item Distribution of 2005 Colloquium Proceedings
  \item Wide distribution of any proposed resolutions well before IAU GA in 2006
  \end{itemize}

\end{enumerate}

\section{Discussion}

\subsection{General Discussion}

\begin{description}
\item[K.Johnston] said some issues might not be solved earlier than 2006.
\item[K.Seidelmann] asked do you want to introduce the new system or not? If so why
  wait another 3 years?
\item[P.Wallace] said that we need to get away from the "one or the other" concept;
  I can't imagine not to publish sidereal time in an Almanac. Algorithms are
  now available for both paradigms, the user can choose.
\item[K.Johnston] asked "What does the average user need?"
\item[Chr.Jacobs] said they need more education, let people know how complex they
  need to go for a given goal in accuracy.
\item[K.Johnston] said please do this for us, the user does not want to be
  involved.
\item[P.Wallace] replied there is a mis-conception that the "new paradigm = more
  complex". This is not true. The non-rotating origin procedure is simpler in
  concept; like spherical trigonometry and vectors, they are both there to choose.
\item[Someone] said that this is difficult at the moment, there is a need for
  education, currently we don't even know what to call things.
\item[K.Johnston] suggested that there is no need for the general astronomers to
  make any change, since at the accuracy they need it doesn't matter. It is
  only the IERS and space astrometry applications that are concerned about 
  sub-milliarcsecond accuracies and need to use the new more accurate system.
\item[It was pointed out] that this was true, but people needed to realize that the
  basic values were coming from new definitions and methods.

\end{description}


\vspace*{6mm}

\subsection{Division I Organization}

At this point there was a break in the discussion and T.Fukushima, President
of Division I, presented some details regarding the future structure of
Division I in the context of this discussion:

\begin{itemize}
\item The ICRS WG has been dissolved (it was too large, with too many
  sub-tasks). As a follow-up for two of the sub-tasks, two new WGs have 
  been established:

  \begin{enumerate}
  \item precession, chair is J.Hilton
  \item nomenclature in fundamental astronomy, chair is N.Capitaine
  \end{enumerate}

\item The new IAU structure puts more emphasis on the Divisions. Commissions
are now dynamic, with a finite life time.  It is possible to terminate
commissions and to create new commissions upon request by the Division and
approval by the EC. Working groups can be established by approval of the
Division, without EC contact.  The approval process is fast, and can be done
by e-mail, with no need to wait for a GA.

\item T.Fukushima initiated a "special committee" for the re-organization of
Division I.  Members are T.Fukushima (chair), Fr.Mignard, I. Platais, G.
Pettit, and K. Seidelmann.

\item For the 2003-2006 period (i.e. before the special committee issues their
  recommendations), the proposal for ICRS related issues is:

   \begin{itemize}
   \item Celestial Reference Frame related issues $\rightarrow$ direct to Commission  8
   \item Earth Rotation related issues $\rightarrow$ direct to Commission 19
   \item 2 new working groups (see above)
   \end{itemize}
\end{itemize}


\vspace*{6mm}

\subsection{Commissions 8 and 19}

\begin{description}
\item[C.Ma] asked what is meant by "directed to" Comm. 8 or 19 ?
\item[D.McCarthy] replied that the commissions take over the organizing role, 
  not the former ICRS WG.
\item[T.Fukushima] said that the ICRS WG activities are more permanent, as an
  intermediate step Comm. 8 and 19 take over, later the special committee will
  suggest a future organization.
\item[Someone] asked what exactly is for Comm.8, and what is for Comm. 19?
\item[K.Johnston] replied that most of it goes into Comm. 19.
\item[Chr.Jacobs] said that I have some ICRF issues, to whom shall I 
  send e-mail now?
\item[T.Fukushima] replied Comm. 8 and 19 presidents take over for now.
\item[B.van Altena] stated I am not directly involved in the ICRS matters, 
  but I would like to give some comments here as an "outsider". 
  Everybody needs to step back and re-think matters; what's happening is not 
  right for Comm. 8 and 19.
\item[I.Platais] said we need to make issues public and transparent for all
  members of the commissions involved.
\item[K.Johnston] said we have an evolution here, we need to look at new fields 
  and assign new commissions, this will evolve and change. There could be a new
  commission about "space astrometry".
\item[B.van Altena] said this discussion "is driven" too much, the users 
  are not happy.
\item[K.Seidelmann] said things will be clarified after the process of 
  assigning terms of reference and revising the organization.
\item[V.Dehant] asked if I have an education items, where to address it?
\item[Fr.Mignard] answered the WG of nomenclature.
\item[D.McCarthy] added plus the special committee.
\end{description}

\vspace*{6mm}

\subsection{ICRS  WG}

\begin{description}
\item[A.Fey] asked about the current members of ICRS WG; is their expertise lost
  until a re-organization ?
\item[F.Mignard] replied that it is not decided yet, some will be members 
  in both new WGs.
\item[K.Johnston] said part of the confusion in the reference frames WG is 
  that now people don't know where they fit in, people have to be put somewhere.
  One suggestion would be a 6 months "grace period" for the current structure to
  continue. This is only a transition problem.
\item[N.Capitaine] said changes like the definition of a WG are now possible at 
  any time by the Division president.
\item[T.Fukushima] responded that the "WG ICRS" shall continue to exist for 
  the next 6 months. 
  During that time a re-arrangement can be organized by e-mail.
\item[F.Mignard] said the ICRS WG was not like a WG, rather like a commission.
\item[T.Fukushima] said the creation of a commission is more formal.
\item[I.Kumkova] said from the 7 sub-tasks of the ICRS WG, 4 tasks are directly
  related to Comm. 8, the 6 months transition period is a good idea.
\item[D.McCarthy] said the job of the special committee is important, it needs 
  to cover all topics.
\end{description}

\end{document}