************************************************************************
IERS Message No. 99                                     November 9, 2006
************************************************************************


IAU Commission 52 Relativity in Fundamental Astronomy


Dear Colleagues,

We would like to inform you that the IAU has created a new Commission
which could be of interest for the IERS community: IAU Commission 52
Relativity in Fundamental Astronomy (RIFA). The web page of the
Commission can be found at

http://astro.geo.tu-dresden.de/RIFA

Please find below a manifesto setting out the long-term and short-term
goals of the Commission. 

On behalf of the Organizing Committee I would like to invite 
all interested IAU members to join the new IAU Commission 52.
In order to join the Commission please send an e-mail to 

Sergei.Klioner@tu-dresden.de


Sincerely Yours,

Sergei Klioner
President of the IAU Commission 52

------------------------------------------------------------------------

IAU Commission 52 "Relativity in Fundamental Astronomy"


Relativity has been becoming increasingly important for modelling and
interpretation of high accuracy astronomical observations during at
least the last 20 years. Relativistic questions have been playing an
important role in the work of several IAU Commissions and Working Groups
for an extended period of time. For these reasons the 26th General
Assembly of the IAU, held in Prague in August 2006, has decided to
establish IAU Commission 52 "Relativity in Fundamental Astronomy"
(RIFA). The general scientific goals of the new Commission are:

- clarify geometrical and dynamical concepts of Fundamental Astronomy 
  within a relativistic framework,

- provide adequate mathematical and physical formulations to be used in
  Fundamental Astronomy,

- deepen the understanding of the above results among astronomers and 
  students in astronomy,

- promote research needed to accomplish these tasks.

Although these goals of the Commission are long-term ones, it seems
reasonable that the following important issues be actively addressed
by the Commission within the next three years (before the 27th General
Assembly of the IAU in 2009):


1. "Units of measurements" for astronomical quantities in the
   relativistic context

In the literature (including very recent papers) one can find different
units used in precise work: "TDB units", "TCB units", "TT units" along
with "SI units". The co-existence of these units is related to the
relativistic scaling of time and space coordinates. On the other hand,
the IAU 1991 resolutions clearly state that only SI units without any
additional relativistic scaling should be used for all astronomical
quantities (astronomical units like AU are not meant here). A balanced
approach to this issue should be suggested and discussed. This would
help us to unify the notations and numerical values of astronomical
constants throughout the literature.


2. "Astronomical units" in the relativistic framework

It is known that a significant freedom exists in the definition of the
system of astronomical units in the framework of relativity. This
freedom has been discussed in a number of recent publications, but up to
now no standard choice has been agreed upon. Moreover, the complexity of
relativistic modification of the current system of astronomical units
together with the fact that the original reasons for astronomical units
are no longer important for current practice naturally invokes a
discussion on a possible simplification of the system of astronomical
units. This question is clearly a delicate one since it concerns many
parts of astronomy. Nevertheless, it seems to be right time to discuss
these issues at the level of IAU Commissions and Working Groups.


3. Time-dependent ecliptic in the GCRS

An improved definition of an ecliptic adopted by the 26th General
Assembly of the IAU is given in the Barycentric Celestial Reference
System. On the other hand, theories of Earth rotation for which ecliptic
plays an important role should be defined in the GCRS. Therefore, a GCRS
ecliptic has to be discussed. It has to be clarified if relativistic
effects can affect the definition of the ecliptic at some perceptible
level of accuracy.


************************************************************************
IERS Messages are edited and distributed by the IERS Central Bureau.
To subscribe or unsubscribe, please write to <central_bureau@iers.org>.
Archives: http://www.iers.org/iers/publications/messages/
************************************************************************