Nicole Capitaine

Proposed terminology in fundamental astronomy based on IAU 2000 resolutions

We present the proposals of the IAU Division I Working Group on Nomenclature for Fundamental Astronomy (NFA) that was formed at the IAU XXV General Assembly in 2003.

Systèmes de référence en astronomie

Dynamical theories imply the existence of reference systems in time and space. When applied to physical systems, these theories require physical representations of the references: realized time scales and reference frames. We review recent developments in such realizations matching the quality of modern observations. Whereas international atomic time (TAI), in existence since 1955, is the basis of quasi-ideal representations of time coordinates needed in dynamics, the problem of realizing a non-rotating spatial reference remained critical until recently. The modern solution appeared in the 1970s with the astrometric application of very long baseline radio-interferometry (VLBI), which allows us to tie the orientation of the celestial frame to the directions of the most distant bodies we know, the quasars. Improvements in this technique as well as the realization of the optical counterpart of the celestial radio frame, thanks to the success of the astrometric mission Hipparcos, allowed the adoption of a new conventional celestial reference frame that will become official from 1 January 1998. The old dream of an absolute reference for measuring the angular motion of celestial bodies has become a reality. This opens a new era for fundamental astronomy and for astrophysics.

Micro-arcsecond Celestial Reference Frames: definition and realization - Impact of the recent IAU Resolutions

The adoption of the International Celestial Reference System (ICRS),based on Very Long Baseline Interferometry (VLBI) observations of extragalactic radiosources by the International Astronomical Union (IAU) since 1998 January 1,opened a new era for astronomy.The ICRS and the corresponding frame,the International Celestial Reference Frame (ICRF),replaced the Fundamental Catalog (FK5) based on positions and proper motions of bright stars,with the Hipparcos catalog being adopted as the primary realization of the ICRS in optical wavelengths.According to its definition,the ICRS is such that the barycentric directions of distant extragalactic objects show no global rotation with respect to these objects; this provides a quasi-inertial reference for measuring the positions and angular motions of the celestial objects.Other resolutions on reference systems were passed by the IAU in 2000 and 2006 and endorsed by the International Union of Geodesy and Geophysics (IUGG) in 2003 and 2007,respectively.These especially concern the definition and realization of the astronomical reference systems in the framework of general relativity and transformations between them.First,the IAU 2000 resolutions refined the concepts and definition of the astronomical reference systems and parameters for Earths rotation,and adopted the IAU 2000 precession-nutation.Then,the IAU 2006 resolutions adopted a new precession model that is consistent with dynamical theories; they also addressed definition,terminology or orientation issues relative to reference systems and time scales that needed to be specified after the adoption of the IAU 2000 resolutions.An additional IUGG 2007 resolution defined the International Terrestrial Reference System (ITRS) so that it strictly complies with the IAU recommendations.Finally,the IAU 2009 resolutions adopted a new system of astronomical constants and an improved realization of the ICRF.These fundamental changes have led to significant improvements in the fields of astrometry,celestial mechanics,geodynamics,geodesy,etc.Of special interest are the improvements in the model for variations in Earths rotation,which,in turn,can provide better knowledge of the dynamics of the Earths interior.These have also contributed to a significant improvement in the accuracy of the ephemerides of the solar system bodies as determined from modern measurements,with a large number of scientific applications.This paper recalls the main aspects of the recent IAU resolutions on reference systems as well as their consequences on the concepts,definitions,nomenclature and models that are suitable for the definition,realization and transformation of reference frames at a microarcsecond level.

DIVISION I: FUNDAMENTAL ASTRONOMY

The goal of the division is to address the scientific issues that were developed at the 2009 IAU General Assembly in Rio de Janeiro. These are: • Astronomical constants —Gaussian gravitational constant, Astronomical Unit, GM Sun , geodesic precession-nutation • Astronomical software • Solar System Ephemerides —Pulsar research —Comparison of dynamical reference frames • Future Optical Reference Frame • Future Radio Reference Frame • Exoplanets —Detection —Dynamics • Predictions of Earth orientation • Units of measurements for astronomical quantities in relativistic context • Astronomical units in the relativistic framework • Time-dependent ecliptic in the GCRS • Asteroid masses • Review of space missions • Detection of gravitational waves • VLBI on the Moon • Real time electronic access to UT1-UTC In pursuit of these goals Division I members have made significant scientific and organizational progress, and are organizing a Joint Discussion on Space-Time Reference Systems for Future Research at the 2012 IAU General Assembly. The details of Division activities and references are provided in the individual Commission and Working Group reports in this volume. A comprehensive list of references related to the work of the Division is available at the IAU Division I website at http://maia.usno.navy.mil/iaudiv1/ .

DIVISION I / WORKING GROUP NOMENCLATURE FOR FUNDAMENTAL ASTRONOMY

The IAU Division I Working Group on Nomenclature for Fundamental Astronomy (NFA) was established by the IAU XXV General Assembly with the task of providing proposals for new nomenclature associated with the implementation of the IAU XXIV GA resolutions (2000) and to make related educational efforts for bringing the issue to the notice of scientists in the community.

The 3D representation of the new transformation from the terrestrial to the celestial system

We offer a 3D representation of the new transformation from the terrestrial to the celestial system.

The Use of the Celestial Pole Coordinates in the Transformation Between the Celestial and Terrestrial Reference Frames

This paper emphasizes the use of the celestial direction cosines of the pole in the matrix transformation of vector components from the Celestial Reference System (CRS) to the Terrestrial Reference System (TRS). These two fundamental quantities are shown to advantageously replace the large number of the usual precession and nutation quantities. Such a modification in the representation of the celestial displacement of the pole has to be associated with the use of the non-rotating origin on the instantaneous equator (Guinot 1979, Capitaine et al. 1986) and of a quasi ideal celestial reference frame as realized by the positions of extragalactic sources.