(Last updated on 27 June 2003)
MODELS FOR THE IMPLEMENTATION OF THE IAU RESOLUTIONS
Capitaine, Nicole
Syrte, Observatoire de Paris, France
The implementation of the IAU 2000 resolutions requires the adoption of (i) the IAU 2000 model (Resolution B1.6) to replace the IAU 1976/1980 precession-nutation for the motion of the Celestial Intermediate Pole (CIP) in the Geocentric Celestial Reference System (GCRS), (ii) a conventional model (Resolution B1.7) for high frequency motions of the CIP in the International Terrestrial Reference System (ITRS) and (iii) the conventional relationship for defining UT1 (Resolution B1.8) as proportional to the Earth Rotation Angle (ERA) between the Celestial and Terrestrial Ephemeris Origins (CEO and TEO). Two equivalent ways of implementing these resolutions in the transformation from ITRS to GCRS can be used, namely (a) the new paradigm, based on a direct use of the CEO and the ERA and (b) the classical paradigm, based on the direct use of the equinox and GST, but using the CEO and the ERA indirectly. Implementation of the resolutions has required computation of expressions compliant with the new precession-nutation model, to be used in the ITRS/GCRS transformation once the relationship ERA (UT1) is adopted. Equivalent expressions have been provided for all the models necessary to implement the IAU 2000 system, using either the CEO-based or the equinox-based transformations.
STATUS OF THE IERS IMPLEMENTATION OF THE IAU RESOLUTIONS
McCarthy, Dennis D.
U. S. Naval Observatory
The International Earth rotation and Reference system Service (IERS) has implemented the IAU resolutions of the 24th General Assembly in its products. The IERS Conventions now provides an outline of the procedures to be used along with software consistent with those procedures. IERS Bulletins A and B have made the data required to implement the resolutions available since January, 2003. To assist users in the transition period between the previous system and that recommended by the 24th General Assembly, the IERS also continues to provide data consistent with the previous system. It also plans to make available a file of frequently asked questions to assist users in the transition between systems.
IMPLEMENTING THE IAU 2000 RESOLUTIONS IN ALMANACS
Bangert, John A.
Astronomical Applications Department
U.S. Naval Observatory
Almanacs provide practical astronomical data in an accessible form to satisfy the needs of a wide variety of user applications such as navigation, pointing a telescope, planning an observing session, or scientific research. Many users expect that the general content and format of the almanacs will remain the same from year to year. Thus, changes to the almanacs are made as infrequently as possible, and only after careful deliberation. The almanac makers implement a proposed change when the change (1) will result in more accurate information in the almanac, (2) is based on sound scientific underpinnings, and (3) will result in data or information relevant to the users of the almanac. The last criterion is the most important. The IAU 2000 resolutions must be considered in the context of these criteria before they are implemented in the almanacs. In addition, even under ideal circumstances there is a considerable lag between the time a resolution is adopted and the time that it is implemented in the almanacs. This lag is due to the time needed to develop, implement, and test new production software, and to the normal almanac publication schedule.
CHANGES IN THE ASTRONOMICAL ALMANAC
Kaplan, George H.
Astronomical Applications Department
U.S. Naval Observatory
The Astronomical Almanac must satisfy the needs of a variety of users around the world, who represent a wide range of interests and sophistication levels. The book, prepared jointly by the US and UK nautical almanac offices, is based to the greatest extent possible on IAU-endorsed and other internationally recognized standards. The IAU resolutions on reference systems and Earth rotation adopted in 1997 and 2000 represent a significant change in approach for both subject areas. To implement these resolutions in contents of The Astronomical Almanac, both the reference data and algorithms used must be changed, and some new tabulations added. Some of the required modifications have already been made and others will be introduced into the editions now in preparation. The specific changes will be described, along with issues for the future.
IMPLEMENTATION OF THE IAU RESOLUTIONS IN THE FRENCH EPHEMERIDES
Thuillot, William
IMCCE, Observatoire de Paris, France
Institut de mécanique céleste et de calcul des éphémérides (IMCCE), formerly Service des calculs et de mécanique céleste du Bureau des longitudes, is in charge, at Paris Observatory and under the auspices of Bureau des longitudes, of the making and the diffusion of the French ephemerides. Various ephemerides are provided as well by means of books as by means of on-line electronic facilities. The implementation of the IAU resolutions, together with the introduction of new dynamical models that we project, will then require important efforts. These projects will be presented. It appears that the improved models and the new constantsmust be adopted, therefore the use of new models of precession and nutation will be done at first. On the other hand, the changes in the systems of coordinates which will imply too hard disruptions to the general users will only be introduced in parallel with the usual systems.
REVISIONS OF JAPANESE ALMANACS
Fukushima, Toshio
National Astronomical Observatory of Japan
Japan publishes two kinds of national almanacs; the more precise and comprehensive one for the nautical use, the Japanese Ephemeris (JE) by the Maritime Safety Agency (MSA), and the more compact for the civil use, the Ephemeris Year Book (EYB) by the National Astronomical Observatory of Japan (NAOJ). As for JE, it is still in the same style and the same contents since the last major revision in 1984. The MSA will make no revision of the JE until all the required procedures for the changes are clear. As for EYB, we have already done a major revision from the edition of Year 2003. The contents of the revisions are (1) the change of base planetary/lunar ephemeris from DE200 to DE405, (2) the change of nutation theory from IAU 1980 to Shirai and Fukushima (2001), and (3) the change of geodetic datum from Tokyo datum to the new Tokyo datum, being almost the same as the latest ITRF.
THE RUSSIAN ASTRONOMICAL YEARBOOKS: MODERN STATE AND IAU RESOLUTIONS
Sveshnikov Michael L.
Institute of Applied astronomy, RAS, St.-Petersburg, Russia
IAA RAS produces several printed yearbooks. Their structure and contents are changed regularity to satisfy resolutions of IAU and requirements of users. In this paper the current plan of the implementing the IAU 2000 resolutions in Russian yearbooks is given. Future reform of yearbooks includes the replace of planetary ephemeredes, precession-nutation model, stellar catalogue and transfer to the new CEO concept. It will be carried out during 2003-2006.
Co-Authors: Glebova N.I., Lukashova M.V., Malkov A.A.
NEW PRECESSION FORMULAE
Fukushima, Toshio
National Astronomical Observatory of Japan
We modified Williams' formulation (Williams 1994) of the 3-1-3-1 rotation matrix to express the precession and the precession-nutation matrices with respect to the ICRF in a robust form. By using the latest determination of the planetary precession of DE405 in the inertial sense (Harada and Fukushima 2003) and one of the recent nutation theories (Shirai and Fukushima 2001), we determined the luni-solar precession from the VLBI observation of celestial pole offsets during 1979 to 2000. We also determined the best estimate of the geodesic precession and nutation. As by-products, we obtained the new determinations of (1) the mean equatorial pole offset at J2000.0, (2) the speed of general precession in longitude at J2000.0, (3) the mean obliquity of ecliptic at J2000.0, and (4) the dynamical flattening of the Earth.
EXPRESSIONS FOR IAU 2000 PRECESSION QUANTITIES
Capitaine, Nicole
Syrte, Observatoire de Paris, France
We discuss precession models consistent with the IAU 2000 precession-nutation
and a range of products that implement them. We first present the expressions
for the currently used precession quantities, in agreement with the MHB corrections
to the precession rates, that appear in the IERS Conventions 2000. We then
discuss a more sophisticated method that we used to develop precession expressions
that are dynamically consistent. We obtained expressions for the precession
of the ecliptic based on most recent theories for the Earth and the Moon and
the most precise numerical ephemerides. We then used these new expressions
for the ecliptic together with the MHB corrections to precession rates to
solve the precession equations for providing a new solution for the precession
of the equator that is dynamically consistent and compliant with IAU 2000.
A number of perturbing effects have first been removed from the MHB estimates
in order to get the physical quantities needed in the equations as integration
constants. We also discuss the most suitable precession quantities to be considered
in order to be based on the minimum number of variables and to be the best
adapted to the most recent models and observations.
Co-Authors: Wallace P., H.M. Nautical Almanac Office, RAL, United Kingdom; Chapront J., Observatoire de Paris, France
PRECESSION EXPRESSIONS CONSISTENT WITH THE IAU 2000A MODEL
Thuillot, William
Institut de Mécanique Céleste
Observatoire de Paris, France
Since the adoption by the XXIVth General Assembly of an accurate model of nutation, the IAU encourages the development of new expressions for precession consistent with the new model. We present here the new precession quantities given in Bretagnon et al., 2003. These expressions are issued from the analytical solution of the rotation of the rigid Earth SMART97 (Bretagnon et al., 1998) which provides together the precession and the nutation. These expressions include the new value of the precession rate of the equator in longitude. As the SMART97 series are close to the Souchay et al. (1999) series used to build the new model, they are consistent with the IAU 2000 Precession-Nutation Model. We give also the differences between our expressions and the Lieske et al. model ones (1977) improved by McCArthy (2002) and show that those differences are superior to the precision of the low-precision model IAU200B. At last, we give the derivatives of our expressions with respect to the precession constant and to the obliquity in order to compute the corrections of the precession quantities given by future improvements of these constants.
Co-Authors: Bretagnon Pierre (1),
Fienga Agnès , Simon Jean-Louis
(1) Pierre Bretagnon, died on November 17, 2002
CONSIDERATION ABOUT THE EOP AND A CONVENTIONAL ECLIPTIC
Thuillot, William
Institut de Mécanique Céleste
Observatoire de Paris, France
Following the model of the Earth rotation given by Bretagnon et al (1998, 2003), we give a complete decription of the Earth rotation parameters as seen as variations of the Euler angles of the Earth rotation. We show that the celestial pole offsets as well as the polar motion can be included with the precession and the nutation in a global modelling of the Earth rotation, thanks to the Euler angles. The use of such Euler angles in IERS publications is discussed: In the one hand, they will be easier to use than the present 5 EOP for astronomers who are not Earth rotation specialists , in the other hand, such tabulated coefficients would face the problem of the high number of coefficients needed to allow interpolations. In the end, we propose the definition of a conventional ecliptic plane close to the mean ecliptic J2000 and with a non-rotating origin.
Co-Authors: Fienga Agnès , Bretagnon Pierre
(1), Simon Jean-Louis
(1) Pierre Bretagnon, died on November 17, 2002
FUTURE DIRECTIONS IN PRECESSION AND NUTATION
Hilton, James L.
U.S. Naval Observatory
The IAU 2000A precession-nutation theory is computationally expensive, requiring over one thousand evaluations of sine and cosine functions required to evaluate IAU 2000A just once. In response to this another precession-nutation theory, IAU 2000B, was adopted at the same time. However, IAU 2000B has a reduced precision and was designed to cover only a limited time span around the epoch J2000.0. At the same time, applications such as the Multiyear Interactive Computer Almanac (MICA), are being developed that require long coverage periods and the abil-ity to reach the accuracy of modern day observations. To address this deficiency future preces-sion and nutation theories will need to do one or more of the following: (a) make a serious effort to optimize the code; (b) reduce its precision to match the accuracy with which the Earth orien-tation can accurately be determined; (c) no longer separate terms that are so close together in frequency space that their individual contributions cannot be determined at the level of accuracy of the observations; (d) move from representation as an analytic theory to a numerically inte-grated representation.
THE SOURCES AND USES OF ASTRONOMICAL CONSTANTS
Standish, E M
Caltech/JPL
This paper discusses the use of ephemerides based upon the independent variable, "Teph", and compares it with the use of ephemerides based upon the recent IAU-defined "TCB". Teph has been used over the past three decades for the ephemerides created at JPL, CfA, and IPA; it has been referred to, somewhat erroneously, as "ET" and/or "TDB" in the past. Teph and TCB are mathematically equivalent; they are both relativistic coordinate times. Proper use of a Teph-based ephemeris should give results identical to those obtained using a TCB-based ephemeris. However, care must be taken in some circumstances, such as navigating a spacecraft in orbit around a remote planet while timing the dynamics on an earth-based clock. This paper discusses such situations.
WORKING GROUP ON THE CELESTIAL REFERENCE
Mignard François
OCA/CERGA, France
Creation and Organization
The WGICRS was created as a Working Group of IAU Division I as a continuation
of similar groups since 1991 whose activities led to the adoption of the ICRF
as a realization of the ICRS in 1997 and to an extensive set of resolutions
in 2000 which provides the general framework of modern astrometry. The Working
Group was renewed at IAU General Assembly 24 in August 2000 for a term of
three years, with the goal of coordinating the work of astronomers to qualify,use,
extend and promote the ICRS and prepare the recommendations relevant to these
topics to be submitted to the IAU General Assembly in 2003. The Working Group
comprises 39 members and is organized around six well identified tasks directed
by a task leader. Each member of the working group has expressed personal
interest in at least one of these task.
Tasks and task leaders of the Working Group
Maintenance and extension of the ICRS (C. Ma), Densification at optical and
IR wavelengths (S. Urban), Space Astrometry and Reference Frames (F. Mignard),
Link to the dynamical system (M. Standish), Computational tools (P. Wallace),
Astronomical Standards (T. Fukushima), Relation with IERS (F. Arias).
IAU WORKING GROUP REPORT RELATIVITY FOR ASTROMETRY, CELESTIAL MECHANICS AND METROLOGY
Soffel, Michael
TU Dresden, Germany
After the last IAU GA in Manchester and the adoption of the new IAU'2000 resolutions concerning relativity the work of the WG focused upon the realisation and practical use of these resolutions. The role of the two Celestial Reference Systems, BCRS and GCRS, and of astrometric places of stars (e.g., apparent places) was discussed. Further points of intensive discussions were: the coordinate transformations between the BCRS and the GCRS, the mass multipole moments (potential coefficients) and especially the spin moments. Such spin moments, related with the rotation of the Earth play no role in Newton's theory of gravity. Orders of magnitude of these spin moments of the Earth were derived in the framework of a model. Further work concerns the implementation of these resolutions in the various models for VLBI, LLR, SLR etc. and in the IERS conventions. A detailed explanatory supplement concerning these resolutions was prepared for publication.
WORKING GROUP REPORT ON CARTOGRAPHIC COORDINATES AND ROTATIONAL ELEMENTS OF THE PLANETS AND SATELLITES
Seidelmann, Ken
University of Virginia, USA
The IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites report of 2000 was published in Celestial Mechanics and Dynamical Astronomy 82: 83-110, 2002. The authors are P.K. Seidelmann, V.K. Abalakin, M. Bursa, M.E. Davies, C. DeBergh, J.H. Lieske, J. Oberst, J.L. Simon, E.M. Standish, P. Stooke, and P.C. Thomas.The working group is reformulated for this triennium and preparing the next report in its continuing series. A draft of the report will be presented for discussion and completion at the IAU General Assembly in Sydney, Australia, in 2003.
REDEFINITION OF UTC
McCarthy Dennis D.
U. S. Naval Observatory
The International Telecommunications Union - Radiocommunications (ITU-R) has formed a Special Rapporteur Group to investigate possible changes in the current definition of Coordinated Universal Time (UTC). These possible changes range from a possible change in the allowed difference between UT1 and UTC to a redefinition of the second. A Colloquium on the subject was held in May, 2003 to assist in the formulation of a possible recommendation to the ITU-R. The results of that Colloquium and the recommendations of the IAU Working Group on the definition of UTC are outlined.
WORKING GROUP "THE FUTURE DEVELOPMENT OF GROUND-BASED ASTROMETRY
Stavinschi, Magda
Astronomical Institute of the Romanian Academy, http://www.astro.ro/wg.html
In its last General Assembly (Aug 2000), the IAU has set up a Working Group on The Future Development of Ground-based Astrometry to suggest scientific programs that can still be successfully performed with such instruments. The first meeting of the WG was held during JENAM 2001, in Munich. The success of the Hipparcos, and the perspective of future astrometric space missions bring serious problems to many observatories and Universities that possess small instruments and used to do research in astrometry. The JENAM JD3 was the first occasion to exchange experience and reflect on the best use of existing instruments. The second meeting has been held during the international colloquium "Journées 2002. Systèmes de référence spatio-temporels" (Sept 2002, Bucharest). J. Kovalevsky presented a Synthesis of Possible Programs Using Small Ground-Based Astrometric Instruments. Other communications were devoted to different programs or techniques, as the limit capabilities of ground-based optical astrometry instruments and to the international campaign PHEMU 2003 for the mutual phenomena of jovian satellites which just started. The participants concluded that their discussion could continue on the WG site and that the group has to be enlarged.
Members: D. Wyn Evans (UK),C. Lopez (Argentina), D. Pascu
(USA), A. Pugliano (Italy), M. Sanchez (Spain), R. Teixeira (Brazil), A. Upgren
(USA); Consultant members and subgroups : A. Humberto Andrei (Solar diameter),
J-E. Arlot (Mutual phenomena), W. Brouw (Radio-astronomy), J. A.Mattei (Photometry),
G. Pinigin (Reference systems), T. Rafferty (Double stars).
Co-Chair: Kovalevsky Jean, OCA, CERGA, France
FAQS AS AN EDUCATIONAL DEVICE
Ma, Chopo
GSFC, USA
FAQs have become common on the worldwide web as an introduction to a specific topic. An initial set of FAQs on the recent IAU resolutions has been prepared for linking from relevant web sites. The content, future refinement and expansion, and distribution will be discussed.