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Report on the activities of the IAU/IAG Joint Working Group on Theory of Earth Rotation
JosИ M. FerrАndiz and Richard S. Gross

JSR 2014, St. Petersburg, 23 September

IAU/IAG JWG on Theory of Earth Rotation

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Introduction: purpose of the IAU/IAG JWG_ThER
· The International Association of Geodesy (IAG) and the International Astronomical Union (IAU) set up a new Joint Working Group on Theory of Earth Rotation in April 2013. · The purpose of this JWG is:

To promote the development of theories of Earth rotation that are fully consistent and that agree with observations and provide predictions of the ERP (Earth Rotation Parameters) with the accuracy required to meet the needs of the near future as recommended by, e.g., GGOS, the Global Geodetic Observing System of the IAG.
· Let us recall that GGOS accuracy goals are 1 mm in position and 0.1 mm/yr in velocity on global scales for the ITRF. The accuracy in position corresponds roughly to 30 as and 3 as/yr - angles measured from the Earth's centre.
JSR 2014, St. Petersburg, 23 September IAU/IAG JWG on Theory of Earth Rotation 3

Desired Outcomes of the JWG
1. Contribute to improving the accuracy of precessionnutation and ERP theoretical models by proposing both new models and additional corrections to existing models. 2. Clarify the issue of consistency among conventional ERP, their definitions in various theoretical approaches, and their practical determination. 3. Establish guidelines or requirements for future theoretical developments with improved accuracy.
The overall goals of the WG cannot be achieved within only 2 years.

The first term should be used to develop a solid concept of how to reach its aims.

Reports must be presented at the next General Assemblies of both IAU and IAG, in 2015.
JSR 2014, St. Petersburg, 23 September IAU/IAG JWG on Theory of Earth Rotation 4

Structure of the JWG
The structure of this JWG follows the characteristics of the current set of EOP (Earth Orientation Parameters) as well as the fields of specialization of researchers. People in charge: · Chair: Jose M. FerrАndiz (IAU) · Vice-Chair: Richard Gross (IAG) The WG is structured in three Sub-Working Groups (SWG): 1. Precession/Nutation (Chair: Juan Getino) 2. Polar Motion and UT1 (Chair: Aleksander Brzezinski) 3. Numerical Solutions and Validation (Chair: Robert Heinkelmann) SWG 3 will be dedicated to numerical theories and solutions, relativity and new concepts and validation by comparisons among theories and observational series.
JSR 2014, St. Petersburg, 23 September IAU/IAG JWG on Theory of Earth Rotation 5

General organization of activities
· Each SWG is entrusted with its own tasks and goals. · The three SWG should work in parallel for the sake of efficiency and they should be coordinated and linked together as closely as the needs of consistency demand. · The JWG is intended on an open basis and the cooperation of non-members is welcome. All people interested in Earth rotation is invited to contribute · If you do not know how to contribute, please contact any of the chair persons or visit our website

http://web.ua.es/en/wgther

JSR 2014, St. Petersburg, 23 September

IAU/IAG JWG on Theory of Earth Rotation

6

Initial activities and previous meetings
· Related JWG meetings, most of them open to attendants, were held on occasion of different conferences: IAG 2013 Scientific Assembly, in Potsdam JournИes 2013, in Paris 8th IVS General Meeting 2014, in Shanghai EGU 2014, in Vienna · Most of the presentations there and other documents memos, ToRs, member list,etc. are available at the JWG web site, hosted by the Institution of the Chair University of Alicante, Spain http://web.ua.es/en/wgther
· Consistency has been one of the mains concerns in the discussions, according to the JWG Terms of Reference and other IAG initiatives - e.g. the GGOS Bureau for Standards and Conventions (BSC)
JSR 2014, St. Petersburg, 23 September IAU/IAG JWG on Theory of Earth Rotation 7