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proposed by Joint Discussion N. 3

The XXIIIrd International Astronomical Union General Assembly


(1) that the International Astronomical Union (IAU) has adopted an International Celestial Reference System (ICRS) in which the axes are fixed relative to the distant background as implied by observations of extragalactic sources,

(2) that the realization of the ICRS is based on observations made from the Earth, the axes of which precess and nutate relative to the ICRS,

(3) that there are significant differences between the nutation adopted by the IAU in 1980 and astronomical observations,

(4) that a rate of variation of the obliquity is observed, which is not predicted by the 1980 IAU precession-nutation theory,

(5) that there is a difference in the precession rate of about -3.0 milliarcseconds per year (mas/year) between the observed and adopted values,


(1) that Division I form a new Working Group to report to the IAU General Assembly in 2000 which will

a. examine and clarify the effects on astrometric computations, of changes such as the adoption of the International Celestial Reference System, the availability of the Hipparcos catalogue, and the change expected in the conventional precession-nutation model, and

b. make recommendations regarding the algorithms to be used,

(2) that this Working Group study these questions jointly with the International Earth Rotation Service (IERS) and maintain a close connection with the IAU Working Group on Reference Frames, the IAU Working Group on Astronomical Constants, and the IAU-IUGG Working Group on Non-rigid Earth Nutation Theory (up to its discontinuation at the 1999 IUGG General Assembly), through exchange of representatives.

Created: 1 Jan 2001. Text provided by the former Central Bureau.

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