USNO Circular 163
The IAU Resolutions on Astronomical Constants,
Timescales, and the Fundamental Reference Frame
George H. Kaplan (editor)
U.S. Naval Observatory
This Circular describes the series of resolutions adopted by the International
Astronomical Union (IAU) in 1976 and 1979 on astronomical constants, timescales,
and the fundamental reference frame (FK5). These recommendations formed the
basis for the reduction of astrometric observations (including those used for
geodesy and Earth rotation measurements) and the computation of ephemerides from
1984 through at least 2002; they are the foundation for much software that is
still in use. These resolutions provided a self-consistent and interrelated
system of changes that were necessary to improve astronomical reference data at a
time when new observational techniques and theories were revolutionizing many
aspects of fundamental astronomy.
As presented herein, related resolutions have been grouped together for
convenience. Each group of resolutions is accompanied by notes that offer brief
explanations. The appendices provide additional tools for implementing these
Note: These recommendations have been superceded by
resolutions adopted by the IAU in 1997 and 2000. USNO
Circular 179 (2005) describes the more recent resolutions and their
implementation and effectively replaces Circular 163. Circular 163 may, however,
be useful for those who are converting software from the old paradigm to the new.
Obtaining a Copy:
Download a Free Copy. USNO Circular 163 is available free of charge as a
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