USNO Circular 163
Astronomical Applications Dept. Astronomical Applications Dept.
Skip navigation

The IAU Resolutions on Astronomical Constants, Timescales, and the Fundamental Reference Frame

George H. Kaplan (editor)
U.S. Naval Observatory
December 1981


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 resolutions.

Note: These recommendations have been superceded by resolutions adopted by the IAU in 1997 and 2000.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 PDF document. Click on the link below to download the publication.

Order a Printed Booklet. A bound, printed version of the circular is available from the U.S. Naval Observatory for $2. To order this, or any USNO special publications, use the order form provided: print it out, fill it in, and mail it to the Naval Observatory with your payment. We cannot accept telephone, fax, or e-mail orders. Only checks or money orders (in U.S. currency, drawn on a U.S. bank) can be accepted for payment. Payment must be received before orders are filled.


Please report any errors via our help desk.