USNO Circular 179 - U.S. Naval Observatory

Introduction
The series of resolutions passed by the International Astronomical Union at its General Assemblies
in 1997 and 2000 are the most significant set of international agreements in positional astronomy in
several decades and arguably since the Paris conference of 1896. The approval of these resolutions
culminated a process — not without controversy — that began with the formation of an intercommission
Working Group on Reference Systems at the 1985 IAU General Assembly in Delhi.
The resolutions came at the end of a remarkable decade for astrometry, geodesy, and dynamical
astronomy. That decade witnessed the successes of the Hipparcos satellite and the Hubble Space
Telescope (in both cases, after apparently fatal initial problems), the completion of the Global
Positioning System, 25-year milestones in the use of very long baseline interferometry (VLBI) and
lunar laser ranging (LLR) for astrometric and geodetic measurements, the discovery of Kuiper
Belt objects and extra-solar planets, and the impact of comet Shoemaker-Levy 9 onto Jupiter.
At the end of the decade, interest in near-Earth asteroids and advances in sensor design were
motivating plans for rapid and deep all-sky surveys. Significant advances in theory also took place,
facilitated by inexpensive computer power and the Internet. Positional and dynamical astronomy
were enriched by a deeper understanding of chaos and resonances in the solar system, advances in
the theory of the rotational dynamics of the Earth, and increasingly sophisticated models of how
planetary and stellar systems form and evolve. It is not too much of an exaggeration to say that
as a result of these and similar developments, the old idea that astrometry is an essential tool of
astrophysics was rediscovered. The IAU resolutions thus came at a fortuitous time, providing a
solid framework for interpreting the modern high-precision measurements that are revitalizing so
many areas of astronomy.
This circular is an attempt to explain these resolutions and provide guidance on their implementation.
This publication is the successor to USNO Circular 163 (1982), which had a similar
purpose for the IAU resolutions passed in 1976, 1979, and 1982. Both the 1976–1982 resolutions
and those of 1997–2000 provide the specification of the fundamental astronomical reference system,
the definition of time scales to be used in astronomy, and the designation of conventional models
for Earth orientation calculations (involving precession, nutation, and Universal Time). It will
certainly not go unnoticed by readers familiar with Circular 163 that the current publication is
considerably thicker. This reflects both the increased complexity of the subject matter and the
wider audience that is addressed.
Of course, the IAU resolutions of 1997–2000 did not arise in a vacuum. Many people participated
in various IAU working groups, colloquia, and symposia in the 1980s and 1990s on these topics,
and some important resolutions were in fact passed by the IAU in the early 1990s. Furthermore,
any set of international standards dealing with such fundamental matters as space and time must
to some extent be based on, and provide continuity with, existing practice. Therefore, many of the
new resolutions carry “baggage” from the past, and there is always the question of how much of this
history (some of it quite convoluted) is important for those who simply wish to implement the latest
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