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Astronomical Applications Department, U.S. Naval Observatory - NOVAS 2004 Overview (Page 6)

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Astronomical Applications Department, U.S. Naval Observatory - NOVAS 2004 Overview
6
a 1365-term trigonometric series is evaluated for each unique date. Neither the models nor current
observations are accurate at this level, however, so much of the increased computational burden is
unproductive. A call to LOACC sets the Earth rotation computations (and only those computations) in
NOVAS to an accuracy of 0.1 milliarcsecond. The computation time for these calculations is thereby
reduced by about 2/3.

Finally, another of the new Earth-rotation-related subroutines is worth mentioning. For a given TDB
date, CEORA provides the right ascension of the CEO with respect to the true equator and equinox of
date. With a sign reversal, this quantity is the equation of the origins, the direction of the true equinox
measured in the equator eastward (+) from the CEO. The equinox and CEO can be considered
different right ascension origins on the instantaneous equator, and as such they define separate
equatorial systems for the equinox-based and CEO-based paradigms. CEORA therefore provides the
angular difference between the origins of these two systems.

Some Terminology

Not surprisingly, the IAU resolutions related to Earth rotation have spawned a lot of new terminology,
not all of which has become universally accepted. An IAU Working Group on Nomenclature for
Fundamental Astronomy has been appointed to try to sort it all out. The most commonly used terms
and abbreviations now appear in comment statements in some of the new NOVAS subroutines,
including the preambles where the input and output arguments are described. A brief summary of
these terms is therefore in order here. The celestial ephemeris origin (CEO) and terrestrial ephemeris
origin (TEO) have already been described; these terms are mentioned specifically in the 2000 IAU
resolutions. Another term specifically introduced in those resolutions is the celestial intermediate pole
(CIP), which is the celestial pole defined by the new precession and nutation models. The true equator
of date is thus a plane orthogonal to the CIP. The coordinate system defined by the true equator of
date and the CEO is widely referred to as the intermediate system (or the celestial intermediate
system), because it is in a sense midway between the rapidly rotating terrestrial latitude-longitude
system and the completely non-rotating ICRS. The right-ascension-like coordinate in the intermediate
system (the azimuthal coordinate measured in the equatorial plane eastward from the CEO) will
probably be called something like CEO right ascension (or right ascension with respect to the CEO).

How NOVAS Implements the CEO-Based Paradigm
The NOVAS implementation of the CEO-based Earth rotation paradigm for a given date is based on
the construction of the intermediate system for that date, using vectors toward the celestial inter-
mediate pole (CIP) and the celestial ephemeris origin (CEO). These two directions define,
respectively, the z-axis and x-axis of the intermediate system. The direction toward the CIP in the
ICRS can be computed by passing the vector (0,0,1) through subroutines NUTATE, PRECES, and
FRAME. Given the direction of the CIP, the only remaining piece of required information is the ICRS
right ascension of the CEO for the same date, which is provided by CEORAI. The basis vectors of the
intermediate system, with respect to the ICRS, are assembled by CEOBAS. Having these basis vectors
available allows NOVAS to easily transform any vector in the ICRS to the intermediate system. The
only other quantity used in the CEO-based paradigm is the Earth rotation angle, which is trivial to
compute and provided by EROT.

The only tricky part of this process is obtaining the ICRS right ascension of the CEO, which is a
unique quantity derived from an integration. CEORAI obtains the right ascension of the CEO for a

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