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