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Astronomical Applications Department, U.S. Naval Observatory - NOVAS 2004 Overview (Page 1)

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Astronomical Applications Department, U.S. Naval Observatory - NOVAS 2004 Overview
Overview of the New NOVAS (Fortran)
George Kaplan, USNO


The 2004 version of Naval Observatory Vector Astrometry Subroutines (NOVAS version F2.9, in
Fortran) implements the resolutions on astronomical reference systems and Earth rotation models
passed at the IAU General Assemblies in 1997 and 2000. Although the IAU resolutions provided
basic guidance, many of the models and parameter values needed to implement the resolutions in
practice did not become available until late 2002. The International Earth Rotation and Reference
Systems Service (IERS), which had been the driving force for many of the resolutions, implemented
these models for their own data processing at the beginning of 2003. (A dynamically consistent
precession theory is still in preparation. The precession in NOVAS version F2.9 is a preliminary
algorithm adopted by the IERS. The final precession will go into NOVAS version F3.0.) This version
of NOVAS also improves the accuracy of its star and planet position calculations (apparent places) by
including several small effects not previously implemented in the code. A number of new convenience
functions have also been added.

A detailed list of the changes in NOVAS from the previous version (F2.0 of 1998) is given in the
Appendix. The following paragraphs are meant to provide some perspective for people who are
already familiar with NOVAS. To the greatest extent possible, the calling sequences for the highest-
level (and most used) functions from the previous versions of NOVAS have been preserved -- but
there are a few important exceptions. There are many new calls.

Important Changes in Existing Calls
Probably the most important change to existing NOVAS calls is the change of proper motion and
parallax units in the calls to APSTAR, VPSTAR, and ASSTAR, CATRAN, and GETHIP, all of which
deal with star positions. The units have been changed as follows:

proper motion in right ascension: from seconds of RA per century to milliarcseconds per year
proper motion in declination: from arcseconds per century to milliarcseconds per year
parallax: from arcseconds to milliarcseconds

These changes have been made to conform to the units used in most modern star catalogs (e.g.,
Hipparcos, Tycho-2, or the FK6), which in turn follow from the observational techniques now used in
the construction of such catalogs. Obviously, star data previously used with NOVAS must either be
replaced or transformed. The transformation equations from "old" to "new" units are as follows:
PMRNEW = PMROLD * 150.D0 * DCOS ( DEC0 * DEGRAD ) ; proper motion in RA
PMDNEW = PMDOLD * 10.D0 ; proper motion in dec
PAXNEW = PAXOLD * 1000.D0 ; parallax

where
DEC0
is the catalog declination (J2000.0 or ICRS) of the star in degrees and
DEGRAD
is the
degrees-to-radians conversion factor (0.01745329...).

The other major change to a high-level subroutine is that PNSW has been renamed to TERCEL (it
carries out the terrestrial-to-celestial transformation), with a change to the time argument. All other

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