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Astronomical Applications Department, U.S. Naval Observatory - NOVAS 2004 Overview (Page 2)

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Astronomical Applications Department, U.S. Naval Observatory - NOVAS 2004 Overview
2
changes to existing NOVAS calls involve lower-level routines not frequently invoked by most users;
these are detailed in the Appendix.

PLACE: A New General-Purpose "Place" Subroutine

All computational code to compute apparent, topocentric, virtual, astrometric, etc., places of stars or
planets has now been consolidated into a single new subroutine call PLACE. The familiar calls to
APSTAR, APPLAN, TPSTAR, etc., are now just "front-ends" to PLACE. This has eliminated much
duplicate code and also provides more flexibility and possible future additions (such as binary star
orbits or nonlinear terms in proper motion). PLACE can also provide star or planet positions within
the "intermediate" coordinate system that is part of the new paradigm for Earth rotation calculations
(see below). PLACE provides its output position both in spherical coordinates (right ascension,
declination, and, for solar system bodies, geometric distance) and as a unit vector. PLACE accepts the
specification of solar system bodies by name, e.g., `MARS', `SATURN', or `SUN', thus increasing the
readability of code. You may want to consider changing your calls to APSTAR, APPLAN, etc., to the
equivalent calls to PLACE.

The International Celestial Reference System (ICRS)

Reference data for positional astronomy, such as the data in astrometric star catalogs (e.g., Hipparcos)
or barycentric planetary ephemerides (e.g., JPL's DE405) are now specified within the International
Celestial Reference System (ICRS). The ICRS is a coordinate system whose origin is at the solar
system barycenter and whose axis directions are effectively defined by the adopted coordinates of
about 600 extragalactic radio sources observed by VLBI (see Section H of The Astronomical
Almanac
). These radio sources (quasars and active galactic nuclei) are assumed to have no observable
intrinsic angular motions. Thus, the ICRS is a "space-fixed" system (more precisely, a kinematically
non-rotating system) without an associated epoch. However, the ICRS closely matches the
conventional dynamical system defined by the Earth's mean equator and equinox of J2000.0; the
alignment difference is at the 0.02 arcsecond level, negligible for many applications.

NOVAS now assumes that input reference data, such as catalog star positions and proper motions, and
the basic solar system ephemerides, are provided in the ICRS, or at least are consistent with it to
within the data's inherent accuracy
. The latter case will probably apply to most FK5-compatible data
specified in the dynamical system of J2000.0. The distinction between the ICRS and the dynamical
system of J2000.0 becomes important only when an accuracy of 0.02 arcsecond or better is important.
Nevertheless, because NOVAS is designed for the highest accuracy applications, you will now see the
ICRS mentioned as the reference system of choice for many input arguments to NOVAS subroutines.

NOVAS now contains a subroutine called FRAME that transforms vectors from the ICRS to the
dynamical system of J2000.0 or vice versa. This transformation is a very small fixed rotation.
FRAME is called many times, in both directions, within the NOVAS code. That is because precession
(and nutation) can properly be applied only to vectors in a dynamical system; vectors in the ICRS
must be transformed, via FRAME, to the dynamical system (equator and equinox) of J2000.0 before
PRECES is used. If your code only interacts with the highest level NOVAS subroutines, all this is
transparent to you. However, if you use PRECES within your own code, you should precede it by a
call to FRAME (with the middle argument K>0) if your input vector is expressed in the ICRS.



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