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NOVAS-C
Naval Observatory Vector Astrometry Subroutines
C Language Version 2.0
John A. Bangert
Astronomical Applications Department
U. S. Naval Observatory
Based on the algorithms and Fortran code by:
George H. Kaplan
Astronomical Applications Department
U. S. Naval Observatory
1.0 Introduction
The Naval Observatory Vector Astrometry Subroutines, NOVAS, is an integrated
package of source-code modules for computing a wide variety of common astrometric
quantities and transformations. The package can provide, in one function call, the
instantaneous coordinates (apparent, topocentric, or astrometric place) of any star or
planet. At a lower level, NOVAS also provides general astrometric utility
transformations, such as those for precession, nutation, aberration, parallax, and the
gravitational deflection of light. The computations are highly precise. The NOVAS
algorithms are, in fact, virtually identical to those now used in the production of The
Astronomical Almanac. NOVAS is easy-to-use and can be incorporated into data
reduction programs, telescope control systems, and simulations.
The first version of NOVAS was released in 1988 as a package of Fortran subroutines
[1]. The Fortran package proved to be very popular, but there were requests for a C-
language version of the software. In the early 1990s, members of the U.S. Naval
Observatory (USNO)/Naval Research Laboratory Optical Interferometer group converted
parts of NOVAS to C for use in their project. Their work was returned to USNO's
Astronomical Applications Department for further development. This work lead to the
first complete edition of NOVAS in ANSI-standard C (designated NOVAS-C Version
1.0), released in 1996. A major revision of the NOVAS Fortran code took place in 1998,
with the primary goal of supporting data conforming to the International Celestial
Reference System (ICRS) [2]. Shortly thereafter, NOVAS-C was updated to reflect the
changes in the Fortran code and to add additional capabilities. The result was NOVAS-C
Version 2.0, described in this document.
NOVAS-C uses, as input, astrometric reference data that is expressed in the
International Astronomical Union (IAU) J2000.0 system. In particular, NOVAS-C 2.0
supports (but is not limited to) data that conforms to the ICRS. ICRS-compatible data
includes the Hipparcos and Tycho Catalogues [3], the ACT Reference Catalog [4], the
International Celestial Reference Frame (ICRF) [5], the Jet Propulsion Laboratory's
