Naval Observatory Vector Astrometry Software (NOVAS)
Naval Observatory Vector Astrometry Software (NOVAS)
Version 3.1 Fortran,
C, and
Python Editions
NOVAS
is currently available at the
Astrophysics Source Code Library
.
NOVAS is an integrated package of routines for computing
various commonly needed quantities in positional astronomy. The package can
provide, in one or two subroutine or function calls, the instantaneous
coordinates of any star or planet in a variety of coordinate systems. At a lower
level, NOVAS also supplies astrometric utility
transformations, such as those for precession, nutation, aberration,
parallax, and the gravitational deflection of light. The computations
are accurate to better than one milliarcsecond. The NOVAS package is an
easy-to-use facility that can be incorporated into data reduction programs,
telescope control systems, and simulations. The U.S. parts of
The Astronomical Almanac are
prepared using NOVAS. Three editions of NOVAS are available:
Fortran,
C, and
Python.
The algorithms used by NOVAS are based on a vector and matrix formulation
that is rigorous and does not use spherical trigonometry at any point. Objects inside
and outside the solar system are treated
similarly. The position vectors formed and operated on by NOVAS place each object at
its relevant distance (in au) from the solar system barycenter. Objects at
unknown distance (parallax zero or undetermined) are placed on the "celestial sphere,"
herein defined to be at a radius of 1 gigaparsec (2 x 1014 au).
NOVAS implements the resolutions on positional astronomy adopted by the International
Astronomical Union (IAU) beginning in 1997, including new reference system definitions
and new models for precession and nutation. The paper by
Kaplan et
al. (1989, Astron. J. 97, 1197)
describes the overall computational strategy used
by NOVAS, although many of the individual algorithms described there have been updated and
improved. USNO Circular 179
and The Explanatory
Supplement to the Astronomical Almanac describe
the IAU recommendations that underpin much of NOVAS and are the basic references for
NOVAS algorithms relating to time, Earth orientation, and the transformations between
various astronomical reference systems.
NOVAS assumes that input data are provided in the International
Celestial Reference System (ICRS), or at least are consistent with it to within the
data's inherent accuracy. This requirement allows NOVAS to make proper use of reference
data from modern sources such as
Hipparcos
,
Tycho-2
,
UCAC
, the
VLBA Calibrator Survey
,
2MASS
, and the
JPL planetary ephemerides
.
NOVAS includes three levels of routines: basic, utility, and
supervisory. Basic-level routines supply the values of fundamental
variables, such as the nutation angles and the heliocentric positions of
solar system bodies, for specific epochs. Utility-level routines perform
transformations corresponding to precession, nutation, aberration, etc.
Supervisory-level routines call the basic and utility routines in the
proper order to compute apparent, topocentric, or astrometric places of stars
or solar system bodies for specific dates and times. If desired, the user
can interact exclusively with the supervisory-level routines without becoming
concerned with the details of the geometry or physical models involved in the
computation.
Some of the basic- and utility-level routines are provided in several
versions to accommodate users with a need for alternative algorithms for some
of the computations. For example, different nutation formulations are available for
applications where speed of computation, but not the highest accuracy, is a requirement.
Similarly, several versions of the routines that supply information on the
positions of solar system bodies are included. For those routines that exist
in multiple versions, only the internal logic differs; the argument list and usage are
identical.
If you use NOVAS, please send us an email
that outlines your application. This information helps justify further improvements to NOVAS.
Your comments and suggestions are also welcome.
For guidance on citing the current version of NOVAS, please see the
NOVAS FAQ or appropriate user's guide.
