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Astronomical Applications Department, U.S. Naval Observatory - NOVAS 2004 Overview (Page 3)

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Astronomical Applications Department, U.S. Naval Observatory - NOVAS 2004 Overview
3
New Features

Subroutines have been added to NOVAS that provide new functionality and convenience:

PLACE: A new general-purpose apparent place subroutine (see paragraph above).

IDSS: An integer function that returns a solar system body's identification number (which is used in
various NOVAS subroutine calls), given the body's name as a character string. For example,
IDSS(`MARS')
usually equals 4. Because IDSS is a function, it can be referred to within calls to other
NOVAS subroutines, e.g.,
CALL APPLAN ( TTJD, IDSS(`JUPITER'), IDSS(`EARTH'), RAJUP, DECJUP, DISJUP )
(If you supply your own version of subroutine SOLSYS, you must also now supply a corresponding
version of IDSS.)

GETVEC: A subroutine that returns the last NOVAS-computed celestial position (apparent or
astrometric place, etc.) as a unit vector. The vector is expressed in the same reference system as the
previously supplied spherical coordinates.

EQECL: Converts right ascension and declination to ecliptic longitude and latitude.

EQGAL: Converts ICRS right ascension and declination to galactic longitude and latitude.

ICRSEQ: Converts ICRS right ascension and declination to one of the equatorial systems of date.

ASTCON: Supplies the value of an astronomical constant, given its name as a character string. The
values of all fundamental astronomical constants used by NOVAS are stored within this subroutine and
nowhere else. The names of the constants available and the units used for each are listed in the
subroutine's preamble. For example,
CALL ASTCON (`ERAD',1.D0,RADIUS)
returns, in argument
RADIUS
, the value of the equatorial radius of the Earth in meters
.

New Models for Precession and Nutation

It has been known for over a decade that the standard models for the precession and nutation of the
Earth's rotation axis have been in need of revision. The value of the angular rate of precession in lon-
gitude adopted by the IAU in 1976 -- and incorporated into the widely used Lieske et al. (1977) pre-
cession formulation -- is too large by about 0.3 arcsecond per century (3 mas/yr). The amplitudes of a
number of the largest nutation components specified in the 1980 IAU Theory of Nutation are also
known to be in error by several milliarcseconds. Both the precession and nutation errors are signifi-
cant relative to current observational capabilities.

Thus, the resolutions passed by the IAU in 2000 mandated an improvement to the precession and
nutation formulations. The new version of NOVAS incorporates the models adopted by the IERS in
response to these resolutions. (As mentioned above, the precession model is still a work in progress,
and an IAU Working Group is in the process of finalizing a dynamically consistent model. NOVAS
version F2.9 contains a preliminary IERS formulation of precession and NOVAS version F3.0 will
contain the final IAU model. The difference in results, for dates within several decades of the year
2000, will be negligible for practical purposes.) Despite the new models, from a programming point of
view, the subroutines that directly involve precession and nutation -- PRECES, NUTATE, ETILT,
NOD, and SIDTIM -- work the same as before, but with slightly different results. It should be noted

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