5
The following table summarizes the two equivalent procedures for hour angle and the NOVAS
subroutines that would be used for each, assuming that polar motion is neglected. The procedures
outlined here provide the Greenwich hour angle of a star.
Equinox-Based Method
CEO-Based Method
Use
subroutine
APSTAR followed by TPSTAR
-- or --
PLACE with OBJECT=
STAR,
LOCATN=1, and ICOORD=1
PLACE with OBJECT=
STAR,
LOCATN=1, and ICOORD=2
... to obtain
RA and DEC, the topocentric
apparent right ascension and
declination of the star with respect to
the equator and equinox of date (in
hours and degrees, respectively)
RA and DEC, the topocentric
apparent right ascension and
declination of the star with respect to
the equator and CEO of date (in
in hours and degrees, respectively)
Then use
subroutine
SIDTIM with K=1
EROT
... to obtain
GST, Greenwich apparent sidereal
time (in hours)
THETA, the Earth rotation angle,
(in degrees)
Compute
Greenwich
hour angle
GHA = GST RA,
(in hours)
GHA = THETA / 15.D0 RA,
(in hours)
The computed GHA may have to be reduced to the range
-12
h
to +12
h
. Subroutines APSTAR and
PLACE require time arguments in the TT time scale, while TPSTAR, SIDTIM, and EROT require
time arguments in the UT1 time scale. The two procedures should yield the same value of GHA to
within several microarcseconds and identical values for DEC.
Two high-level NOVAS subroutines that involve Earth rotation, SIDTIM and TERCEL (the latter
replaces the old PNSW) can actually perform their internal calculations using either the equinox-based
paradigm or the CEO-based paradigm.
2
(Note: ZDAZ is also affected because it calls TERCEL.) The
method used is selected by a prior call to either EQINOX or CEOTEO (without arguments), which
remains in effect until changed. Since there is no external difference in how SIDTIM or TERCEL are
used, and the two computational paradigms yield answers that are consistent within a few micro-
arcseconds over many centuries, there is seldom a practical basis for a choice. However, the equinox
method must be used for dates before 1700 or after 2300, and is much more efficient if mean sidereal
time is to be computed. The equinox-based paradigm is the default, that is, it is used unless CEOTEO
has been called. That will, of course, be the case for any existing programs that are not updated to
make this choice explicit.
Another choice is now available that has a more practical effect: Earth rotation calculations can be per-
formed in either high- or low-accuracy mode. A call to either HIACC or LOACC (without arguments)
sets the accuracy, which remains in effect until changed. High-accuracy mode is the default, with the
various models evaluated at the few-microarcsecond level. For nutation, for example, this means that
2
It may seem odd that sidereal time can be computed using the CEO-based paradigm, but all that is
needed is the angle between the equinox and the CEO (both of which lie in the equatorial plane), and this
is straightforward to compute if we know the location of both points in the ICRS.
