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.