Correct for polar motion palPolmo
"
Mean"
longitude and latitude are the (fixed) values for the site’
s location with respect to the IERS
terrestrial reference frame; the latitude is geodetic. TAKE CARE WITH THE LONGITUDE SIGN
CONVENTION. The longitudes used by the present routine are east-positive, in accordance with
geographical convention (and right-handed). In particular, note that the longitudes returned by the
sla_OBS routine are west-positive, following astronomical usage, and must be reversed in sign before
use in the present routine.
XP and YP are the (changing) coordinates of the Celestial Ephemeris Pole with respect to the IERS Reference Pole. XP is positive along the meridian at longitude 0 degrees, and YP is positive along the meridian at longitude 270 degrees (i.e. 90 degrees west). Values for XP,YP can be obtained from IERS circulars and equivalent publications; the maximum amplitude observed so far is about 0.3 arcseconds.
"
True"
longitude and latitude are the (moving) values for the site’
s location with respect to the
celestial ephemeris pole and the meridian which corresponds to the Greenwich apparent sidereal
time. The true longitude and latitude link the terrestrial coordinates with the standard celestial models
(for precession, nutation, sidereal time etc).
The azimuths produced by sla_AOP and sla_AOPQK are with respect to due north as
defined by the Celestial Ephemeris Pole, and can therefore be called "
celestial azimuths"
. However, a telescope fixed to the Earth measures azimuth essentially with respect to
due north as defined by the IERS Reference Pole, and can therefore be called "
terrestrial
azimuth"
. Uncorrected, this would manifest itself as a changing "
azimuth zero-point error"
.
The value DAZ is the correction to be added to a celestial azimuth to produce a terrestrial
azimuth.
The present routine is rigorous. For most practical purposes, the following simplified formulae provide an adequate approximation:
elong = elongmxpcos(elongm)-ypsin(elongm) phi = phim(xpsin(elongm)ypcos(elongm))tan(phim) daz = -sqrt(xpxpypyp)cos(elongm-atan2(xp,yp))/cos(phim)
An alternative formulation for DAZ is:
x = cos(elongm)cos(phim) y = sin(elongm)cos(phim) daz = atan2(-xyp-yxp,xxyy)
Reference: Seidelmann, P.K. (ed), 1992. "
Explanatory Supplement to the Astronomical Almanac"
,
ISBN 0-935702-68-7, sections 3.27, 4.25, 4.52.