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diff --git a/math/slalib/doc/polmo.hlp b/math/slalib/doc/polmo.hlp new file mode 100644 index 00000000..b3f68383 --- /dev/null +++ b/math/slalib/doc/polmo.hlp @@ -0,0 +1,87 @@ +.help polmo Jun99 "Slalib Package" +.nf + + SUBROUTINE slPLMO ( ELONGM, PHIM, XP, YP, ELONG, PHI, DAZ ) + + - - - - - - + P L M O + - - - - - - + + Polar motion: correct site longitude and latitude for polar + motion and calculate azimuth difference between celestial and + terrestrial poles. + + Given: + ELONGM d mean longitude of the observer (radians, east +ve) + PHIM d mean geodetic latitude of the observer (radians) + XP d polar motion x-coordinate (radians) + YP d polar motion y-coordinate (radians) + + Returned: + ELONG d true longitude of the observer (radians, east +ve) + PHI d true geodetic latitude of the observer (radians) + DAZ d azimuth correction (terrestrial-celestial, radians) + + Notes: + + 1) "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 slOBS routine are west-positive, following + astronomical usage, and must be reversed in sign before use in + the present routine. + + 2) 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. + + 3) "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). + + 4) The azimuths produced by slAOP and slAOPQ 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. + + 5) The present routine is rigorous. For most practical + purposes, the following simplified formulae provide an + adequate approximation: + + ELONG = ELONGM+XP*COS(ELONGM)-YP*SIN(ELONGM) + PHI = PHIM+(XP*SIN(ELONGM)+YP*COS(ELONGM))*TAN(PHIM) + DAZ = -SQRT(XP*XP+YP*YP)*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(-X*YP-Y*XP,X*X+Y*Y) + + Reference: Seidelmann, P.K. (ed), 1992. "Explanatory Supplement + to the Astronomical Almanac", ISBN 0-935702-68-7, + sections 3.27, 4.25, 4.52. + + P.T.Wallace Starlink 22 February 1996 + + Copyright (C) 1995 Rutherford Appleton Laboratory + Copyright (C) 1995 Association of Universities for Research in Astronomy Inc. + +.fi +.endhelp |