PRO altaz2hadec, alt, az, lat, ha, dec ;+ ; NAME: ; ALTAZ2HADEC ; PURPOSE: ; Convert Horizon (Alt-Az) coordinates to Hour Angle and Declination. ; EXPLANATION:: ; Can deal with the NCP singularity. Intended mainly to be used by ; program hor2eq.pro ; CALLING SEQUENCE: ; ALTAZ2HADEC, alt, az, lat, ha, dec ; ; INPUTS ; alt - the local apparent altitude, in DEGREES, scalar or vector ; az - the local apparent azimuth, in DEGREES, scalar or vector, ; measured EAST of NORTH!!! If you have measured azimuth west-of-south ; (like the book MEEUS does), convert it to east of north via: ; az = (az + 180) mod 360 ; ; lat - the local geodetic latitude, in DEGREES, scalar or vector. ; ; OUTPUTS ; ha - the local apparent hour angle, in DEGREES. The hour angle is the ; time that right ascension of 0 hours crosses the local meridian. ; It is unambiguously defined. ; dec - the local apparent declination, in DEGREES. ; ; EXAMPLE: ; Arcturus is observed at an apparent altitude of 59d,05m,10s and an ; azimuth (measured east of north) of 133d,18m,29s while at the ; latitude of +43.07833 degrees. ; What are the local hour angle and declination of this object? ; ; IDL> altaz2hadec, ten(59,05,10), ten(133,18,29), 43.07833, ha, dec ; ===> Hour angle ha = 336.683 degrees ; Declination, dec = 19.1824 degrees ; ; The widely available XEPHEM code gets: ; Hour Angle = 336.683 ; Declination = 19.1824 ; ; REVISION HISTORY: ; Written Chris O'Dell Univ. of Wisconsin-Madison May 2002 ;- if N_params() LT 4 then begin print,'Syntax - ALTAZ2HADEC, alt, az, lat, ha, dec' return endif d2r = !dpi/180.0d alt_r = alt*d2r az_r = az*d2r lat_r = lat*d2r ;****************************************************************************** ; find local HOUR ANGLE (in degrees, from 0. to 360.) ha = atan( -sin(az_r)*cos(alt_r), \$ -cos(az_r)*sin(lat_r)*cos(alt_r)+sin(alt_r)*cos(lat_r)) ha = ha / d2r w = where(ha LT 0.) if w[0] ne -1 then ha[w] = ha[w] + 360. ha = ha mod 360. ; Find declination (positive if north of Celestial Equator, negative if south) sindec = sin(lat_r)*sin(alt_r) + cos(lat_r)*cos(alt_r)*cos(az_r) dec = asin(sindec)/d2r ; convert dec to degrees END