pro planet_coords, date, ra, dec, planet=planet, jd = jd, jpl = jpl ;+ ; NAME: ; PLANET_COORDS ; PURPOSE: ; Find low-precision RA and DEC for the planets given a date ; ; EXPLANATION: ; This routine uses HELIO to get the heliocentric ecliptic coordinates ; of the planets at the given date, then converts these to geocentric ; ecliptic coordinates ala "Astronomical Algorithms" by Jean Meeus ; (1991, p 209). These are then converted to RA and Dec using EULER. ; The accuracy between the years 1800 and 2050 is better than 1 arcminute for ; the terrestial planets, but reaches 10 arcminutes for Saturn. Before ; 1850 or after 2050 the accuracy can get much worse. ; ; A /JPL option is also available to use the full JPL ephemeris for much ; higher accuracy. ; CALLING SEQUENCE: ; PLANET_COORDS, DATE, RA, DEC, [ PLANET = , /JD, /JPL] ; ; INPUTS: ; DATE - If /JD is not set, then date is a 3-6 element vector containing ; year,month (1-12), day, and optionally hour, minute, & second. ; If /JD is set then DATE is a Julian date. An advantage of the ; /JD option is that it allows the use of vector dates. ; OUTPUTS: ; RA - right ascension of planet(s), J2000 degrees, double precision ; DEC - declination of planet(s), J2000 degrees, double precision ; ; OPTIONAL INPUT KEYWORD: ; PLANET - scalar string giving name of a planet, e.g. 'venus'. Default ; is to compute coords for all of them (except Earth). ; /JD - If set, then the date parameter should be supplied as Julian date ; JPL - if /JPL set, then PLANET_COORDS will call the procedure ; JPLEPHINTERP to compute positions using the full JPL ephemeris. ; The JPL ephemeris FITS file JPLEPH.405 must exist in either the ; current directory, or in the directory specified by the ; environment variable ASTRO_DATA. Alternatively, the JPL keyword ; can be set to the full path and name of the ephemeris file. ; A copy of the JPL ephemeris FITS file JPLEPH.405 is available in ; http://idlastro.gsfc.nasa.gov/ftp/data/ ; EXAMPLES: ; (1) Find the RA, Dec of Venus on 1992 Dec 20 ; IDL> planet_coords, [1992,12,20], ra,dec ;Compute for all planets ; IDL> print,adstring(ra[1],dec[1],1) ;Venus is second planet ; ====> RA = 21 05 2.66 Dec = -18 51 45.7 ; This position is 40" from the full DE2000 ephemeris position of ; RA = 21 05 5.38 -18 51 35.6 ; ; (2) Return the current RA and Dec of all 8 planets using JPL ephemeris ; IDL> get_juldate, jd ;Get current Julian Date ; IDL> planet_coords,jd,ra,dec,/jd,/jpl ;Find positions of all planets ; IDL> forprint,adstring(ra,dec,0) ;Display positions ; ; (3) Plot the declination of Mars for every day in the year 2001 ; IDL> jdcnv,2001,1,1,0,jd ;Get Julian date of midnight on Jan 1 ; Now get Mars RA,Dec for 365 consecutive days ; IDL> planet_coords,jd+indgen(365),ra,dec,/jd, planet = 'mars' ; IDL> plot,indgen(365)+1,dec ; NOTES: ; HELIO is based on the two-body problem and neglects interactions ; between the planets. This is why the worst results are for ; Saturn. Use the /JPL option or the online ephemeris generator ; http://ssd.jpl.nasa.gov/cgi-bin/eph for more accuracy. ; ; The procedure returns astrometric coordinates, i.e. no correction ; for aberration. A correction for light travel time is applied ; when /JPL is set, but not for the default low-precision calculation. ; PROCEDURES USED: ; JULDATE ; EULER, HELIO - if /JPL is not set ; JPLEPHREAD, JPLEPHINTERP - if /JPL is set ; REVISION HISTORY: ; Written P.Plait & W. Landsman August 2000 ; Fixed Julian date conversion W. Landsman August 2000 ; Added /JPL keyword W. Landsman July 2001 ;- On_error,2 if N_params() LT 1 then begin print,'Syntax - planet_coords, date, ra,dec, [PLANET =, /JD , JPL= ]' print,' date - either 3-6 element date or Julian date (if /JD is set)' print,' ra,dec - output ra and dec in degrees' print,' PLANET - name of planet (optional)' return endif radeg = 180.0d/!DPI c = 2.99792458d5 ;convert input date to real JD if keyword_set(jd) then begin jj = date if N_elements(jj) GT 0 then if N_elements(planet) GT 1 then $ message,'ERROR - A planet name must be supplied for vector dates' endif else begin juldate,date,jj jj = jj + 2400000.0d endelse ;make output arrays to include each planet ; note that we need Earth to convert from heliocentric ; ecliptic coordinates to geocentric and then to RA and DEC if keyword_set(planet) then begin planetlist = ['MERCURY','VENUS','MARS', $ 'JUPITER','SATURN','URANUS','NEPTUNE','PLUTO'] index = 1+ where(planetlist eq strupcase(strtrim(planet,2)), Nfound) if index[0] GE 3 then index = index + 1 if Nfound EQ 0 then message,'Unrecognized planet of ' + planet endif else index = [1,2,4,5,6,7,8,9] if keyword_set(JPL) then begin if size(jpl,/TNAME) EQ 'STRING' then jplfile = jpl else $ jplfile = find_with_def('JPLEPH.405','ASTRO_DATA') if jplfile EQ '' then message,'ERROR - Cannot find JPL ephemeris file' ;Read ephemeris FITS file JPLEPHREAD,jplfile, pinfo, pdata, [long(min(jj)-1), long(max(jj)+1)] n = N_elements(index) ra = dblarr(n) & dec = dblarr(n) for i=0, N_elements(index)-1 do begin JPLEPHINTERP, pinfo, pdata, jj, x,y,z, objectname=index[i] JPLEPHINTERP, pinfo, pdata, jj, xe,ye,ze, /EARTH x = x-xe & y = y-ye & z = z-ze ra[i] = atan(y,x) * radeg g = where(ra LT 0, Ng) if Ng GT 0 then ra[g] = ra[g] + 360.0d dec[i] = atan(z,sqrt(x*x + y*y)) * radeg print,adstring(ra[i],dec[i],1) ; Compute distance to planet(s) and adjust Julian date for light travel time ; and recompute planet positions dis = sqrt(x^2 + y^2 + z^2) jj1 = jj - dis/c/86400.0d JPLEPHINTERP, pinfo, pdata, jj1, x,y,z, objectname=index[i] x = x-xe & y = y-ye & z = z-ze ra[i] = atan(y,x) * radeg g = where(ra LT 0, Ng) if Ng GT 0 then ra[g] = ra[g] + 360.0d dec[i] = atan(z,sqrt(x*x + y*y)) * radeg print,adstring(ra[i],dec[i],1) endfor return endif helio,jj,index,rad,lon,lat,/radian ; extract Earth's info helio,jj,3,rade,lone,late,/radian ;get rectangular coords of planets x = rad * cos(lat) * cos(lon) - rade * cos(late) * cos(lone) y = rad * cos(lat) * sin(lon) - rade * cos(late) * sin(lone) z = rad * sin(lat) - rade * sin(late) ;get geocentric longitude lambda and geo latitude, beta lambda = atan(y,x) * radeg beta = atan(z,sqrt(x*x + y*y)) * radeg ;convert to Ra and Dec euler, lambda, beta, ra, dec, 4 return end