binarypos,time,pos,asini=asini,porb=porb,eccentricity=ecc,omega=omega_d, $
              t90=t90,t0=t0, pporb=pporb,limit=limit

               time   : event time (MJD)

       Mandatory:
               porb   : Orbital period at the epoch [same units as
                        time argument]
               eccentricity : Eccentricity, (0<=e<1)
               omega  : Longitude of periastron [degrees]
       mandatory for spectroscopic binaries:
               asini  : Projected semi-major axis (a sin i),
                        Mandatory for spectroscopic binaries,
                        give it in light seconds (a sini/c) if you
                        want to compute a time correction
       mandatory for visual binaries:
               semi   : semimajor axis
               bigomega: Longitude of the ascending node [degrees]
               incl   : inclination [degrees]
       the epoch is given by one of the following:
               T0     : epoch for mean longitude of 0 degrees
                        (periastron, MJD)
               T90    : epoch for mean longitude of 90 degrees (MJD)
           T90 and T0 have to be in days and in the same time
           system as time (e.g. JD or MJD)
         Circular orbits:
              * if time of lower conjunction Tlow is known, set
                T0=Tlow and omega=0
              * if time of ascending node is known, Tasc, set
                T90=Tasc and omega=0
              * if time of mid eclipse is known, Tecl, set
                T0=Tecl-0.5*porb and omega=0

               pporb: rate of change of the orbital period (s/s)

                pos : spectroscopic binaries:
                            1d array containing the z position of the object
                            (negative: closer to Earth)
                      visual binaries:
                            3d array containing the xyz position of
                            the object in a right handed coordinate
                            system where the z axis points away from
                            Earth, the x axis points towards
                            the North Celestial Pole, and where the
                            y-axis increases towards East.

   Follows equations from Hilditch's book

 ;; reproduce Hilditch, Fig. 2.11
 ;; (without the pulse delay
 ;; B1913+16
 asini=2.3371 ;; lt-sec
 ecc=0.627
 porb=27906.98d0 ;; seconds
 omega=178.9  ;; degrees
 t0=0d0       ;; arbitrary
 tstart=-0.2*porb
 tstop=+1.2*porb
 npt=200
 time=tstart+(tstop-tstart)*findgen(npt)/(npt-1)
 binarypos,time,pos,asini=asini,eccentricity=ecc,omega=omega, $
   t0=t0,porb=porb
 plot,time/porb,pos,xstyle=1,yrange=[-4.,+4.],ystyle=1
 ;; reproduce Hilditch Fig. 214
 ;; (there is a sign problem!)
 ecc=0.6
 semi=1.
 incl=30.
 omega=0.
 bigomega=135.
 porb=1.
 t0=0.
 npt=100.
 time=dindgen(npt)/(npt-1)
 binarypos,time,pos,eccentricity=ecc,omega=omega, $
   t0=t0,porb=porb,semi=semi,bigomega=bigomega,incl=incl
 ;; NB: x-coordinate is towards NORTH!
 ;; plot such that East is on the left (telescopic sight)
 plot,pos[1,*],-pos[0,*], $
   xrange=[-1.6,+1.6], $
   yrange=[-1.6,+1.6], $
   xstyle=1,ystyle=1

 $Log: binarypos.pro,v $
 Revision 1.1  2005/05/25 16:08:22  wilms
 initial release

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