ECI2GEO

     Convert Earth-centered inertial coordinates to geographic spherical coords

     Converts from ECI (Earth-Centered Inertial) (X,Y,Z) rectangular
     coordinates to geographic spherical coordinates (latitude, longitude,
     altitude).    JD time is also needed as input.
     ECI coordinates are in km from Earth center.
     Geographic coordinates are in degrees/degrees/km
     Geographic coordinates assume the Earth is a perfect sphere, with radius
     equal to its equatorial radius.

     gcoord=eci2geo(ECI_XYZ,JDtime)

       ECI_XYZ : the ECI [X,Y,Z] coordinates (in km), can be an array [3,n]
                 of n such coordinates.
       JDtime: the Julian Day time, double precision. Can be a 1-D array of n
                 such times.

       None

       a 3-element array of geographic [latitude,longitude,altitude], or an
         array [3,n] of n such coordinates, double precision

       None

       CT2LST - Convert Local Civil Time to Local Mean Sidereal Time

       IDL> gcoord=eci2geo([6378.137+600,0,0], 2452343.38982663D)
       IDL> print,gcoord
       0.0000000       232.27096       600.00000
       (The above is the geographic direction of the vernal point on
       2002/03/09 21:21:21.021, in geographic coordinates. The chosen
       altitude was 600 km.)
       gcoord can be further transformed into geodetic coordinates (using
       geo2geodetic.pro) or into geomagnetic coordinates (using geo2mag.pro)

       Written by Pascal Saint-Hilaire ([email protected]) on
              2001/05/13
       Modified on 2002/05/13, PSH : vectorization + use of JD times

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