SubL1Accel:

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   Find a sail acceleration to maintain a Sub-L1 station. 
   L1 is at about 0.01 AU, equivalent to about 237 Earth radii.
   Assumes a circular orbit of the Earth-Moon barycenter at 1 AU.
   The angles theta and phi measure the displacement of the sail from the
   Sun-Earth line.

   Note: Unlike Carl Sauer's plotted results, a nonzero theta or phi seem to 
   push the acceleration completely off the radial direction.

   Since version 7.
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   Form:
   [a, a0, alpha] = SubL1Accel( dX, theta, phi )
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   Inputs
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   dX         Distance from the Earth, AU
   theta      Ecliptic longitude (rad)
   phi        Ecliptic latitude (rad)


   -------
   Outputs
   -------
   a          Sail acceleration at station
   a0         Characteristic accel, km/s2
   alpha      Sail normal angle
 
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   Reference: Carl G. Sauer, The L1 Diamond Affair. 14th AAS/AIAA Space Flight
              Mechanics Conference. Maui, Hawaii, 2004. AAS 04-278
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Children:

Math: Linear/Mag
Math: Linear/Unit