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A sliver of the surface of a Sphere of Radius 
 cut out by two planes through the azimuthal axis with
Dihedral Angle 
.  The Surface Area of the lune is
See also Lune (Plane), Sphere
References
Beyer, W. H.  CRC Standard Mathematical Tables, 28th ed.  Boca Raton, FL: CRC Press, p. 130, 1987.