function EqKepler, E, arg

@compile_opt.pro		; On error, return to caller

;+
; NAME:
;	EqKepler
; PURPOSE:
;	Kepler's equation is used in the determination of orbital positions.
;	It gives the relation between mean and eccentric anomaly (for given
;	orbit eccentricity).
; CATEGORY:
;	smei/gen/idl/ephem
; CALLING SEQUENCE:
;	F = EqKepler(E,arg)
; INPUTS:
;	E		scalar; type: float
;				eccentric anomaly (radians)
;	arg		array[2], type: float
;				arg[0]	mean anomaly 	  (radians)
;				arg[1]	orbit ellipticity (eccentricity)
; OUTPUTS:
;	EqKepler	fnc value
; PROCEDURE:
;	The eccentric anomaly is the root of Kepler's equation:
;	EqKepler = X-arg[1]*sin(X)-arg[0] = 0
;	See O. Montenbruck, "Practical Ephemeris Calculations", Springer
;	(1989), par. 3.1.1.4, p. 44
; MODIFICATION HISTORY:
;	1990, Paul Hick (UCSD)
;-

if arg[1] lt 1 then				$	; Bound orbits
	F = E-arg[1]*sin (E)-arg[0]	$
else							$
	F = E-arg[1]*sinh(E)+arg[0]

return, F  &  end