function cos_f, mean_anom, ecc
;+
; NAME:
;	cos_f
;
; PURPOSE:
;	This function returns the cosine of the true anomaly, using relations given in 
;	Murray & Dermott (1999), ch. 2.4. This routine uses keplereq.pro.
;
; CATEGORY:
;	Celestial Mechanics.
;
; CALLING SEQUENCE:
;	Result = cos_f(mean_anom, ecc)
;
; INPUTS:
;	mean_anom: orbital mean anomaly
;	ecc: orbital eccentricity
;
; OUTPUTS:
;	Cosine of the orbital true anomaly	
;
; EXAMPLE:
;       ;make mean anomaly array
;	num = 101
;	mean_anom = 2.*!pi*dindgen(num)
;	;orbital eccentricity of 0.1
;	ecc = 0.1
;	cos_f = cos_f(mean_anom, ecc)
;
; MODIFICATION HISTORY:
; 	Written by:	Brian Jackson (decaelus@gmail.com), 2011 Jul 25.
;-

  ;Call keplereq to calculate eccentric anomaly from mean anomaly
  ecc_anom = keplereq(mean_anom, ecc)
  return, (cos(ecc_anom)-ecc)/(1.-ecc*cos(ecc_anom))

end;function