function sol_zen_ang,doy,time,lat,lon

;-------------------------------------------------------------------
; Routine calculates the solar zenith angle (SZA)  
; 
;
; Input:
;   doy......day of year
;   time.....time of day, hours GMT
;   lat......latitude, degrees
;   lon......longitude, degrees
;
; Output:
;   SZA......solar zenith angle, degrees
;
; Requirements: solar_time.pro
;
; Source: Mark Hervig
;-------------------------------------------------------------------

;- calculate the solar declination angle

   sdec =  23.5* cos(2.* !pi* doy /365.)  ; deg
   
;- calculate the LST

   LST = solar_time(time,lon)  ; hrs

;- calculate the SZA

   arg1_ang  = sin(lat* !pi/180.)*sin(sdec* !pi/180.)
   
   arg2_ang  = cos(2.*!pi*(LST - 12.)/24.) * cos(lat*!pi/180.) * cos(sdec*!pi/180.)
   
   chi       = acos(arg1_ang + arg2_ang)
   
   SZA       = 180.* chi / !pi

;- done

   return,SZA
   
   end