;;;
;;; Calculation sunrise/sunset
;;; example from Nautical Almanac Office, United States Naval Observatory
;;;
;;; Inputs:
;;;	day, month, year:      date of sunrise/sunset
;;;	latitude, longitude:   location for sunrise/sunset
;;;	zenith:                Sun's zenith for sunrise/sunset
;;;	  offical      = 90 degrees 50'
;;;	  civil        = 96 degrees
;;;	  nautical     = 102 degrees
;;;	  astronomical = 108 degrees
;;;
(define (day-of-year dd mm yyyy) 
 (let* ((N1 (floor (/ (* 275 mm) 9))) 
        (N2 (floor (/ (+ 9 mm) 12))) 
        (N3 (+ 1 (floor (/ (- yyyy (* 4 (floor (/ yyyy 4))) -2) 3)))) 
        (N (- N1 (* N2 N3) (- dd) 30))) 
   N))
(define (hrmin T offset) 
  (let* ((ohr (floor T)) 
         (h (modulo (+ ohr offset) 24)) 
         (m (floor (* 60 (- T ohr))))) 
    (string-append (if (> 10 h) "0" "") (number->string h) ":" 
                   (if (> 10 m) "0" "") (number->string m) 
                   " UTC" (if (> 0 offset) "-" "+") 
                   (number->string (abs offset)))))  
(define (inbound L) 
  (cond ((> L 360) (- L 360)) 
        ((< L 360) (+ L 360)) 
        (else L)))
(define (sind x) (sin (* 0.01745329 x)))
(define (cosd x) (cos (* 0.01745329 x)))
(define (tand x) (tan (* 0.01745329 x)))
(define (asind x) (* 57.29578 (asin x)))
(define (acosd x) (* 57.29578 (acos x)))
(define (atand x) (* 57.29578 (atan x)))

(define (sunrise dd mm yyyy latitude longitude rise zenith offset)  
 "sunrise dd mm yyyy lat long rise [rise/set] zenith [90 5/6] offset [10]"
 (let* ((lngHour (/ longitude 15))  
        (N (day-of-year dd mm yyyy))  
        (t (if (string=? rise "rise")  
              (+ N (/ (- 6 lngHour) 24))  
              (+ N (/ (- 18 lngHour) 24))))  
        (M (- (* 0.9856 t) 3.289))  
        (L (inbound (+ M (* 1.916  (sind M)) (* 0.020 (sin (* 2 M))) 282.634)))  
        (RA (atand (* 0.91764 (tand L))))  
        (Lquadrant  (* (floor (/ L 90))  90))  
        (Rquadrant (* (floor (/ RA 90))  90))  
        (RAH  (/ (+ RA (- Lquadrant Rquadrant)) 15))  
        (sinDec (* 0.39782 (sind L)))  
        (cosDec (cosd (asind sinDec)))  
        (cosH (/ (- (cosd zenith) (* sinDec (sind latitude)))  
                 (* cosDec (cosd latitude))))  
        (H  (if (string=? rise "rise")  
               (/ (- 360 (acosd cosH)) 15)  
               (/ (acosd cosH) 15)))  
        (T (+ H RAH (- (* 0.06571 t)) (- 6.622)))  
        (UT (- T lngHour)))  
    (hrmin UT offset)))

(define (sunrise-today)  
 (let* ((dy (substring (date) 0 8))  
        (yyyy (string->number (substring dy 0 4)))  
        (mm (string->number (substring dy 4 6)))  
        (dd (string->number (substring dy 6 8)))   
        (latitude -27.5)   
        (longitude 153)   
        (zenith (+ 90 (/ 5 6)))  
        (offset 10))  
   (display (string-append "Date   : " dy)) (newline)  
   (display (string-append "Sunrise: " (sunrise dd mm yyyy latitude longitude "rise" zenith offset)))  
   (newline)  
   (display (string-append "Sunset : " (sunrise dd mm yyyy latitude longitude "set" zenith offset)))))