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+;;; solar.el --- calendar functions for solar events.
+
+;; Copyright (C) 1992 Free Software Foundation, Inc.
+
+;; Author: Edward M. Reingold <reingold@cs.uiuc.edu>
+;; Keywords: sunrise, sunset, equinox, solstice, calendar, diary, holidays
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY.  No author or distributor
+;; accepts responsibility to anyone for the consequences of using it
+;; or for whether it serves any particular purpose or works at all,
+;; unless he says so in writing.  Refer to the GNU Emacs General Public
+;; License for full details.
+
+;; Everyone is granted permission to copy, modify and redistribute
+;; GNU Emacs, but only under the conditions described in the
+;; GNU Emacs General Public License.   A copy of this license is
+;; supposed to have been given to you along with GNU Emacs so you
+;; can know your rights and responsibilities.  It should be in a
+;; file named COPYING.  Among other things, the copyright notice
+;; and this notice must be preserved on all copies.
+
+;;; Commentary:
+
+;; This collection of functions implements the features of calendar.el and
+;; diary.el that deal with sunrise/sunset and eqinoxes/solstices.
+
+;; Based on the ``Almanac for Computers 1984,'' prepared by the Nautical
+;; Almanac Office, United States Naval Observatory, Washington, 1984 and
+;; on ``Astronomical Formulae for Calculators,'' 3rd ed., by Jean Meeus,
+;; Willmann-Bell, Inc., 1985.
+;;
+;; WARNINGS:
+;;    1. SUNRISE/SUNSET calculations will be accurate only to +/- 2 minutes.
+;;       Locations should be between +/- 65 degrees of latitude.
+;;       Dates should be in the latter half of the 20th century.
+;;
+;;    2. Equinox/solstice times will be accurate only to +/- 15 minutes.
+
+;; The author would be delighted to have an astronomically more sophisticated
+;; person rewrite the code for the solar calculations in this file!
+
+;; Comments, corrections, and improvements should be sent to
+;;  Edward M. Reingold               Department of Computer Science
+;;  (217) 333-6733                   University of Illinois at Urbana-Champaign
+;;  reingold@cs.uiuc.edu             1304 West Springfield Avenue
+;;                                   Urbana, Illinois 61801
+
+;;; Code:
+
+(if (fboundp 'atan)
+    (require 'lisp-float-type)
+  (error "Solar calculations impossible since floating point is unavailable."))
+
+(require 'calendar)
+
+(defun solar-setup ()
+  "Prompt user for latitude, longitude, and time zone."
+  (beep)
+  (if (not calendar-longitude)
+      (setq calendar-longitude
+            (solar-get-number
+             "Enter longitude (decimal fraction; + east, - west): ")))
+  (if (not calendar-latitude)
+      (setq calendar-latitude
+            (solar-get-number
+             "Enter latitude (decimal fraction; + north, - south): ")))
+  (if (not calendar-time-zone)
+      (setq calendar-time-zone
+            (solar-get-number
+             "Enter difference from Universal Time (in minutes): "))))
+
+(defun solar-get-number (prompt)
+  "Return a number from the minibuffer, prompting with PROMPT.
+Returns nil if nothing was entered."
+  (let ((x (read-string prompt "")))
+    (if (not (string-equal x ""))
+        (string-to-int x))))
+
+(defun solar-sin-degrees (x)
+  (sin (degrees-to-radians x)))
+
+(defun solar-cosine-degrees (x)
+  (cos (degrees-to-radians x)))
+
+(defun solar-tangent-degrees (x)
+  (tan (degrees-to-radians x)))
+
+(defun solar-xy-to-quadrant (x y)
+  "Determines the quadrant of the point X, Y."
+  (if (> x 0)
+      (if (> y 0) 1 4)
+      (if (> y 0) 2 3)))
+
+(defun solar-degrees-to-quadrant (angle)
+  "Determines the quadrant of ANGLE."
+  (1+ (truncate (/ (solar-mod angle 360.0) 90.0))))
+
+(defun solar-arctan (x quad)
+  "Arctangent of X in quadrant QUAD."
+  (let ((deg (radians-to-degrees (atan x))))
+    (cond ((equal quad 2)   (+ deg 180))
+          ((equal quad 3)   (+ deg 180))
+          ((equal quad 4)   (+ deg 360))
+          (t                deg))))
+
+(defun solar-arccos (x)
+  (let ((y (sqrt (- 1 (* x x)))))
+    (solar-arctan (/ y x) (solar-xy-to-quadrant x y))))
+
+(defun solar-arcsin (y)
+  (let ((x (sqrt (- 1 (* y y)))))
+    (solar-arctan (/ y x) (solar-xy-to-quadrant x y))))
+
+(defun solar-mod (x y)
+  "Returns X mod Y; value is *always* non-negative."
+  (let ((v (% x y)))
+    (if (> 0 v)
+        (+ v y)
+      v)))
+
+(defconst solar-earth-inclination 23.441884 
+  "Inclination of earth's equator to its solar orbit in degrees.")
+
+(defconst solar-cos-inclination (solar-cosine-degrees solar-earth-inclination) 
+  "Cosine of earth's inclination.")
+
+(defconst solar-sin-inclination (solar-sin-degrees solar-earth-inclination)
+  "Sine of earth's inclination.")
+
+(defconst solar-earth-orbit-eccentricity 0.016718
+  "Eccentricity of orbit of the earth around the sun.")
+
+(defmacro solar-degrees-to-hours (deg)
+  (list '/ deg 15))
+
+(defmacro solar-hours-to-days (hour)
+  (list '/ hour 24))
+
+(defun solar-longitude-of-sun (day)
+  "Longitude of the sun at DAY in the year."
+  (let ((mean-anomaly (- (* 0.9856 day) 3.289)))
+    (solar-mod (+ mean-anomaly 
+                  (* 1.916 (solar-sin-degrees mean-anomaly))
+                  (* 0.020 (solar-sin-degrees (* 2 mean-anomaly)))
+                  282.634)
+               360)))
+
+(defun solar-right-ascension (longitude)
+  "Right ascension of the sun, given its LONGITUDE."
+  (solar-degrees-to-hours
+   (solar-arctan
+    (* solar-cos-inclination (solar-tangent-degrees longitude))
+    (solar-degrees-to-quadrant longitude))))
+
+(defun solar-declination (longitude)
+  "Declination of the sun, given its LONGITUDE."
+  (solar-arcsin
+   (* solar-sin-inclination
+      (solar-sin-degrees longitude))))
+
+(defun solar-sunrise (date)
+  "Calculates the *standard* time of sunrise for Gregorian DATE for location
+given by `calendar-latitude' and `calendar-longitude'.  Returns a decimal fraction
+of hours.  Returns nil if the sun does not rise at that location on that day."
+  (let* ((day-of-year (calendar-day-number date))
+         (approx-sunrise
+          (+ day-of-year
+             (solar-hours-to-days
+              (-  6 (solar-degrees-to-hours calendar-longitude)))))
+         (solar-longitude-of-sun-at-sunrise
+          (solar-longitude-of-sun approx-sunrise))
+         (solar-right-ascension-at-sunrise
+          (solar-right-ascension solar-longitude-of-sun-at-sunrise))
+         (solar-declination-at-sunrise
+          (solar-declination solar-longitude-of-sun-at-sunrise))
+         (cos-local-sunrise
+          (/ (- (solar-cosine-degrees (+ 90 (/ 50.0 60.0)))
+                (* (solar-sin-degrees solar-declination-at-sunrise)
+                   (solar-sin-degrees calendar-latitude)))
+             (* (solar-cosine-degrees solar-declination-at-sunrise)
+                (solar-cosine-degrees calendar-latitude)))))
+    (if (<= (abs cos-local-sunrise) 1);; otherwise, no sunrise that day
+      (let* ((local-sunrise (solar-degrees-to-hours
+                             (- 360 (solar-arccos cos-local-sunrise))))
+             (local-mean-sunrise
+              (solar-mod (- (+ local-sunrise solar-right-ascension-at-sunrise)
+                            (+ (* 0.065710 approx-sunrise)
+                               6.622))
+                         24)))
+        (+ (- local-mean-sunrise (solar-degrees-to-hours calendar-longitude))
+           (/ calendar-time-zone 60.0))))))
+
+(defun solar-sunset (date)
+  "Calculates the *standard* time of sunset for Gregorian DATE for location
+given by `calendar-latitude' and `calendar-longitude'.  Returns a decimal fractions
+of hours.  Returns nil if the sun does not set at that location on that day."
+  (let* ((day-of-year (calendar-day-number date))
+         (approx-sunset
+          (+ day-of-year
+             (solar-hours-to-days
+              (- 18 (solar-degrees-to-hours calendar-longitude)))))
+         (solar-longitude-of-sun-at-sunset
+          (solar-longitude-of-sun approx-sunset))
+         (solar-right-ascension-at-sunset
+          (solar-right-ascension solar-longitude-of-sun-at-sunset))
+         (solar-declination-at-sunset
+          (solar-declination solar-longitude-of-sun-at-sunset))
+         (cos-local-sunset
+          (/ (- (solar-cosine-degrees (+ 90 (/ 50.0 60.0)))
+                (* (solar-sin-degrees solar-declination-at-sunset)
+                   (solar-sin-degrees calendar-latitude)))
+             (* (solar-cosine-degrees solar-declination-at-sunset)
+                (solar-cosine-degrees calendar-latitude)))))
+    (if (<= (abs cos-local-sunset) 1);; otherwise, no sunset that day
+      (let* ((local-sunset (solar-degrees-to-hours
+                            (solar-arccos cos-local-sunset)))
+             (local-mean-sunset
+              (solar-mod (- (+ local-sunset solar-right-ascension-at-sunset)
+                            (+ (* 0.065710 approx-sunset) 6.622))
+                         24)))
+        (+ (- local-mean-sunset (solar-degrees-to-hours calendar-longitude))
+           (/ calendar-time-zone 60.0))))))
+
+(defun solar-time-string (time date)
+  "Printable form for decimal fraction standard TIME on DATE.
+Format used is given by `calendar-time-display-form'.  Converted to daylight
+savings time according to `calendar-daylight-savings-starts' and
+`calendar-daylight-savings-ends', if those variables are not nil."
+  (let* ((year (extract-calendar-year date))
+         (abs-date (calendar-absolute-from-gregorian date))
+         (dst (and calendar-daylight-savings-starts
+                   calendar-daylight-savings-ends
+                   (<= (calendar-absolute-from-gregorian
+                        (eval calendar-daylight-savings-starts))
+                       abs-date)
+                   (< abs-date
+                      (calendar-absolute-from-gregorian
+                       (eval calendar-daylight-savings-ends)))))
+         (time (if dst (1+ time) time))
+         (time-zone (if dst
+                        calendar-daylight-time-zone-name
+                        calendar-standard-time-zone-name))
+         (24-hours (truncate time))
+         (12-hours (format "%d" (if (> 24-hours 12)
+                                    (- 24-hours 12)
+                                  (if (= 24-hours 0) 12 24-hours))))
+         (am-pm (if (>= 24-hours 12) "pm" "am"))
+         (minutes (format "%02d" (round (* 60 (- time 24-hours)))))
+         (24-hours (format "%02d" 24-hours)))
+    (mapconcat 'eval calendar-time-display-form "")))
+
+(defun solar-sunrise-sunset (date)
+  "String giving local times of sunrise and sunset on Gregorian DATE."
+  (let ((rise (solar-sunrise date))
+        (set (solar-sunset date)))
+    (format "%s, %s at %s"
+            (if rise
+                (concat "Sunrise " (solar-time-string rise date))
+              "No sunrise")
+            (if set
+                (concat "sunset " (solar-time-string set date))
+              "no sunset")
+            (eval calendar-location-name))))
+
+(defun solar-apparent-longitude-of-sun (date)
+  "Apparent longitude of the sun on Gregorian DATE."
+  (let* ((time (/ (- (calendar-absolute-from-gregorian date) 
+                     (calendar-absolute-from-gregorian '(1 0.5 1900)))
+                  36525))
+         (l (+ 279.69668
+               (* 36000.76892 time)
+               (* 0.0003025 time time)))
+         (m (+ 358.47583
+               (* 35999.04975 time)
+               (* -0.000150 time time)
+               (* -0.0000033 time time time)))
+         (c (+ (* (+ 1.919460
+                     (* -0.004789 time)
+                     (* -0.000014 time time))
+                  (solar-sin-degrees m))
+               (* (+ 0.020094
+                     (* -0.000100 time))
+                  (solar-sin-degrees (* 2 m)))
+               (* 0.000293
+                  (solar-sin-degrees (* 3 m)))))
+         (L (+ l c))
+         (omega (+ 259.18
+                   (* -1934.142 time)))
+         (app (+ L
+                 -0.00569
+                 (* -0.00479
+                    (solar-sin-degrees omega)))))
+    app))
+
+(defun solar-ephemeris-correction (year)
+  "Difference in minutes between Ephemeris time an Universal time in YEAR.
+Value is only an approximation."
+  (let ((T (/ (- year 1900) 100.0)))
+    (+ 0.41 (* 1.2053 T) (* 0.4992 T T))))
+
+(defun solar-equinoxes/solstices (k year)
+  "Date of equinox/solstice K for YEAR.  K=0, spring equinox; K=1, summer
+solstice; K=2, fall equinox; K=3, winter solstice.  Accurate to within
+several minutes."
+  (let ((date (list (+ 3 (* k 3)) 21 year))
+        (correction 1000))
+    (while (> correction 0.00001)
+      (setq app (solar-mod (solar-apparent-longitude-of-sun date) 360.0))
+      (setq correction (* 58 (solar-sin-degrees (- (* k 90) app))))
+      (setq date (list (extract-calendar-month date)
+                       (+ (extract-calendar-day date) correction)
+                       year)))
+    (list (extract-calendar-month date)
+          (+ (extract-calendar-day date) (/ calendar-time-zone 60.0 24.0)
+             (- (/ (solar-ephemeris-correction year) 60.0 24.0)))
+          year)))
+
+;;;###autoload
+(defun sunrise-sunset (&optional arg)
+  "Local time of sunrise and sunset for today.  Accurate to +/- 2 minutes.
+If called with an optional prefix argument, prompts for date.
+
+If called with an optional double prefix argument, prompts for longitude,
+latitude, time zone, and date.
+
+This function is suitable for execution in a .emacs file."
+ (interactive "p")
+ (if (< arg 16)
+     (if (not (and calendar-latitude calendar-longitude calendar-time-zone))
+         (solar-setup)))
+ (let* ((calendar-longitude
+         (if (< arg 16)
+             calendar-longitude
+           (solar-get-number
+            "Enter longitude (decimal fraction; + east, - west): ")))
+        (calendar-latitude
+         (if (< arg 16)
+             calendar-latitude
+           (solar-get-number
+            "Enter latitude (decimal fraction; + north, - south): ")))
+        (calendar-time-zone
+         (if (< arg 16)
+             calendar-time-zone
+           (solar-get-number
+            "Enter difference from Universal Time (in minutes): ")))
+        (calendar-location-name
+         (let ((float-output-format "%.1f"))
+           (format "%s%s, %s%s"
+                   (abs calendar-latitude)
+                   (if (> calendar-latitude 0) "N" "S")
+                   (abs calendar-longitude)
+                   (if (> calendar-longitude 0) "E" "W"))))
+        (calendar-standard-time-zone-name
+         (cond ((= calendar-time-zone 0) "UT")
+               ((< calendar-time-zone 0) (format "UT%dmin" calendar-time-zone))
+               (t                   (format "UT+%dmin" calendar-time-zone))))
+        (calendar-daylight-savings-starts nil)
+        (calendar-daylight-savings-ends nil)
+        (date (if (< arg 4)
+                  (calendar-current-date)
+                (calendar-read-date)))
+        (date-string (calendar-date-string date t))
+        (time-string (solar-sunrise-sunset date))
+        (msg (format "%s: %s" date-string time-string))
+        (one-window (one-window-p t)))
+   (if (<= (length msg) (screen-width))
+       (message msg)
+     (with-output-to-temp-buffer "*temp*"
+       (princ (concat date-string "\n" time-string)))
+     (message (substitute-command-keys
+               (if one-window
+                   (if pop-up-windows
+                       "Type \\[delete-other-windows] to remove temp window."
+                     "Type \\[switch-to-buffer] RET to remove temp window.")
+                 "Type \\[switch-to-buffer-other-window] RET to restore old contents of temp window."))))))
+ 
+(defun calendar-sunrise-sunset ()
+  "Local time of sunrise and sunset for date under cursor.
+Accurate to +/- 2 minutes."
+  (interactive)
+  (if (not (and calendar-latitude calendar-longitude calendar-time-zone))
+      (solar-setup))
+  (message
+   (solar-sunrise-sunset
+    (or (calendar-cursor-to-date)
+        (error "Cursor is not on a date!")))))
+
+(defun diary-sunrise-sunset ()
+  "Local time of sunrise and sunset as a diary entry.
+Accurate to +/- 2 minutes."
+  (if (not (and calendar-latitude calendar-longitude calendar-time-zone))
+      (solar-setup))
+  (solar-sunrise-sunset date))
+
+(defun diary-sabbath-candles ()
+  "Local time of candle lighting diary entry--applies if date is a Friday.
+No diary entry if there is no sunset on that date."
+  (if (not (and calendar-latitude calendar-longitude calendar-time-zone))
+      (solar-setup))
+  (if (= (% (calendar-absolute-from-gregorian date) 7) 5);;  Friday
+      (let* ((sunset (solar-sunset date))
+             (light (if sunset (- sunset (/ 18.0 60.0)))))
+        (if light (format "%s Sabbath candle lighting"
+                          (solar-time-string light date))))))
+
+(defun calendar-holiday-function-solar-equinoxes-solstices ()
+  "Date and time of equinoxes and solstices, if visible in the calendar window.
+Requires floating point."
+  (let* ((m displayed-month)
+         (y displayed-year))
+    (increment-calendar-month m y (cond ((= 1 (% m 3)) -1)
+                                        ((= 2 (% m 3))  1)
+                                        (t              0)))
+    (let* ((calendar-standard-time-zone-name
+            (if calendar-time-zone calendar-standard-time-zone-name "UT"))
+           (calendar-daylight-savings-starts
+            (if calendar-time-zone calendar-daylight-savings-starts))
+           (calendar-daylight-savings-ends
+            (if calendar-time-zone calendar-daylight-savings-ends))
+           (calendar-time-zone (if calendar-time-zone calendar-time-zone 0))
+           (k (1- (/ m 3)))
+           (date (solar-equinoxes/solstices k y))
+           (day (extract-calendar-day date))
+           (time (* 24 (- day (truncate day))))
+           ;; Time zone/DST can't move the date out of range,
+           ;; so let solar-time-string do the conversion.
+           (date (list (extract-calendar-month date)
+                       (truncate day)
+                       (extract-calendar-year date))))
+      (list (list date
+                  (format "%s %s"
+                          (cond ((= k 0)  "Vernal Equinox")
+                                ((= k 1)  "Summer Solstice")
+                                ((= k 2)  "Fall Equinox")
+                                ((= k 3)  "Winter Solstice"))
+                          (solar-time-string time date)))))))
+
+(provide 'solar)
+
+;;; solar.el ends here