Function: calendar-bahai-twin-holy-birthdays-for-year

calendar-bahai-twin-holy-birthdays-for-year is a byte-compiled function defined in cal-bahai.el.gz.

Signature

(calendar-bahai-twin-holy-birthdays-for-year BAHAI-YEAR)

Documentation

Calculate Gregorian dates of the Twin Holy Birthdays for Bahá’í YEAR.

Returns a list of two Gregorian dates: (BAB-DATE BAHA-DATE). The dates are determined by the eighth new moon after Naw-Rúz, calculated for Tehran's timezone. The first day following the eighth new moon is the Birth of the Báb, and the second day is the Birth of Bahá’u’lláh.

Source Code

;; Defined in /usr/src/emacs/lisp/calendar/cal-bahai.el.gz
(defun calendar-bahai-twin-holy-birthdays-for-year (bahai-year)
  "Calculate Gregorian dates of the Twin Holy Birthdays for Bahá’í YEAR.
Returns a list of two Gregorian dates: (BAB-DATE BAHA-DATE).
The dates are determined by the eighth new moon after Naw-Rúz,
calculated for Tehran's timezone.  The first day following the
eighth new moon is the Birth of the Báb, and the second day is
the Birth of Bahá’u’lláh."
  (let* ((calendar-time-zone calendar-bahai-tehran-timezone)
         ;; Disable DST for Tehran
         (calendar-daylight-savings-starts nil)
         (calendar-daylight-savings-ends nil)
         ;; Get absolute date of Naw-Rúz for this Bahá’í year
         (nawruz-absolute (calendar-bahai-nawruz bahai-year))
         ;; Naw-Rúz starts at sunset on this date in Tehran
         ;; We need to find new moons that occur AFTER sunset on Naw-Rúz
         (nawruz-greg (calendar-gregorian-from-absolute nawruz-absolute))
         ;; Get sunset time on Naw-Rúz in Tehran
         (nawruz-sunset-data (let ((calendar-latitude calendar-bahai-tehran-latitude)
                                   (calendar-longitude calendar-bahai-tehran-longitude))
                              (solar-sunrise-sunset nawruz-greg)))
         (nawruz-sunset (or (car (cadr nawruz-sunset-data)) 18.0)) ; default 6pm
         ;; Convert Naw-Rúz sunset to Julian day
         ;; Absolute date is at midnight, add fraction for sunset time
         (nawruz-julian (+ (calendar-astro-from-absolute nawruz-absolute)
                           (/ nawruz-sunset 24.0)))
         ;; Find the 8th new moon after Naw-Rúz sunset
         (current-julian nawruz-julian)
         eighth-new-moon)
    ;; Find 8 new moons after Naw-Rúz by iterating
    (dotimes (_ 8)
      (setq current-julian (lunar-new-moon-on-or-after current-julian))
      (setq eighth-new-moon current-julian)
      ;; Move forward by at least one day to find the next new moon
      ;; (lunar cycle is ~29.5 days, so +1 is safe)
      (setq current-julian (+ current-julian 1)))

    ;; Convert the eighth new moon to absolute date and time
    (let* ((new-moon-abs-with-frac (calendar-astro-to-absolute eighth-new-moon))
           (new-moon-absolute (floor new-moon-abs-with-frac))
           (new-moon-time-frac (- new-moon-abs-with-frac new-moon-absolute))
           (new-moon-time-hours (* 24 new-moon-time-frac))
           (new-moon-greg (calendar-gregorian-from-absolute new-moon-absolute))
           ;; Get sunset time in Tehran for the day of the new moon
           (sunset-data (let ((calendar-latitude calendar-bahai-tehran-latitude)
                              (calendar-longitude calendar-bahai-tehran-longitude)
                              (calendar-time-zone calendar-bahai-tehran-timezone))
                          (solar-sunrise-sunset new-moon-greg)))
           ;; solar-sunrise-sunset returns ((sunrise-time zone) (sunset-time zone) daylight)
           (sunset-time (car (cadr sunset-data)))
           ;; Determine which civil day is the "first day following" the new moon
           ;; In Bahá’í calendar, days run from sunset to sunset.
           ;; If new moon occurs before sunset, the Bahá’í day starting at
           ;; sunset that evening begins the "first day following"
           ;; If new moon occurs after sunset, we're already in the next Bahá’í
           ;; day, so the "first day following" starts at the next sunset
           (bab-absolute (if (and sunset-time (< new-moon-time-hours sunset-time))
                             (1+ new-moon-absolute)  ; Next civil day
                           (+ new-moon-absolute 2))) ; Civil day after next
           (baha-absolute (1+ bab-absolute)))
      ;; Return both dates as Gregorian dates
      (list (calendar-gregorian-from-absolute bab-absolute)
            (calendar-gregorian-from-absolute baha-absolute)))))