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; Simplify eclipse calculation in calendar/lunar.el

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* lisp/calendar/lunar.el (eclipse-check): Do the calculation in
degrees, and simplify.
This commit is contained in:
Ulrich Müller 2023-02-14 19:51:37 +01:00
parent 1e0f4c554b
commit 80b34d165d
1 changed files with 9 additions and 16 deletions
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25
lisp/calendar/lunar.el
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@ -155,25 +155,18 @@ remainder mod 4 gives the phase: 0 new moon, 1 first quarter, 2 full moon,
;; from "Astronomy with your Personal Computer", Subroutine Eclipse
;; Line 7000 Peter Duffett-Smith Cambridge University Press 1990
(defun eclipse-check (moon-lat phase)
(let* ((moon-lat (* (/ float-pi 180) moon-lat))
;; For positions near the ascending or descending node,
;; calculate the absolute angular distance from that node.
(moon-lat (abs (- moon-lat (* (floor (/ moon-lat float-pi))
float-pi))))
(moon-lat (if (> moon-lat 0.37) ; FIXME (* 0.5 float-pi)
(- float-pi moon-lat)
moon-lat))
(let* ((node-dist (mod moon-lat 180))
;; Absolute angular distance from the ascending or descending
;; node, whichever is nearer.
(node-dist (min node-dist (- 180 node-dist)))
(phase-name (cond ((= phase 0) "Solar")
((= phase 2) "Lunar")
(t ""))))
(cond ((string= phase-name "")
"")
((< moon-lat 2.42600766e-1)
(concat "** " phase-name " Eclipse **"))
((< moon-lat 0.37)
(concat "** " phase-name " Eclipse possible **"))
(t
""))))
(cond
((string= phase-name "") "")
((< node-dist 13.9) (concat "** " phase-name " Eclipse **"))
((< node-dist 21.2) (concat "** " phase-name " Eclipse possible **"))
(t ""))))
(defconst lunar-cycles-per-year 12.3685 ; 365.25/29.530588853
"Mean number of lunar cycles per 365.25 day year.")