Subject: Re: bug#61460: 30.0.50; Calendar shows eclipse for quarter moon

From: Ulrich Müller <[email-masked]>
Date: Fri, 17 Feb 2023 08:03:47 +0100

>>>>> On Fri, 17 Feb 2023, Michael Heerdegen wrote:

> I have some more cosmetic changes:

> [...]

>         (node-dist (funcall (lambda (x) (min x (- 180 x)))
>                             (mod moon-lat 180))))

I didn't go for this one, because IMHO it is too clever and would
worsen readability of the code. (Also, there is precedent for
successive rebinding in function lunar-phase.)

> In particular, the name of the global function should start with
> "lunar-".  Does that look ok?

Thank you. Updated patch below.

>From 4a3abeac5c6614c9b0b346767627f978b2dd138a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ulrich=20M=C3=BCller?= <[email-masked]>
Date: Thu, 16 Feb 2023 20:09:22 +0100
Subject: [PATCH] ; Adjust limit for eclipse in calendar/lunar.el, rename
 function

* lisp/calendar/lunar.el (lunar-check-for-eclipse): Renamed from
'eclipse-check'; thanks to Michael Heerdegen for the suggestion.
Slightly adjust the upper limit for the distance from the node to
the value found in literature. (bug#61460)
---
 lisp/calendar/lunar.el | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/lisp/calendar/lunar.el b/lisp/calendar/lunar.el
index 1f827ca34b0..fe8129ec511 100644
--- a/lisp/calendar/lunar.el
+++ b/lisp/calendar/lunar.el
@@ -94,7 +94,7 @@ remainder mod 4 gives the phase: 0 new moon, 1 first quarter, 2 full moon,
                        (* -0.0016528 time time)
                        (* -0.00000239 time time time))
                     360.0))
-	 (eclipse (eclipse-check moon-lat phase))
+         (eclipse (lunar-check-for-eclipse moon-lat phase))
          (adjustment
           (if (memq phase '(0 2))
               (+ (* (- 0.1734 (* 0.000393 time))
@@ -154,19 +154,18 @@ 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)
+(defun lunar-check-for-eclipse (moon-lat phase)
   (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 "") "")
-     ((< node-dist 13.9) (concat "** " phase-name " Eclipse **"))
-     ((< node-dist 21.2) (concat "** " phase-name " Eclipse possible **"))
-     (t ""))))
+         (type (cond ((= phase 0) "Solar")
+                     ((= phase 2) "Lunar"))))
+    (cond ((not type) "")
+          ;; Limits 13.9° and 21.0° from Meeus (1991), page 350.
+          ((< node-dist 13.9) (concat "** " type " Eclipse **"))
+          ((< node-dist 21.0) (concat "** " type " Eclipse possible **"))
+          (t ""))))
 
 (defconst lunar-cycles-per-year 12.3685 ; 365.25/29.530588853
   "Mean number of lunar cycles per 365.25 day year.")
-- 
2.39.2