GNU bug report logs - #61460
30.0.50; Calendar shows eclipse for quarter moon

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From: Michael Heerdegen <michael_heerdegen <at> web.de>
To: Ulrich Mueller <ulm <at> gentoo.org>
Cc: 61460 <at> debbugs.gnu.org, Nicholas Strauss <nicholas.strauss <at> gmail.com>
Subject: bug#61460: 30.0.50; Calendar shows eclipse for quarter moon
Date: Tue, 14 Feb 2023 10:15:59 +0100

Ulrich Mueller <ulm <at> gentoo.org> writes:

> I wonder why the angles in above explanation are completely different
> from the ones given in eclipse-check:
>
> 	  ((< moon-lat 2.42600766e-1)
> 	   (concat "** " phase-name " Eclipse **"))
> 	  ((< moon-lat 0.37)
> 	   (concat "** " phase-name " Eclipse possible **"))
>
> These are in radians. The first angle is 13.9° (exactly) and the
> second one is 21.2°. They are in argument of latitude, while (IIUC)
> the angles in "Finsternis-Limit" are given in ecliptic longitude.
> Still, with the moon's orbital inclination being only 5°, one wouldn't
> expect much of a difference there. Or am I missing something?

Indeed, I wondered too about those numbers (also a bit about the format
and the number of digits) and don't have an explanation.


AFAIU (rectangular spherical triangle) we should have something like

 tan 𝚫u = tan 𝚫λ / cos i  [did I get it right?]

(with u: argument of latitude, λ: ecliptic longitude, 𝚫 meaning distance
from lunar node, i inclination (~ 5 degrees)).  The conversion does not
make much of a difference.


Michael.

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