From 139b2ea2f156f5e968d116fc41e8e2b88732ea0d Mon Sep 17 00:00:00 2001
From: Brandon Rhodes <brandon@rhodesmill.org>
Date: Thu, 27 Jan 2022 14:27:21 -0500
Subject: [PATCH] Revert "Astronomical Almanac 2020 Pluto elements (#2 #22)"
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This reverts commit 26e5d1abc3e9a0a74ed7b569baf0c08b788fc898.  It broke
Pluto’s motion, which users noticed as a big jump at the dates where we
switch from the short-term solution to these long-term orbital elements.
(Reported to the mailing list on groups.io: thank you, Maxime Gommeaux!)
---
 libastro/plans.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/libastro/plans.c b/libastro/plans.c
index 9680c2a..08481e3 100644
--- a/libastro/plans.c
+++ b/libastro/plans.c
@@ -45,17 +45,17 @@ pluto_ell (
 double mj,	/* epoch */
 double *ret)	/* ecliptic coordinates {l,b,r} at equinox of date */
 {
-	/* Mean orbital elements of Pluto.
-	 * Astronomical Almanac 2020 page G3.
+	/* mean orbital elements of Pluto.
+	 * The origin of these is somewhat obscure.
 	 */
-	double	a = 39.789,			/* semimajor axis, au */
-		e = 0.252,			/* excentricity */
-		inc0 = 17.097,			/* inclination, deg */
-		Om0 = 110.297,			/* long asc node, deg */
-		omeg0 = 115.058,		/* arg of perihel, deg */
-		mjp = 2459000.5 - MJD0,		/* epoch of perihel */
+	double	a = 39.543,			/* semimajor axis, au */
+		e = 0.2490,			/* excentricity */
+		inc0 = 17.140,			/* inclination, deg */
+		Om0 = 110.307,			/* long asc node, deg */
+		omeg0 = 113.768,		/* arg of perihel, deg */
+		mjp = 2448045.539 - MJD0,	/* epoch of perihel */
 		mjeq = J2000,			/* equinox of elements */
-		n = 0.0039;			/* daily motion, deg */
+		n = 144.9600/36525.;            /* daily motion, deg */
 
 	double inc, Om, omeg;	/* orbital elements at epoch of date */
 	double ma, ea, nu;	/* mean, excentric and true anomaly */