diff -Naur ./orig/xephem-3.7.7/libastro/refract.c ./patched/xephem-3.7.7/libastro/refract.c
--- ./orig/xephem-3.7.7/libastro/refract.c	2005-03-20 12:04:52.000000000 +0100
+++ ./patched/xephem-3.7.7/libastro/refract.c	2020-09-19 17:57:41.050022062 +0200
@@ -62,6 +62,11 @@

 	double d, t, t0, a;

+	if (isnan (ta)) {
+	    *aa = ta;
+	    return;
+	}
+
 	/* first guess of error is to go backwards.
 	 * make use that we know delta-apparent is always < delta-true.
 	 */