Fix phase of moon calculation

The reported phase was often one day early because calculations were
based on midnight. For full and new moon this is changed to noon. For
first and last quarter a direct check of the appearance of the half moon
during the selected day is performed.

Comments are relocated and expanded.

Signed-off-by: Lars Henriksen <LarsHenriksen@get2net.dk>
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

1 files changed, 30 insertions, 27 deletions

diff --git a/src/ui-calendar.c b/src/ui-calendar.c
index 419f7da..9ea31e1 100644
--- a/src/ui-calendar.c
+++ b/src/ui-calendar.c
@@ -843,8 +843,22 @@ static void adj360(double *deg)
 }
 
 /*
+ * Phase of the Moon.  Calculates the current phase of the moon.
+ * Based on routines from `Practical Astronomy with Your Calculator',
+ * by Duffett-Smith.  Comments give the section from the book that
+ * particular piece of code was adapted from.
+ *
+ * -- Keith E. Brandt  VIII 1984
+ *
+ * Updated to the Third Edition of Duffett-Smith's book, IX 1998
+ *
+ */
+
+/*
  * potm --
- *	return phase of the moon
+ *	given number of days since January 1st 1990, 00:00
+ *	(incl. hours and minutes as a fraction of a day),
+ *	return phase of the moon as a percentage.
  */
 static double potm(double days)
 {
@@ -877,17 +891,6 @@ static double potm(double days)
 	return 50.0 * (1 - cos(dtor(D)));	/* sec 67 #3 */
 }
 
-/*
- * Phase of the Moon.  Calculates the current phase of the moon.
- * Based on routines from `Practical Astronomy with Your Calculator',
- * by Duffett-Smith.  Comments give the section from the book that
- * particular piece of code was adapted from.
- *
- * -- Keith E. Brandt  VIII 1984
- *
- * Updated to the Third Edition of Duffett-Smith's book, IX 1998
- *
- */
 static double pom(time_t tmpt)
 {
 	struct tm *GMT;
@@ -908,30 +911,30 @@ static double pom(time_t tmpt)
 
 /*
  * Return a pictogram representing the current phase of the moon.
- * Careful: date is the selected day in calendar at 00:00, so it represents
- * the phase of the moon for previous day.
+ * Careful: date is the selected day in calendar at 00:00,
+ * and so represents the phase of the moon at midnight.
  */
 const char *ui_calendar_get_pom(time_t date)
 {
 	const char *pom_pict[MOON_PHASES] =
 	    { "   ", "|) ", "(|)", "(| ", " | " };
 	enum pom phase = NO_POM;
-	double pom_today, relative_pom, pom_yesterday, pom_tomorrow;
 	const double half = 50.0;
 
-	pom_yesterday = pom(date);
-	pom_today = pom(date + DAYINSEC);
-	relative_pom = abs(pom_today - half);
-	pom_tomorrow = pom(date + 2 * DAYINSEC);
-	if (pom_today > pom_yesterday && pom_today > pom_tomorrow)
+	date += DAYINSEC / 2;				/* adjust to noon */
+	double pom_n_b = pom(date - DAYINSEC);	 	/* noon before */
+	double pom_m_b = pom(date - DAYINSEC / 2);	/* midnight before */
+	double pom_n   = pom(date); 			/* noon */
+	double pom_m_a = pom(date + DAYINSEC / 2);	/* midnight after */
+	double pom_n_a = pom(date + DAYINSEC);	 	/* noon after */
+
+	if (pom_n > pom_n_b && pom_n > pom_n_a)
 		phase = FULL_MOON;
-	else if (pom_today < pom_yesterday && pom_today < pom_tomorrow)
+	else if (pom_n < pom_n_b && pom_n < pom_n_a)
 		phase = NEW_MOON;
-	else if (relative_pom < abs(pom_yesterday - half)
-		 && relative_pom < abs(pom_tomorrow - half))
-		phase =
-		    (pom_tomorrow >
-		     pom_today) ? FIRST_QUARTER : LAST_QUARTER;
-
+	else if (pom_m_b < half && half < pom_m_a)
+		phase = FIRST_QUARTER;
+	else if (pom_m_b > half && half > pom_m_a)
+		phase = LAST_QUARTER;
 	return pom_pict[phase];
 }