[avr-libc-commit] [2341] Modified 'solar_declination' to conform more closely to the zeros of the 'full' equation.

[Mike Rice]

Revision: 2341
          http://svn.sv.gnu.org/viewvc/?view=rev&root=avr-libc&revision=2341
Author:   swfltek
Date:     2013-04-08 20:24:58 +0000 (Mon, 08 Apr 2013)
Log Message:
-----------
Modified 'solar_declination' to conform more closely to the zeros of the 'full' 
equation.

Modified Paths:
--------------
    trunk/avr-libc/libc/time/solar_declination.c

Modified: trunk/avr-libc/libc/time/solar_declination.c
===================================================================
--- trunk/avr-libc/libc/time/solar_declination.c        2013-04-08 20:21:57 UTC 
(rev 2340)
+++ trunk/avr-libc/libc/time/solar_declination.c        2013-04-08 20:24:58 UTC 
(rev 2341)
@@ -28,49 +28,44 @@
 
 /* $Id$ */
 
-/*
-    Basic Solar Declination calculation.
-*/
-
 #include <time.h>
 #include <math.h>
+#include "ephemera_common.h"
 
-#define TROP_YEAR 31556925
-#define ANOM_YEAR 31558433
-#define INCLINATION 0.409105176667471 /* Earths axial tilt */
-#define PERIHELION 31218166 /* perihelion of 1999 */
-#define SOLSTICE 836160 /* winter solstice of 1999 */
-#define TWO_PI 6.283185307179586
-#define DELTA_V 0.0167
+#define DELTA_V 0.033
+#define LAG 37800
 
 double
 solar_declination(time_t * timer)
 {
 
-    uint32_t   fT, oV;
+    uint32_t        fT, oV;
     double          dV, dT;
 
-    /* fractional anomalistic year */
+    /*
+     * Determine approximate orbital angle, relative to the winter
+     * solstice
+     */
+    fT = *timer % TROP_YEAR;
+    fT += SOLSTICE;
+    fT += LAG;
+    dT = fT;
+    dT /= TROP_YEAR;
+    dT *= TWO_PI;
+
+    /* Determine approximate orbital angle relative to perihelion */
     oV = *timer % ANOM_YEAR;
     oV += PERIHELION;
-
-    /* convert to angle */
     dV = oV;
     dV /= ANOM_YEAR;
     dV *= TWO_PI;
 
-    /* orbital velocity correction */
+    /* Derive a velocity correction factor from the perihelion angle */
     dV = sin(dV);
     dV *= DELTA_V;
 
-     /* fractional tropical year */
-    fT = *timer % TROP_YEAR;
-    fT += SOLSTICE;
-    dT = fT;
-    dT /= TROP_YEAR;
-    dT *= TWO_PI;
+    /* compute declination */
+    dT = cos(dT + dV) * INCLINATION;
 
-    dT = cos(dT+dV) * INCLINATION;
-
     return -dT;
 }