[avr-libc-commit] [2345] Updated equation_of_time.c, solar_declination.c, and ephemera_common .h, for a better fit to USNO data.

[Mike Rice]

Revision: 2345
          http://svn.sv.gnu.org/viewvc/?view=rev&root=avr-libc&revision=2345
Author:   swfltek
Date:     2013-04-11 23:58:48 +0000 (Thu, 11 Apr 2013)
Log Message:
-----------
Updated equation_of_time.c, solar_declination.c, and ephemera_common.h, for a 
better fit to USNO data. 

I had been using data from both USNO and NOAA, before discovering that they do 
not agree on some important data points. 

This update resolves the issues I have been having with these to functions.

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

Modified: trunk/avr-libc/libc/time/ephemera_common.h
===================================================================
--- trunk/avr-libc/libc/time/ephemera_common.h  2013-04-10 19:52:09 UTC (rev 
2344)
+++ trunk/avr-libc/libc/time/ephemera_common.h  2013-04-11 23:58:48 UTC (rev 
2345)
@@ -39,5 +39,6 @@
 #define TWO_PI 6.283185307179586
 #define TROP_CYCLE 5022440.6025
 #define ANOM_CYCLE 5022680.6082
+#define DELTA_V 0.03342044    /* 2x orbital eccentricity */
 
 #endif

Modified: trunk/avr-libc/libc/time/equation_of_time.c
===================================================================
--- trunk/avr-libc/libc/time/equation_of_time.c 2013-04-10 19:52:09 UTC (rev 
2344)
+++ trunk/avr-libc/libc/time/equation_of_time.c 2013-04-11 23:58:48 UTC (rev 
2345)
@@ -41,24 +41,30 @@
 equation_of_time(time_t * timer)
 {
     int32_t         s, p;
-    double          pf, sf;
+    double          pf, sf, dV;
 
-    /* compute orbital positions */
+    /* compute orbital position relative to perihelion */
     p = *timer % ANOM_YEAR;
     p += PERIHELION;
     pf = p;
     pf /= ANOM_CYCLE;
+    pf = sin(pf);
 
+    /* Derive a velocity correction factor from the perihelion angle */
+    dV = pf * DELTA_V;
+
+    /* compute approximate position relative to solstice */
     s = *timer % TROP_YEAR;
     s += SOLSTICE;
+    s *= 2;
     sf = s;
+
     sf /= TROP_CYCLE;
-    sf *= 2.0;        /* 2x tropical angle */
+    sf += dV;
 
-    pf = sin(pf);
     sf = sin(sf);
-    pf *= 458.0;
-    sf *= 592.0;
+    pf *= 459.6;
+    sf *= 592.2;
 
     s = pf + sf;
     return -s;

Modified: trunk/avr-libc/libc/time/solar_declination.c
===================================================================
--- trunk/avr-libc/libc/time/solar_declination.c        2013-04-10 19:52:09 UTC 
(rev 2344)
+++ trunk/avr-libc/libc/time/solar_declination.c        2013-04-11 23:58:48 UTC 
(rev 2345)
@@ -32,8 +32,7 @@
 #include <math.h>
 #include "ephemera_common.h"
 
-#define DELTA_V 0.033
-#define LAG 37800
+#define LAG 38520
 
 double
 solar_declination(time_t * timer)